LazyFrame#

This page gives an overview of all public LazyFrame methods.

class polars.LazyFrame(
data: FrameInitTypes | None = None,
schema: SchemaDefinition | None = None,
*,
schema_overrides: SchemaDict | None = None,
strict: bool = True,
orient: Orientation | None = None,
infer_schema_length: int | None = 100,
nan_to_null: bool = False,
)[source]

Representation of a Lazy computation graph/query against a DataFrame.

This allows for whole-query optimisation in addition to parallelism, and is the preferred (and highest-performance) mode of operation for polars.

Parameters:
datadict, Sequence, ndarray, Series, or pandas.DataFrame

Two-dimensional data in various forms; dict input must contain Sequences, Generators, or a range. Sequence may contain Series or other Sequences.

schemaSequence of str, (str,DataType) pairs, or a {str:DataType,} dict

The LazyFrame schema may be declared in several ways:

  • As a dict of {name:type} pairs; if type is None, it will be auto-inferred.

  • As a list of column names; in this case types are automatically inferred.

  • As a list of (name,type) pairs; this is equivalent to the dictionary form.

If you supply a list of column names that does not match the names in the underlying data, the names given here will overwrite them. The number of names given in the schema should match the underlying data dimensions.

schema_overridesdict, default None

Support type specification or override of one or more columns; note that any dtypes inferred from the schema param will be overridden.

The number of entries in the schema should match the underlying data dimensions, unless a sequence of dictionaries is being passed, in which case a partial schema can be declared to prevent specific fields from being loaded.

strictbool, default True

Throw an error if any data value does not exactly match the given or inferred data type for that column. If set to False, values that do not match the data type are cast to that data type or, if casting is not possible, set to null instead.

orient{‘col’, ‘row’}, default None

Whether to interpret two-dimensional data as columns or as rows. If None, the orientation is inferred by matching the columns and data dimensions. If this does not yield conclusive results, column orientation is used.

infer_schema_lengthint or None

The maximum number of rows to scan for schema inference. If set to None, the full data may be scanned (this can be slow). This parameter only applies if the input data is a sequence or generator of rows; other input is read as-is.

nan_to_nullbool, default False

If the data comes from one or more numpy arrays, can optionally convert input data np.nan values to null instead. This is a no-op for all other input data.

Notes

Initialising LazyFrame(...) directly is equivalent to DataFrame(...).lazy().

Examples

Constructing a LazyFrame directly from a dictionary:

>>> data = {"a": [1, 2], "b": [3, 4]}
>>> lf = pl.LazyFrame(data)
>>> lf.collect()
shape: (2, 2)
┌─────┬─────┐
│ a   ┆ b   │
│ --- ┆ --- │
│ i64 ┆ i64 │
╞═════╪═════╡
│ 1   ┆ 3   │
│ 2   ┆ 4   │
└─────┴─────┘

Notice that the dtypes are automatically inferred as Polars Int64:

>>> lf.collect_schema().dtypes()
[Int64, Int64]

To specify a more detailed/specific frame schema you can supply the schema parameter with a dictionary of (name,dtype) pairs…

>>> data = {"col1": [0, 2], "col2": [3, 7]}
>>> lf2 = pl.LazyFrame(data, schema={"col1": pl.Float32, "col2": pl.Int64})
>>> lf2.collect()
shape: (2, 2)
┌──────┬──────┐
│ col1 ┆ col2 │
│ ---  ┆ ---  │
│ f32  ┆ i64  │
╞══════╪══════╡
│ 0.0  ┆ 3    │
│ 2.0  ┆ 7    │
└──────┴──────┘

…a sequence of (name,dtype) pairs…

>>> data = {"col1": [1, 2], "col2": [3, 4]}
>>> lf3 = pl.LazyFrame(data, schema=[("col1", pl.Float32), ("col2", pl.Int64)])
>>> lf3.collect()
shape: (2, 2)
┌──────┬──────┐
│ col1 ┆ col2 │
│ ---  ┆ ---  │
│ f32  ┆ i64  │
╞══════╪══════╡
│ 1.0  ┆ 3    │
│ 2.0  ┆ 4    │
└──────┴──────┘

…or a list of typed Series.

>>> data = [
...     pl.Series("col1", [1, 2], dtype=pl.Float32),
...     pl.Series("col2", [3, 4], dtype=pl.Int64),
... ]
>>> lf4 = pl.LazyFrame(data)
>>> lf4.collect()
shape: (2, 2)
┌──────┬──────┐
│ col1 ┆ col2 │
│ ---  ┆ ---  │
│ f32  ┆ i64  │
╞══════╪══════╡
│ 1.0  ┆ 3    │
│ 2.0  ┆ 4    │
└──────┴──────┘

Constructing a LazyFrame from a numpy ndarray, specifying column names:

>>> import numpy as np
>>> data = np.array([(1, 2), (3, 4)], dtype=np.int64)
>>> lf5 = pl.LazyFrame(data, schema=["a", "b"], orient="col")
>>> lf5.collect()
shape: (2, 2)
┌─────┬─────┐
│ a   ┆ b   │
│ --- ┆ --- │
│ i64 ┆ i64 │
╞═════╪═════╡
│ 1   ┆ 3   │
│ 2   ┆ 4   │
└─────┴─────┘

Constructing a LazyFrame from a list of lists, row orientation specified:

>>> data = [[1, 2, 3], [4, 5, 6]]
>>> lf6 = pl.LazyFrame(data, schema=["a", "b", "c"], orient="row")
>>> lf6.collect()
shape: (2, 3)
┌─────┬─────┬─────┐
│ a   ┆ b   ┆ c   │
│ --- ┆ --- ┆ --- │
│ i64 ┆ i64 ┆ i64 │
╞═════╪═════╪═════╡
│ 1   ┆ 2   ┆ 3   │
│ 4   ┆ 5   ┆ 6   │
└─────┴─────┴─────┘

Methods:

approx_n_unique

Approximate count of unique values.

bottom_k

Return the k smallest rows.

cache

Cache the result once the execution of the physical plan hits this node.

cast

Cast LazyFrame column(s) to the specified dtype(s).

clear

Create an empty copy of the current LazyFrame, with zero to 'n' rows.

clone

Create a copy of this LazyFrame.

collect

Materialize this LazyFrame into a DataFrame.

collect_async

Collect DataFrame asynchronously in thread pool.

collect_schema

Resolve the schema of this LazyFrame.

count

Return the number of non-null elements for each column.

describe

Creates a summary of statistics for a LazyFrame, returning a DataFrame.

deserialize

Read a logical plan from a file to construct a LazyFrame.

drop

Remove columns from the DataFrame.

drop_nulls

Drop all rows that contain null values.

explain

Create a string representation of the query plan.

explode

Explode the DataFrame to long format by exploding the given columns.

fetch

Collect a small number of rows for debugging purposes.

fill_nan

Fill floating point NaN values.

fill_null

Fill null values using the specified value or strategy.

filter

Filter the rows in the LazyFrame based on a predicate expression.

first

Get the first row of the DataFrame.

gather_every

Take every nth row in the LazyFrame and return as a new LazyFrame.

group_by

Start a group by operation.

group_by_dynamic

Group based on a time value (or index value of type Int32, Int64).

head

Get the first n rows.

inspect

Inspect a node in the computation graph.

interpolate

Interpolate intermediate values.

join

Add a join operation to the Logical Plan.

join_asof

Perform an asof join.

last

Get the last row of the DataFrame.

lazy

Return lazy representation, i.e. itself.

limit

Get the first n rows.

map_batches

Apply a custom function.

max

Aggregate the columns in the LazyFrame to their maximum value.

mean

Aggregate the columns in the LazyFrame to their mean value.

median

Aggregate the columns in the LazyFrame to their median value.

melt

Unpivot a DataFrame from wide to long format.

merge_sorted

Take two sorted DataFrames and merge them by the sorted key.

min

Aggregate the columns in the LazyFrame to their minimum value.

null_count

Aggregate the columns in the LazyFrame as the sum of their null value count.

pipe

Offers a structured way to apply a sequence of user-defined functions (UDFs).

profile

Profile a LazyFrame.

quantile

Aggregate the columns in the LazyFrame to their quantile value.

rename

Rename column names.

reverse

Reverse the DataFrame.

rolling

Create rolling groups based on a temporal or integer column.

select

Select columns from this LazyFrame.

select_seq

Select columns from this LazyFrame.

serialize

Serialize the logical plan of this LazyFrame to a file or string in JSON format.

set_sorted

Indicate that one or multiple columns are sorted.

shift

Shift values by the given number of indices.

show_graph

Show a plot of the query plan.

sink_csv

Evaluate the query in streaming mode and write to a CSV file.

sink_ipc

Evaluate the query in streaming mode and write to an IPC file.

sink_ndjson

Evaluate the query in streaming mode and write to an NDJSON file.

sink_parquet

Evaluate the query in streaming mode and write to a Parquet file.

slice

Get a slice of this DataFrame.

sort

Sort the LazyFrame by the given columns.

sql

Execute a SQL query against the LazyFrame.

std

Aggregate the columns in the LazyFrame to their standard deviation value.

sum

Aggregate the columns in the LazyFrame to their sum value.

tail

Get the last n rows.

top_k

Return the k largest rows.

unique

Drop duplicate rows from this DataFrame.

unnest

Decompose struct columns into separate columns for each of their fields.

unpivot

Unpivot a DataFrame from wide to long format.

update

Update the values in this LazyFrame with the values in other.

var

Aggregate the columns in the LazyFrame to their variance value.

with_columns

Add columns to this LazyFrame.

with_columns_seq

Add columns to this LazyFrame.

with_context

Add an external context to the computation graph.

with_row_count

Add a column at index 0 that counts the rows.

with_row_index

Add a row index as the first column in the LazyFrame.

Attributes:

columns

Get the column names.

dtypes

Get the column data types.

schema

Get an ordered mapping of column names to their data type.

width

Get the number of columns.

approx_n_unique() LazyFrame[source]

Approximate count of unique values.

Deprecated since version 0.20.11: Use select(pl.all().approx_n_unique()) instead.

This is done using the HyperLogLog++ algorithm for cardinality estimation.

Examples

>>> lf = pl.LazyFrame(
...     {
...         "a": [1, 2, 3, 4],
...         "b": [1, 2, 1, 1],
...     }
... )
>>> lf.approx_n_unique().collect()  
shape: (1, 2)
┌─────┬─────┐
│ a   ┆ b   │
│ --- ┆ --- │
│ u32 ┆ u32 │
╞═════╪═════╡
│ 4   ┆ 2   │
└─────┴─────┘
bottom_k(
k: int,
*,
by: IntoExpr | Iterable[IntoExpr],
reverse: bool | Sequence[bool] = False,
) LazyFrame[source]

Return the k smallest rows.

Non-null elements are always preferred over null elements, regardless of the value of reverse. The output is not guaranteed to be in any particular order, call sort() after this function if you wish the output to be sorted.

Parameters:
k

Number of rows to return.

by

Column(s) used to determine the bottom rows. Accepts expression input. Strings are parsed as column names.

reverse

Consider the k largest elements of the by column(s) (instead of the k smallest). This can be specified per column by passing a sequence of booleans.

See also

top_k

Examples

>>> lf = pl.LazyFrame(
...     {
...         "a": ["a", "b", "a", "b", "b", "c"],
...         "b": [2, 1, 1, 3, 2, 1],
...     }
... )

Get the rows which contain the 4 smallest values in column b.

>>> lf.bottom_k(4, by="b").collect()
shape: (4, 2)
┌─────┬─────┐
│ a   ┆ b   │
│ --- ┆ --- │
│ str ┆ i64 │
╞═════╪═════╡
│ b   ┆ 1   │
│ a   ┆ 1   │
│ c   ┆ 1   │
│ a   ┆ 2   │
└─────┴─────┘

Get the rows which contain the 4 smallest values when sorting on column a and b.

>>> lf.bottom_k(4, by=["a", "b"]).collect()
shape: (4, 2)
┌─────┬─────┐
│ a   ┆ b   │
│ --- ┆ --- │
│ str ┆ i64 │
╞═════╪═════╡
│ a   ┆ 1   │
│ a   ┆ 2   │
│ b   ┆ 1   │
│ b   ┆ 2   │
└─────┴─────┘
cache() LazyFrame[source]

Cache the result once the execution of the physical plan hits this node.

It is not recommended using this as the optimizer likely can do a better job.

cast(
dtypes: Mapping[ColumnNameOrSelector | PolarsDataType, PolarsDataType] | PolarsDataType,
*,
strict: bool = True,
) LazyFrame[source]

Cast LazyFrame column(s) to the specified dtype(s).

Parameters:
dtypes

Mapping of column names (or selector) to dtypes, or a single dtype to which all columns will be cast.

strict

Throw an error if a cast could not be done (for instance, due to an overflow).

Examples

>>> from datetime import date
>>> lf = pl.LazyFrame(
...     {
...         "foo": [1, 2, 3],
...         "bar": [6.0, 7.0, 8.0],
...         "ham": [date(2020, 1, 2), date(2021, 3, 4), date(2022, 5, 6)],
...     }
... )

Cast specific frame columns to the specified dtypes:

>>> lf.cast({"foo": pl.Float32, "bar": pl.UInt8}).collect()
shape: (3, 3)
┌─────┬─────┬────────────┐
│ foo ┆ bar ┆ ham        │
│ --- ┆ --- ┆ ---        │
│ f32 ┆ u8  ┆ date       │
╞═════╪═════╪════════════╡
│ 1.0 ┆ 6   ┆ 2020-01-02 │
│ 2.0 ┆ 7   ┆ 2021-03-04 │
│ 3.0 ┆ 8   ┆ 2022-05-06 │
└─────┴─────┴────────────┘

Cast all frame columns matching one dtype (or dtype group) to another dtype:

>>> lf.cast({pl.Date: pl.Datetime}).collect()
shape: (3, 3)
┌─────┬─────┬─────────────────────┐
│ foo ┆ bar ┆ ham                 │
│ --- ┆ --- ┆ ---                 │
│ i64 ┆ f64 ┆ datetime[μs]        │
╞═════╪═════╪═════════════════════╡
│ 1   ┆ 6.0 ┆ 2020-01-02 00:00:00 │
│ 2   ┆ 7.0 ┆ 2021-03-04 00:00:00 │
│ 3   ┆ 8.0 ┆ 2022-05-06 00:00:00 │
└─────┴─────┴─────────────────────┘

Use selectors to define the columns being cast:

>>> import polars.selectors as cs
>>> lf.cast({cs.numeric(): pl.UInt32, cs.temporal(): pl.String}).collect()
shape: (3, 3)
┌─────┬─────┬────────────┐
│ foo ┆ bar ┆ ham        │
│ --- ┆ --- ┆ ---        │
│ u32 ┆ u32 ┆ str        │
╞═════╪═════╪════════════╡
│ 1   ┆ 6   ┆ 2020-01-02 │
│ 2   ┆ 7   ┆ 2021-03-04 │
│ 3   ┆ 8   ┆ 2022-05-06 │
└─────┴─────┴────────────┘

Cast all frame columns to the specified dtype:

>>> lf.cast(pl.String).collect().to_dict(as_series=False)
{'foo': ['1', '2', '3'],
 'bar': ['6.0', '7.0', '8.0'],
 'ham': ['2020-01-02', '2021-03-04', '2022-05-06']}
clear(n: int = 0) LazyFrame[source]

Create an empty copy of the current LazyFrame, with zero to ‘n’ rows.

Returns a copy with an identical schema but no data.

Parameters:
n

Number of (empty) rows to return in the cleared frame.

See also

clone

Cheap deepcopy/clone.

Examples

>>> lf = pl.LazyFrame(
...     {
...         "a": [None, 2, 3, 4],
...         "b": [0.5, None, 2.5, 13],
...         "c": [True, True, False, None],
...     }
... )
>>> lf.clear().collect()
shape: (0, 3)
┌─────┬─────┬──────┐
│ a   ┆ b   ┆ c    │
│ --- ┆ --- ┆ ---  │
│ i64 ┆ f64 ┆ bool │
╞═════╪═════╪══════╡
└─────┴─────┴──────┘
>>> lf.clear(2).collect()
shape: (2, 3)
┌──────┬──────┬──────┐
│ a    ┆ b    ┆ c    │
│ ---  ┆ ---  ┆ ---  │
│ i64  ┆ f64  ┆ bool │
╞══════╪══════╪══════╡
│ null ┆ null ┆ null │
│ null ┆ null ┆ null │
└──────┴──────┴──────┘
clone() LazyFrame[source]

Create a copy of this LazyFrame.

This is a cheap operation that does not copy data.

See also

clear

Create an empty copy of the current LazyFrame, with identical schema but no data.

Examples

>>> lf = pl.LazyFrame(
...     {
...         "a": [None, 2, 3, 4],
...         "b": [0.5, None, 2.5, 13],
...         "c": [True, True, False, None],
...     }
... )
>>> lf.clone()  
<LazyFrame at ...>
collect(
*,
type_coercion: bool = True,
predicate_pushdown: bool = True,
projection_pushdown: bool = True,
simplify_expression: bool = True,
slice_pushdown: bool = True,
comm_subplan_elim: bool = True,
comm_subexpr_elim: bool = True,
cluster_with_columns: bool = True,
no_optimization: bool = False,
streaming: bool = False,
background: bool = False,
_eager: bool = False,
**_kwargs: Any,
) DataFrame | InProcessQuery[source]

Materialize this LazyFrame into a DataFrame.

By default, all query optimizations are enabled. Individual optimizations may be disabled by setting the corresponding parameter to False.

Parameters:
type_coercion

Do type coercion optimization.

predicate_pushdown

Do predicate pushdown optimization.

projection_pushdown

Do projection pushdown optimization.

simplify_expression

Run simplify expressions optimization.

slice_pushdown

Slice pushdown optimization.

comm_subplan_elim

Will try to cache branching subplans that occur on self-joins or unions.

comm_subexpr_elim

Common subexpressions will be cached and reused.

cluster_with_columns

Combine sequential independent calls to with_columns

no_optimization

Turn off (certain) optimizations.

streaming

Process the query in batches to handle larger-than-memory data. If set to False (default), the entire query is processed in a single batch.

Warning

Streaming mode is considered unstable. It may be changed at any point without it being considered a breaking change.

Note

Use explain() to see if Polars can process the query in streaming mode.

background

Run the query in the background and get a handle to the query. This handle can be used to fetch the result or cancel the query.

Warning

Background mode is considered unstable. It may be changed at any point without it being considered a breaking change.

Returns:
DataFrame

See also

fetch

Run the query on the first n rows only for debugging purposes.

explain

Print the query plan that is evaluated with collect.

profile

Collect the LazyFrame and time each node in the computation graph.

polars.collect_all

Collect multiple LazyFrames at the same time.

polars.Config.set_streaming_chunk_size

Set the size of streaming batches.

Examples

>>> lf = pl.LazyFrame(
...     {
...         "a": ["a", "b", "a", "b", "b", "c"],
...         "b": [1, 2, 3, 4, 5, 6],
...         "c": [6, 5, 4, 3, 2, 1],
...     }
... )
>>> lf.group_by("a").agg(pl.all().sum()).collect()  
shape: (3, 3)
┌─────┬─────┬─────┐
│ a   ┆ b   ┆ c   │
│ --- ┆ --- ┆ --- │
│ str ┆ i64 ┆ i64 │
╞═════╪═════╪═════╡
│ a   ┆ 4   ┆ 10  │
│ b   ┆ 11  ┆ 10  │
│ c   ┆ 6   ┆ 1   │
└─────┴─────┴─────┘

Collect in streaming mode

>>> lf.group_by("a").agg(pl.all().sum()).collect(
...     streaming=True
... )  
shape: (3, 3)
┌─────┬─────┬─────┐
│ a   ┆ b   ┆ c   │
│ --- ┆ --- ┆ --- │
│ str ┆ i64 ┆ i64 │
╞═════╪═════╪═════╡
│ a   ┆ 4   ┆ 10  │
│ b   ┆ 11  ┆ 10  │
│ c   ┆ 6   ┆ 1   │
└─────┴─────┴─────┘
collect_async(
*,
gevent: bool = False,
type_coercion: bool = True,
predicate_pushdown: bool = True,
projection_pushdown: bool = True,
simplify_expression: bool = True,
no_optimization: bool = False,
slice_pushdown: bool = True,
comm_subplan_elim: bool = True,
comm_subexpr_elim: bool = True,
cluster_with_columns: bool = True,
streaming: bool = False,
) Awaitable[DataFrame] | _GeventDataFrameResult[DataFrame][source]

Collect DataFrame asynchronously in thread pool.

Warning

This functionality is considered unstable. It may be changed at any point without it being considered a breaking change.

Collects into a DataFrame (like collect()) but, instead of returning a DataFrame directly, it is scheduled to be collected inside a thread pool, while this method returns almost instantly.

This can be useful if you use gevent or asyncio and want to release control to other greenlets/tasks while LazyFrames are being collected.

Parameters:
gevent

Return wrapper to gevent.event.AsyncResult instead of Awaitable

type_coercion

Do type coercion optimization.

predicate_pushdown

Do predicate pushdown optimization.

projection_pushdown

Do projection pushdown optimization.

simplify_expression

Run simplify expressions optimization.

no_optimization

Turn off (certain) optimizations.

slice_pushdown

Slice pushdown optimization.

comm_subplan_elim

Will try to cache branching subplans that occur on self-joins or unions.

comm_subexpr_elim

Common subexpressions will be cached and reused.

cluster_with_columns

Combine sequential independent calls to with_columns

streaming

Process the query in batches to handle larger-than-memory data. If set to False (default), the entire query is processed in a single batch.

Warning

Streaming mode is considered unstable. It may be changed at any point without it being considered a breaking change.

Note

Use explain() to see if Polars can process the query in streaming mode.

Returns:
If gevent=False (default) then returns an awaitable.
If gevent=True then returns wrapper that has a
.get(block=True, timeout=None) method.

See also

polars.collect_all

Collect multiple LazyFrames at the same time.

polars.collect_all_async

Collect multiple LazyFrames at the same time lazily.

Notes

In case of error set_exception is used on asyncio.Future/gevent.event.AsyncResult and will be reraised by them.

Examples

>>> import asyncio
>>> lf = pl.LazyFrame(
...     {
...         "a": ["a", "b", "a", "b", "b", "c"],
...         "b": [1, 2, 3, 4, 5, 6],
...         "c": [6, 5, 4, 3, 2, 1],
...     }
... )
>>> async def main():
...     return await (
...         lf.group_by("a", maintain_order=True)
...         .agg(pl.all().sum())
...         .collect_async()
...     )
>>> asyncio.run(main())
shape: (3, 3)
┌─────┬─────┬─────┐
│ a   ┆ b   ┆ c   │
│ --- ┆ --- ┆ --- │
│ str ┆ i64 ┆ i64 │
╞═════╪═════╪═════╡
│ a   ┆ 4   ┆ 10  │
│ b   ┆ 11  ┆ 10  │
│ c   ┆ 6   ┆ 1   │
└─────┴─────┴─────┘
collect_schema() Schema[source]

Resolve the schema of this LazyFrame.

Examples

Determine the schema.

>>> lf = pl.LazyFrame(
...     {
...         "foo": [1, 2, 3],
...         "bar": [6.0, 7.0, 8.0],
...         "ham": ["a", "b", "c"],
...     }
... )
>>> lf.collect_schema()
Schema({'foo': Int64, 'bar': Float64, 'ham': String})

Access various properties of the schema.

>>> schema = lf.collect_schema()
>>> schema["bar"]
Float64
>>> schema.names()
['foo', 'bar', 'ham']
>>> schema.dtypes()
[Int64, Float64, String]
>>> schema.len()
3
property columns: list[str][source]

Get the column names.

Returns:
list of str

A list containing the name of each column in order.

Warning

Determining the column names of a LazyFrame requires resolving its schema, which is a potentially expensive operation. Using collect_schema() is the idiomatic way of resolving the schema. This property exists only for symmetry with the DataFrame class.

See also

collect_schema
Schema.names

Examples

>>> lf = pl.LazyFrame(
...     {
...         "foo": [1, 2, 3],
...         "bar": [6, 7, 8],
...         "ham": ["a", "b", "c"],
...     }
... ).select("foo", "bar")
>>> lf.columns  
['foo', 'bar']
count() LazyFrame[source]

Return the number of non-null elements for each column.

Examples

>>> lf = pl.LazyFrame(
...     {"a": [1, 2, 3, 4], "b": [1, 2, 1, None], "c": [None, None, None, None]}
... )
>>> lf.count().collect()
shape: (1, 3)
┌─────┬─────┬─────┐
│ a   ┆ b   ┆ c   │
│ --- ┆ --- ┆ --- │
│ u32 ┆ u32 ┆ u32 │
╞═════╪═════╪═════╡
│ 4   ┆ 3   ┆ 0   │
└─────┴─────┴─────┘
describe(
percentiles: Sequence[float] | float | None = (0.25, 0.5, 0.75),
*,
interpolation: RollingInterpolationMethod = 'nearest',
) DataFrame[source]

Creates a summary of statistics for a LazyFrame, returning a DataFrame.

Parameters:
percentiles

One or more percentiles to include in the summary statistics. All values must be in the range [0, 1].

interpolation{‘nearest’, ‘higher’, ‘lower’, ‘midpoint’, ‘linear’}

Interpolation method used when calculating percentiles.

Returns:
DataFrame

Warning

  • This method does not maintain the laziness of the frame, and will collect the final result. This could potentially be an expensive operation.

  • We do not guarantee the output of describe to be stable. It will show statistics that we deem informative, and may be updated in the future. Using describe programmatically (versus interactive exploration) is not recommended for this reason.

Notes

The median is included by default as the 50% percentile.

Examples

>>> from datetime import date, time
>>> lf = pl.LazyFrame(
...     {
...         "float": [1.0, 2.8, 3.0],
...         "int": [40, 50, None],
...         "bool": [True, False, True],
...         "str": ["zz", "xx", "yy"],
...         "date": [date(2020, 1, 1), date(2021, 7, 5), date(2022, 12, 31)],
...         "time": [time(10, 20, 30), time(14, 45, 50), time(23, 15, 10)],
...     }
... )

Show default frame statistics:

>>> lf.describe()
shape: (9, 7)
┌────────────┬──────────┬──────────┬──────────┬──────┬─────────────────────┬──────────┐
│ statistic  ┆ float    ┆ int      ┆ bool     ┆ str  ┆ date                ┆ time     │
│ ---        ┆ ---      ┆ ---      ┆ ---      ┆ ---  ┆ ---                 ┆ ---      │
│ str        ┆ f64      ┆ f64      ┆ f64      ┆ str  ┆ str                 ┆ str      │
╞════════════╪══════════╪══════════╪══════════╪══════╪═════════════════════╪══════════╡
│ count      ┆ 3.0      ┆ 2.0      ┆ 3.0      ┆ 3    ┆ 3                   ┆ 3        │
│ null_count ┆ 0.0      ┆ 1.0      ┆ 0.0      ┆ 0    ┆ 0                   ┆ 0        │
│ mean       ┆ 2.266667 ┆ 45.0     ┆ 0.666667 ┆ null ┆ 2021-07-02 16:00:00 ┆ 16:07:10 │
│ std        ┆ 1.101514 ┆ 7.071068 ┆ null     ┆ null ┆ null                ┆ null     │
│ min        ┆ 1.0      ┆ 40.0     ┆ 0.0      ┆ xx   ┆ 2020-01-01          ┆ 10:20:30 │
│ 25%        ┆ 2.8      ┆ 40.0     ┆ null     ┆ null ┆ 2021-07-05          ┆ 14:45:50 │
│ 50%        ┆ 2.8      ┆ 50.0     ┆ null     ┆ null ┆ 2021-07-05          ┆ 14:45:50 │
│ 75%        ┆ 3.0      ┆ 50.0     ┆ null     ┆ null ┆ 2022-12-31          ┆ 23:15:10 │
│ max        ┆ 3.0      ┆ 50.0     ┆ 1.0      ┆ zz   ┆ 2022-12-31          ┆ 23:15:10 │
└────────────┴──────────┴──────────┴──────────┴──────┴─────────────────────┴──────────┘

Customize which percentiles are displayed, applying linear interpolation:

>>> with pl.Config(tbl_rows=12):
...     lf.describe(
...         percentiles=[0.1, 0.3, 0.5, 0.7, 0.9],
...         interpolation="linear",
...     )
shape: (11, 7)
┌────────────┬──────────┬──────────┬──────────┬──────┬─────────────────────┬──────────┐
│ statistic  ┆ float    ┆ int      ┆ bool     ┆ str  ┆ date                ┆ time     │
│ ---        ┆ ---      ┆ ---      ┆ ---      ┆ ---  ┆ ---                 ┆ ---      │
│ str        ┆ f64      ┆ f64      ┆ f64      ┆ str  ┆ str                 ┆ str      │
╞════════════╪══════════╪══════════╪══════════╪══════╪═════════════════════╪══════════╡
│ count      ┆ 3.0      ┆ 2.0      ┆ 3.0      ┆ 3    ┆ 3                   ┆ 3        │
│ null_count ┆ 0.0      ┆ 1.0      ┆ 0.0      ┆ 0    ┆ 0                   ┆ 0        │
│ mean       ┆ 2.266667 ┆ 45.0     ┆ 0.666667 ┆ null ┆ 2021-07-02 16:00:00 ┆ 16:07:10 │
│ std        ┆ 1.101514 ┆ 7.071068 ┆ null     ┆ null ┆ null                ┆ null     │
│ min        ┆ 1.0      ┆ 40.0     ┆ 0.0      ┆ xx   ┆ 2020-01-01          ┆ 10:20:30 │
│ 10%        ┆ 1.36     ┆ 41.0     ┆ null     ┆ null ┆ 2020-04-20          ┆ 11:13:34 │
│ 30%        ┆ 2.08     ┆ 43.0     ┆ null     ┆ null ┆ 2020-11-26          ┆ 12:59:42 │
│ 50%        ┆ 2.8      ┆ 45.0     ┆ null     ┆ null ┆ 2021-07-05          ┆ 14:45:50 │
│ 70%        ┆ 2.88     ┆ 47.0     ┆ null     ┆ null ┆ 2022-02-07          ┆ 18:09:34 │
│ 90%        ┆ 2.96     ┆ 49.0     ┆ null     ┆ null ┆ 2022-09-13          ┆ 21:33:18 │
│ max        ┆ 3.0      ┆ 50.0     ┆ 1.0      ┆ zz   ┆ 2022-12-31          ┆ 23:15:10 │
└────────────┴──────────┴──────────┴──────────┴──────┴─────────────────────┴──────────┘
classmethod deserialize(
source: str | Path | IOBase,
*,
format: SerializationFormat = 'binary',
) LazyFrame[source]

Read a logical plan from a file to construct a LazyFrame.

Parameters:
source

Path to a file or a file-like object (by file-like object, we refer to objects that have a read() method, such as a file handler (e.g. via builtin open function) or BytesIO).

format

The format with which the LazyFrame was serialized. Options:

  • "binary": Deserialize from binary format (bytes). This is the default.

  • "json": Deserialize from JSON format (string).

Warning

This function uses pickle if the logical plan contains Python UDFs, and as such inherits the security implications. Deserializing can execute arbitrary code, so it should only be attempted on trusted data.

Notes

Serialization is not stable across Polars versions: a LazyFrame serialized in one Polars version may not be deserializable in another Polars version.

Examples

>>> import io
>>> lf = pl.LazyFrame({"a": [1, 2, 3]}).sum()
>>> bytes = lf.serialize()
>>> pl.LazyFrame.deserialize(io.BytesIO(bytes)).collect()
shape: (1, 1)
┌─────┐
│ a   │
│ --- │
│ i64 │
╞═════╡
│ 6   │
└─────┘
drop(
*columns: ColumnNameOrSelector | Iterable[ColumnNameOrSelector],
strict: bool = True,
) LazyFrame[source]

Remove columns from the DataFrame.

Parameters:
*columns

Names of the columns that should be removed from the dataframe. Accepts column selector input.

strict

Validate that all column names exist in the schema and throw an exception if a column name does not exist in the schema.

Examples

Drop a single column by passing the name of that column.

>>> lf = pl.LazyFrame(
...     {
...         "foo": [1, 2, 3],
...         "bar": [6.0, 7.0, 8.0],
...         "ham": ["a", "b", "c"],
...     }
... )
>>> lf.drop("ham").collect()
shape: (3, 2)
┌─────┬─────┐
│ foo ┆ bar │
│ --- ┆ --- │
│ i64 ┆ f64 │
╞═════╪═════╡
│ 1   ┆ 6.0 │
│ 2   ┆ 7.0 │
│ 3   ┆ 8.0 │
└─────┴─────┘

Drop multiple columns by passing a selector.

>>> import polars.selectors as cs
>>> lf.drop(cs.numeric()).collect()
shape: (3, 1)
┌─────┐
│ ham │
│ --- │
│ str │
╞═════╡
│ a   │
│ b   │
│ c   │
└─────┘

Use positional arguments to drop multiple columns.

>>> lf.drop("foo", "ham").collect()
shape: (3, 1)
┌─────┐
│ bar │
│ --- │
│ f64 │
╞═════╡
│ 6.0 │
│ 7.0 │
│ 8.0 │
└─────┘
drop_nulls(
subset: ColumnNameOrSelector | Collection[ColumnNameOrSelector] | None = None,
) LazyFrame[source]

Drop all rows that contain null values.

The original order of the remaining rows is preserved.

Parameters:
subset

Column name(s) for which null values are considered. If set to None (default), use all columns.

Examples

>>> lf = pl.LazyFrame(
...     {
...         "foo": [1, 2, 3],
...         "bar": [6, None, 8],
...         "ham": ["a", "b", None],
...     }
... )

The default behavior of this method is to drop rows where any single value of the row is null.

>>> lf.drop_nulls().collect()
shape: (1, 3)
┌─────┬─────┬─────┐
│ foo ┆ bar ┆ ham │
│ --- ┆ --- ┆ --- │
│ i64 ┆ i64 ┆ str │
╞═════╪═════╪═════╡
│ 1   ┆ 6   ┆ a   │
└─────┴─────┴─────┘

This behaviour can be constrained to consider only a subset of columns, as defined by name or with a selector. For example, dropping rows if there is a null in any of the integer columns:

>>> import polars.selectors as cs
>>> lf.drop_nulls(subset=cs.integer()).collect()
shape: (2, 3)
┌─────┬─────┬──────┐
│ foo ┆ bar ┆ ham  │
│ --- ┆ --- ┆ ---  │
│ i64 ┆ i64 ┆ str  │
╞═════╪═════╪══════╡
│ 1   ┆ 6   ┆ a    │
│ 3   ┆ 8   ┆ null │
└─────┴─────┴──────┘

This method drops a row if any single value of the row is null.

Below are some example snippets that show how you could drop null values based on other conditions:

>>> lf = pl.LazyFrame(
...     {
...         "a": [None, None, None, None],
...         "b": [1, 2, None, 1],
...         "c": [1, None, None, 1],
...     }
... )
>>> lf.collect()
shape: (4, 3)
┌──────┬──────┬──────┐
│ a    ┆ b    ┆ c    │
│ ---  ┆ ---  ┆ ---  │
│ null ┆ i64  ┆ i64  │
╞══════╪══════╪══════╡
│ null ┆ 1    ┆ 1    │
│ null ┆ 2    ┆ null │
│ null ┆ null ┆ null │
│ null ┆ 1    ┆ 1    │
└──────┴──────┴──────┘

Drop a row only if all values are null:

>>> lf.filter(~pl.all_horizontal(pl.all().is_null())).collect()
shape: (3, 3)
┌──────┬─────┬──────┐
│ a    ┆ b   ┆ c    │
│ ---  ┆ --- ┆ ---  │
│ null ┆ i64 ┆ i64  │
╞══════╪═════╪══════╡
│ null ┆ 1   ┆ 1    │
│ null ┆ 2   ┆ null │
│ null ┆ 1   ┆ 1    │
└──────┴─────┴──────┘
property dtypes: list[DataType][source]

Get the column data types.

Returns:
list of DataType

A list containing the data type of each column in order.

Warning

Determining the data types of a LazyFrame requires resolving its schema, which is a potentially expensive operation. Using collect_schema() is the idiomatic way to resolve the schema. This property exists only for symmetry with the DataFrame class.

See also

collect_schema
Schema.dtypes

Examples

>>> lf = pl.LazyFrame(
...     {
...         "foo": [1, 2, 3],
...         "bar": [6.0, 7.0, 8.0],
...         "ham": ["a", "b", "c"],
...     }
... )
>>> lf.dtypes  
[Int64, Float64, String]
explain(
*,
format: ExplainFormat = 'plain',
optimized: bool = True,
type_coercion: bool = True,
predicate_pushdown: bool = True,
projection_pushdown: bool = True,
simplify_expression: bool = True,
slice_pushdown: bool = True,
comm_subplan_elim: bool = True,
comm_subexpr_elim: bool = True,
cluster_with_columns: bool = True,
streaming: bool = False,
tree_format: bool | None = None,
) str[source]

Create a string representation of the query plan.

Different optimizations can be turned on or off.

Parameters:
format{‘plain’, ‘tree’}

The format to use for displaying the logical plan.

optimized

Return an optimized query plan. Defaults to True. If this is set to True the subsequent optimization flags control which optimizations run.

type_coercion

Do type coercion optimization.

predicate_pushdown

Do predicate pushdown optimization.

projection_pushdown

Do projection pushdown optimization.

simplify_expression

Run simplify expressions optimization.

slice_pushdown

Slice pushdown optimization.

comm_subplan_elim

Will try to cache branching subplans that occur on self-joins or unions.

comm_subexpr_elim

Common subexpressions will be cached and reused.

cluster_with_columns

Combine sequential independent calls to with_columns

streaming

Run parts of the query in a streaming fashion (this is in an alpha state)

Warning

Streaming mode is considered unstable. It may be changed at any point without it being considered a breaking change.

tree_format

Format the output as a tree.

Deprecated since version 0.20.30: Use format="tree" instead.

Examples

>>> lf = pl.LazyFrame(
...     {
...         "a": ["a", "b", "a", "b", "b", "c"],
...         "b": [1, 2, 3, 4, 5, 6],
...         "c": [6, 5, 4, 3, 2, 1],
...     }
... )
>>> lf.group_by("a", maintain_order=True).agg(pl.all().sum()).sort(
...     "a"
... ).explain()  
explode(
columns: str | Expr | Sequence[str | Expr],
*more_columns: str | Expr,
) LazyFrame[source]

Explode the DataFrame to long format by exploding the given columns.

Parameters:
columns

Column names, expressions, or a selector defining them. The underlying columns being exploded must be of the List or Array data type.

*more_columns

Additional names of columns to explode, specified as positional arguments.

Examples

>>> lf = pl.LazyFrame(
...     {
...         "letters": ["a", "a", "b", "c"],
...         "numbers": [[1], [2, 3], [4, 5], [6, 7, 8]],
...     }
... )
>>> lf.explode("numbers").collect()
shape: (8, 2)
┌─────────┬─────────┐
│ letters ┆ numbers │
│ ---     ┆ ---     │
│ str     ┆ i64     │
╞═════════╪═════════╡
│ a       ┆ 1       │
│ a       ┆ 2       │
│ a       ┆ 3       │
│ b       ┆ 4       │
│ b       ┆ 5       │
│ c       ┆ 6       │
│ c       ┆ 7       │
│ c       ┆ 8       │
└─────────┴─────────┘
fetch(
n_rows: int = 500,
*,
type_coercion: bool = True,
predicate_pushdown: bool = True,
projection_pushdown: bool = True,
simplify_expression: bool = True,
no_optimization: bool = False,
slice_pushdown: bool = True,
comm_subplan_elim: bool = True,
comm_subexpr_elim: bool = True,
cluster_with_columns: bool = True,
streaming: bool = False,
) DataFrame[source]

Collect a small number of rows for debugging purposes.

Deprecated since version 1.0: Use collect() instead, in conjunction with a call to head().`

Warning

This is strictly a utility function that can help to debug queries using a smaller number of rows, and should not be used in production code.

Notes

This is similar to a collect() operation, but it overwrites the number of rows read by every scan operation. Be aware that fetch does not guarantee the final number of rows in the DataFrame. Filters, join operations and fewer rows being available in the scanned data will all influence the final number of rows (joins are especially susceptible to this, and may return no data at all if n_rows is too small as the join keys may not be present).

fill_nan(value: int | float | Expr | None) LazyFrame[source]

Fill floating point NaN values.

Parameters:
value

Value to fill the NaN values with.

Warning

Note that floating point NaNs (Not a Number) are not missing values. To replace missing values, use fill_null().

See also

fill_null

Examples

>>> lf = pl.LazyFrame(
...     {
...         "a": [1.5, 2, float("nan"), 4],
...         "b": [0.5, 4, float("nan"), 13],
...     }
... )
>>> lf.fill_nan(99).collect()
shape: (4, 2)
┌──────┬──────┐
│ a    ┆ b    │
│ ---  ┆ ---  │
│ f64  ┆ f64  │
╞══════╪══════╡
│ 1.5  ┆ 0.5  │
│ 2.0  ┆ 4.0  │
│ 99.0 ┆ 99.0 │
│ 4.0  ┆ 13.0 │
└──────┴──────┘
fill_null(
value: Any | Expr | None = None,
strategy: FillNullStrategy | None = None,
limit: int | None = None,
*,
matches_supertype: bool = True,
) LazyFrame[source]

Fill null values using the specified value or strategy.

Parameters:
value

Value used to fill null values.

strategy{None, ‘forward’, ‘backward’, ‘min’, ‘max’, ‘mean’, ‘zero’, ‘one’}

Strategy used to fill null values.

limit

Number of consecutive null values to fill when using the ‘forward’ or ‘backward’ strategy.

matches_supertype

Fill all matching supertypes of the fill value literal.

See also

fill_nan

Examples

>>> lf = pl.LazyFrame(
...     {
...         "a": [1, 2, None, 4],
...         "b": [0.5, 4, None, 13],
...     }
... )
>>> lf.fill_null(99).collect()
shape: (4, 2)
┌─────┬──────┐
│ a   ┆ b    │
│ --- ┆ ---  │
│ i64 ┆ f64  │
╞═════╪══════╡
│ 1   ┆ 0.5  │
│ 2   ┆ 4.0  │
│ 99  ┆ 99.0 │
│ 4   ┆ 13.0 │
└─────┴──────┘
>>> lf.fill_null(strategy="forward").collect()
shape: (4, 2)
┌─────┬──────┐
│ a   ┆ b    │
│ --- ┆ ---  │
│ i64 ┆ f64  │
╞═════╪══════╡
│ 1   ┆ 0.5  │
│ 2   ┆ 4.0  │
│ 2   ┆ 4.0  │
│ 4   ┆ 13.0 │
└─────┴──────┘
>>> lf.fill_null(strategy="max").collect()
shape: (4, 2)
┌─────┬──────┐
│ a   ┆ b    │
│ --- ┆ ---  │
│ i64 ┆ f64  │
╞═════╪══════╡
│ 1   ┆ 0.5  │
│ 2   ┆ 4.0  │
│ 4   ┆ 13.0 │
│ 4   ┆ 13.0 │
└─────┴──────┘
>>> lf.fill_null(strategy="zero").collect()
shape: (4, 2)
┌─────┬──────┐
│ a   ┆ b    │
│ --- ┆ ---  │
│ i64 ┆ f64  │
╞═════╪══════╡
│ 1   ┆ 0.5  │
│ 2   ┆ 4.0  │
│ 0   ┆ 0.0  │
│ 4   ┆ 13.0 │
└─────┴──────┘
filter(
*predicates: IntoExprColumn | Iterable[IntoExprColumn] | bool | list[bool] | np.ndarray[Any, Any],
**constraints: Any,
) LazyFrame[source]

Filter the rows in the LazyFrame based on a predicate expression.

The original order of the remaining rows is preserved.

Rows where the filter does not evaluate to True are discarded, including nulls.

Parameters:
predicates

Expression that evaluates to a boolean Series.

constraints

Column filters; use name = value to filter columns by the supplied value. Each constraint will behave the same as pl.col(name).eq(value), and will be implicitly joined with the other filter conditions using &.

Examples

>>> lf = pl.LazyFrame(
...     {
...         "foo": [1, 2, 3],
...         "bar": [6, 7, 8],
...         "ham": ["a", "b", "c"],
...     }
... )

Filter on one condition:

>>> lf.filter(pl.col("foo") > 1).collect()
shape: (2, 3)
┌─────┬─────┬─────┐
│ foo ┆ bar ┆ ham │
│ --- ┆ --- ┆ --- │
│ i64 ┆ i64 ┆ str │
╞═════╪═════╪═════╡
│ 2   ┆ 7   ┆ b   │
│ 3   ┆ 8   ┆ c   │
└─────┴─────┴─────┘

Filter on multiple conditions:

>>> lf.filter((pl.col("foo") < 3) & (pl.col("ham") == "a")).collect()
shape: (1, 3)
┌─────┬─────┬─────┐
│ foo ┆ bar ┆ ham │
│ --- ┆ --- ┆ --- │
│ i64 ┆ i64 ┆ str │
╞═════╪═════╪═════╡
│ 1   ┆ 6   ┆ a   │
└─────┴─────┴─────┘

Provide multiple filters using *args syntax:

>>> lf.filter(
...     pl.col("foo") == 1,
...     pl.col("ham") == "a",
... ).collect()
shape: (1, 3)
┌─────┬─────┬─────┐
│ foo ┆ bar ┆ ham │
│ --- ┆ --- ┆ --- │
│ i64 ┆ i64 ┆ str │
╞═════╪═════╪═════╡
│ 1   ┆ 6   ┆ a   │
└─────┴─────┴─────┘

Provide multiple filters using **kwargs syntax:

>>> lf.filter(foo=1, ham="a").collect()
shape: (1, 3)
┌─────┬─────┬─────┐
│ foo ┆ bar ┆ ham │
│ --- ┆ --- ┆ --- │
│ i64 ┆ i64 ┆ str │
╞═════╪═════╪═════╡
│ 1   ┆ 6   ┆ a   │
└─────┴─────┴─────┘

Filter on an OR condition:

>>> lf.filter((pl.col("foo") == 1) | (pl.col("ham") == "c")).collect()
shape: (2, 3)
┌─────┬─────┬─────┐
│ foo ┆ bar ┆ ham │
│ --- ┆ --- ┆ --- │
│ i64 ┆ i64 ┆ str │
╞═════╪═════╪═════╡
│ 1   ┆ 6   ┆ a   │
│ 3   ┆ 8   ┆ c   │
└─────┴─────┴─────┘
first() LazyFrame[source]

Get the first row of the DataFrame.

Examples

>>> lf = pl.LazyFrame(
...     {
...         "a": [1, 3, 5],
...         "b": [2, 4, 6],
...     }
... )
>>> lf.first().collect()
shape: (1, 2)
┌─────┬─────┐
│ a   ┆ b   │
│ --- ┆ --- │
│ i64 ┆ i64 │
╞═════╪═════╡
│ 1   ┆ 2   │
└─────┴─────┘
gather_every(n: int, offset: int = 0) LazyFrame[source]

Take every nth row in the LazyFrame and return as a new LazyFrame.

Parameters:
n

Gather every n-th row.

offset

Starting index.

Examples

>>> lf = pl.LazyFrame(
...     {
...         "a": [1, 2, 3, 4],
...         "b": [5, 6, 7, 8],
...     }
... )
>>> lf.gather_every(2).collect()
shape: (2, 2)
┌─────┬─────┐
│ a   ┆ b   │
│ --- ┆ --- │
│ i64 ┆ i64 │
╞═════╪═════╡
│ 1   ┆ 5   │
│ 3   ┆ 7   │
└─────┴─────┘
>>> lf.gather_every(2, offset=1).collect()
shape: (2, 2)
┌─────┬─────┐
│ a   ┆ b   │
│ --- ┆ --- │
│ i64 ┆ i64 │
╞═════╪═════╡
│ 2   ┆ 6   │
│ 4   ┆ 8   │
└─────┴─────┘
group_by(
*by: IntoExpr | Iterable[IntoExpr],
maintain_order: bool = False,
**named_by: IntoExpr,
) LazyGroupBy[source]

Start a group by operation.

Parameters:
*by

Column(s) to group by. Accepts expression input. Strings are parsed as column names.

maintain_order

Ensure that the order of the groups is consistent with the input data. This is slower than a default group by. Setting this to True blocks the possibility to run on the streaming engine.

**named_by

Additional columns to group by, specified as keyword arguments. The columns will be renamed to the keyword used.

Examples

Group by one column and call agg to compute the grouped sum of another column.

>>> lf = pl.LazyFrame(
...     {
...         "a": ["a", "b", "a", "b", "c"],
...         "b": [1, 2, 1, 3, 3],
...         "c": [5, 4, 3, 2, 1],
...     }
... )
>>> lf.group_by("a").agg(pl.col("b").sum()).collect()  
shape: (3, 2)
┌─────┬─────┐
│ a   ┆ b   │
│ --- ┆ --- │
│ str ┆ i64 │
╞═════╪═════╡
│ a   ┆ 2   │
│ b   ┆ 5   │
│ c   ┆ 3   │
└─────┴─────┘

Set maintain_order=True to ensure the order of the groups is consistent with the input.

>>> lf.group_by("a", maintain_order=True).agg(pl.col("c")).collect()
shape: (3, 2)
┌─────┬───────────┐
│ a   ┆ c         │
│ --- ┆ ---       │
│ str ┆ list[i64] │
╞═════╪═══════════╡
│ a   ┆ [5, 3]    │
│ b   ┆ [4, 2]    │
│ c   ┆ [1]       │
└─────┴───────────┘

Group by multiple columns by passing a list of column names.

>>> lf.group_by(["a", "b"]).agg(pl.max("c")).collect()  
shape: (4, 3)
┌─────┬─────┬─────┐
│ a   ┆ b   ┆ c   │
│ --- ┆ --- ┆ --- │
│ str ┆ i64 ┆ i64 │
╞═════╪═════╪═════╡
│ a   ┆ 1   ┆ 5   │
│ b   ┆ 2   ┆ 4   │
│ b   ┆ 3   ┆ 2   │
│ c   ┆ 3   ┆ 1   │
└─────┴─────┴─────┘

Or use positional arguments to group by multiple columns in the same way. Expressions are also accepted.

>>> lf.group_by("a", pl.col("b") // 2).agg(
...     pl.col("c").mean()
... ).collect()  
shape: (3, 3)
┌─────┬─────┬─────┐
│ a   ┆ b   ┆ c   │
│ --- ┆ --- ┆ --- │
│ str ┆ i64 ┆ f64 │
╞═════╪═════╪═════╡
│ a   ┆ 0   ┆ 4.0 │
│ b   ┆ 1   ┆ 3.0 │
│ c   ┆ 1   ┆ 1.0 │
└─────┴─────┴─────┘
group_by_dynamic(
index_column: IntoExpr,
*,
every: str | timedelta,
period: str | timedelta | None = None,
offset: str | timedelta | None = None,
include_boundaries: bool = False,
closed: ClosedInterval = 'left',
label: Label = 'left',
group_by: IntoExpr | Iterable[IntoExpr] | None = None,
start_by: StartBy = 'window',
) LazyGroupBy[source]

Group based on a time value (or index value of type Int32, Int64).

Time windows are calculated and rows are assigned to windows. Different from a normal group by is that a row can be member of multiple groups. By default, the windows look like:

  • [start, start + period)

  • [start + every, start + every + period)

  • [start + 2*every, start + 2*every + period)

where start is determined by start_by, offset, every, and the earliest datapoint. See the start_by argument description for details.

Warning

The index column must be sorted in ascending order. If group_by is passed, then the index column must be sorted in ascending order within each group.

Parameters:
index_column

Column used to group based on the time window. Often of type Date/Datetime. This column must be sorted in ascending order (or, if group_by is specified, then it must be sorted in ascending order within each group).

In case of a dynamic group by on indices, dtype needs to be one of {Int32, Int64}. Note that Int32 gets temporarily cast to Int64, so if performance matters use an Int64 column.

every

interval of the window

period

length of the window, if None it will equal ‘every’

offset

offset of the window, does not take effect if start_by is ‘datapoint’. Defaults to zero.

include_boundaries

Add the lower and upper bound of the window to the “_lower_boundary” and “_upper_boundary” columns. This will impact performance because it’s harder to parallelize

closed{‘left’, ‘right’, ‘both’, ‘none’}

Define which sides of the temporal interval are closed (inclusive).

label{‘left’, ‘right’, ‘datapoint’}

Define which label to use for the window:

  • ‘left’: lower boundary of the window

  • ‘right’: upper boundary of the window

  • ‘datapoint’: the first value of the index column in the given window. If you don’t need the label to be at one of the boundaries, choose this option for maximum performance

group_by

Also group by this column/these columns

start_by{‘window’, ‘datapoint’, ‘monday’, ‘tuesday’, ‘wednesday’, ‘thursday’, ‘friday’, ‘saturday’, ‘sunday’}

The strategy to determine the start of the first window by.

  • ‘window’: Start by taking the earliest timestamp, truncating it with every, and then adding offset. Note that weekly windows start on Monday.

  • ‘datapoint’: Start from the first encountered data point.

  • a day of the week (only takes effect if every contains 'w'):

    • ‘monday’: Start the window on the Monday before the first data point.

    • ‘tuesday’: Start the window on the Tuesday before the first data point.

    • ‘sunday’: Start the window on the Sunday before the first data point.

    The resulting window is then shifted back until the earliest datapoint is in or in front of it.

Returns:
LazyGroupBy

Object you can call .agg on to aggregate by groups, the result of which will be sorted by index_column (but note that if group_by columns are passed, it will only be sorted within each group).

See also

rolling

Notes

  1. If you’re coming from pandas, then

    # polars
    df.group_by_dynamic("ts", every="1d").agg(pl.col("value").sum())
    

    is equivalent to

    # pandas
    df.set_index("ts").resample("D")["value"].sum().reset_index()
    

    though note that, unlike pandas, polars doesn’t add extra rows for empty windows. If you need index_column to be evenly spaced, then please combine with DataFrame.upsample().

  2. The every, period and offset arguments are created with the following string language:

    • 1ns (1 nanosecond)

    • 1us (1 microsecond)

    • 1ms (1 millisecond)

    • 1s (1 second)

    • 1m (1 minute)

    • 1h (1 hour)

    • 1d (1 calendar day)

    • 1w (1 calendar week)

    • 1mo (1 calendar month)

    • 1q (1 calendar quarter)

    • 1y (1 calendar year)

    • 1i (1 index count)

    Or combine them: “3d12h4m25s” # 3 days, 12 hours, 4 minutes, and 25 seconds

    By “calendar day”, we mean the corresponding time on the next day (which may not be 24 hours, due to daylight savings). Similarly for “calendar week”, “calendar month”, “calendar quarter”, and “calendar year”.

    In case of a group_by_dynamic on an integer column, the windows are defined by:

    • “1i” # length 1

    • “10i” # length 10

Examples

>>> from datetime import datetime
>>> lf = pl.LazyFrame(
...     {
...         "time": pl.datetime_range(
...             start=datetime(2021, 12, 16),
...             end=datetime(2021, 12, 16, 3),
...             interval="30m",
...             eager=True,
...         ),
...         "n": range(7),
...     }
... )
>>> lf.collect()
shape: (7, 2)
┌─────────────────────┬─────┐
│ time                ┆ n   │
│ ---                 ┆ --- │
│ datetime[μs]        ┆ i64 │
╞═════════════════════╪═════╡
│ 2021-12-16 00:00:00 ┆ 0   │
│ 2021-12-16 00:30:00 ┆ 1   │
│ 2021-12-16 01:00:00 ┆ 2   │
│ 2021-12-16 01:30:00 ┆ 3   │
│ 2021-12-16 02:00:00 ┆ 4   │
│ 2021-12-16 02:30:00 ┆ 5   │
│ 2021-12-16 03:00:00 ┆ 6   │
└─────────────────────┴─────┘

Group by windows of 1 hour starting at 2021-12-16 00:00:00.

>>> lf.group_by_dynamic("time", every="1h", closed="right").agg(
...     pl.col("n")
... ).collect()
shape: (4, 2)
┌─────────────────────┬───────────┐
│ time                ┆ n         │
│ ---                 ┆ ---       │
│ datetime[μs]        ┆ list[i64] │
╞═════════════════════╪═══════════╡
│ 2021-12-15 23:00:00 ┆ [0]       │
│ 2021-12-16 00:00:00 ┆ [1, 2]    │
│ 2021-12-16 01:00:00 ┆ [3, 4]    │
│ 2021-12-16 02:00:00 ┆ [5, 6]    │
└─────────────────────┴───────────┘

The window boundaries can also be added to the aggregation result

>>> lf.group_by_dynamic(
...     "time", every="1h", include_boundaries=True, closed="right"
... ).agg(pl.col("n").mean()).collect()
shape: (4, 4)
┌─────────────────────┬─────────────────────┬─────────────────────┬─────┐
│ _lower_boundary     ┆ _upper_boundary     ┆ time                ┆ n   │
│ ---                 ┆ ---                 ┆ ---                 ┆ --- │
│ datetime[μs]        ┆ datetime[μs]        ┆ datetime[μs]        ┆ f64 │
╞═════════════════════╪═════════════════════╪═════════════════════╪═════╡
│ 2021-12-15 23:00:00 ┆ 2021-12-16 00:00:00 ┆ 2021-12-15 23:00:00 ┆ 0.0 │
│ 2021-12-16 00:00:00 ┆ 2021-12-16 01:00:00 ┆ 2021-12-16 00:00:00 ┆ 1.5 │
│ 2021-12-16 01:00:00 ┆ 2021-12-16 02:00:00 ┆ 2021-12-16 01:00:00 ┆ 3.5 │
│ 2021-12-16 02:00:00 ┆ 2021-12-16 03:00:00 ┆ 2021-12-16 02:00:00 ┆ 5.5 │
└─────────────────────┴─────────────────────┴─────────────────────┴─────┘

When closed=”left”, the window excludes the right end of interval: [lower_bound, upper_bound)

>>> lf.group_by_dynamic("time", every="1h", closed="left").agg(
...     pl.col("n")
... ).collect()
shape: (4, 2)
┌─────────────────────┬───────────┐
│ time                ┆ n         │
│ ---                 ┆ ---       │
│ datetime[μs]        ┆ list[i64] │
╞═════════════════════╪═══════════╡
│ 2021-12-16 00:00:00 ┆ [0, 1]    │
│ 2021-12-16 01:00:00 ┆ [2, 3]    │
│ 2021-12-16 02:00:00 ┆ [4, 5]    │
│ 2021-12-16 03:00:00 ┆ [6]       │
└─────────────────────┴───────────┘

When closed=”both” the time values at the window boundaries belong to 2 groups.

>>> lf.group_by_dynamic("time", every="1h", closed="both").agg(
...     pl.col("n")
... ).collect()
shape: (4, 2)
┌─────────────────────┬───────────┐
│ time                ┆ n         │
│ ---                 ┆ ---       │
│ datetime[μs]        ┆ list[i64] │
╞═════════════════════╪═══════════╡
│ 2021-12-16 00:00:00 ┆ [0, 1, 2] │
│ 2021-12-16 01:00:00 ┆ [2, 3, 4] │
│ 2021-12-16 02:00:00 ┆ [4, 5, 6] │
│ 2021-12-16 03:00:00 ┆ [6]       │
└─────────────────────┴───────────┘

Dynamic group bys can also be combined with grouping on normal keys

>>> lf = lf.with_columns(groups=pl.Series(["a", "a", "a", "b", "b", "a", "a"]))
>>> lf.collect()
shape: (7, 3)
┌─────────────────────┬─────┬────────┐
│ time                ┆ n   ┆ groups │
│ ---                 ┆ --- ┆ ---    │
│ datetime[μs]        ┆ i64 ┆ str    │
╞═════════════════════╪═════╪════════╡
│ 2021-12-16 00:00:00 ┆ 0   ┆ a      │
│ 2021-12-16 00:30:00 ┆ 1   ┆ a      │
│ 2021-12-16 01:00:00 ┆ 2   ┆ a      │
│ 2021-12-16 01:30:00 ┆ 3   ┆ b      │
│ 2021-12-16 02:00:00 ┆ 4   ┆ b      │
│ 2021-12-16 02:30:00 ┆ 5   ┆ a      │
│ 2021-12-16 03:00:00 ┆ 6   ┆ a      │
└─────────────────────┴─────┴────────┘
>>> lf.group_by_dynamic(
...     "time",
...     every="1h",
...     closed="both",
...     group_by="groups",
...     include_boundaries=True,
... ).agg(pl.col("n")).collect()
shape: (6, 5)
┌────────┬─────────────────────┬─────────────────────┬─────────────────────┬───────────┐
│ groups ┆ _lower_boundary     ┆ _upper_boundary     ┆ time                ┆ n         │
│ ---    ┆ ---                 ┆ ---                 ┆ ---                 ┆ ---       │
│ str    ┆ datetime[μs]        ┆ datetime[μs]        ┆ datetime[μs]        ┆ list[i64] │
╞════════╪═════════════════════╪═════════════════════╪═════════════════════╪═══════════╡
│ a      ┆ 2021-12-16 00:00:00 ┆ 2021-12-16 01:00:00 ┆ 2021-12-16 00:00:00 ┆ [0, 1, 2] │
│ a      ┆ 2021-12-16 01:00:00 ┆ 2021-12-16 02:00:00 ┆ 2021-12-16 01:00:00 ┆ [2]       │
│ a      ┆ 2021-12-16 02:00:00 ┆ 2021-12-16 03:00:00 ┆ 2021-12-16 02:00:00 ┆ [5, 6]    │
│ a      ┆ 2021-12-16 03:00:00 ┆ 2021-12-16 04:00:00 ┆ 2021-12-16 03:00:00 ┆ [6]       │
│ b      ┆ 2021-12-16 01:00:00 ┆ 2021-12-16 02:00:00 ┆ 2021-12-16 01:00:00 ┆ [3, 4]    │
│ b      ┆ 2021-12-16 02:00:00 ┆ 2021-12-16 03:00:00 ┆ 2021-12-16 02:00:00 ┆ [4]       │
└────────┴─────────────────────┴─────────────────────┴─────────────────────┴───────────┘

Dynamic group by on an index column

>>> lf = pl.LazyFrame(
...     {
...         "idx": pl.int_range(0, 6, eager=True),
...         "A": ["A", "A", "B", "B", "B", "C"],
...     }
... )
>>> lf.group_by_dynamic(
...     "idx",
...     every="2i",
...     period="3i",
...     include_boundaries=True,
...     closed="right",
... ).agg(pl.col("A").alias("A_agg_list")).collect()
shape: (4, 4)
┌─────────────────┬─────────────────┬─────┬─────────────────┐
│ _lower_boundary ┆ _upper_boundary ┆ idx ┆ A_agg_list      │
│ ---             ┆ ---             ┆ --- ┆ ---             │
│ i64             ┆ i64             ┆ i64 ┆ list[str]       │
╞═════════════════╪═════════════════╪═════╪═════════════════╡
│ -2              ┆ 1               ┆ -2  ┆ ["A", "A"]      │
│ 0               ┆ 3               ┆ 0   ┆ ["A", "B", "B"] │
│ 2               ┆ 5               ┆ 2   ┆ ["B", "B", "C"] │
│ 4               ┆ 7               ┆ 4   ┆ ["C"]           │
└─────────────────┴─────────────────┴─────┴─────────────────┘
head(n: int = 5) LazyFrame[source]

Get the first n rows.

Parameters:
n

Number of rows to return.

Notes

Consider using the fetch() operation if you only want to test your query. The fetch() operation will load the first n rows at the scan level, whereas the head()/limit() are applied at the end.

Examples

>>> lf = pl.LazyFrame(
...     {
...         "a": [1, 2, 3, 4, 5, 6],
...         "b": [7, 8, 9, 10, 11, 12],
...     }
... )
>>> lf.head().collect()
shape: (5, 2)
┌─────┬─────┐
│ a   ┆ b   │
│ --- ┆ --- │
│ i64 ┆ i64 │
╞═════╪═════╡
│ 1   ┆ 7   │
│ 2   ┆ 8   │
│ 3   ┆ 9   │
│ 4   ┆ 10  │
│ 5   ┆ 11  │
└─────┴─────┘
>>> lf.head(2).collect()
shape: (2, 2)
┌─────┬─────┐
│ a   ┆ b   │
│ --- ┆ --- │
│ i64 ┆ i64 │
╞═════╪═════╡
│ 1   ┆ 7   │
│ 2   ┆ 8   │
└─────┴─────┘
inspect(fmt: str = '{}') LazyFrame[source]

Inspect a node in the computation graph.

Print the value that this node in the computation graph evaluates to and pass on the value.

Examples

>>> lf = pl.LazyFrame({"foo": [1, 1, -2, 3]})
>>> (
...     lf.with_columns(pl.col("foo").cum_sum().alias("bar"))
...     .inspect()  # print the node before the filter
...     .filter(pl.col("bar") == pl.col("foo"))
... )  
<LazyFrame at ...>
interpolate() LazyFrame[source]

Interpolate intermediate values. The interpolation method is linear.

Examples

>>> lf = pl.LazyFrame(
...     {
...         "foo": [1, None, 9, 10],
...         "bar": [6, 7, 9, None],
...         "baz": [1, None, None, 9],
...     }
... )
>>> lf.interpolate().collect()
shape: (4, 3)
┌──────┬──────┬──────────┐
│ foo  ┆ bar  ┆ baz      │
│ ---  ┆ ---  ┆ ---      │
│ f64  ┆ f64  ┆ f64      │
╞══════╪══════╪══════════╡
│ 1.0  ┆ 6.0  ┆ 1.0      │
│ 5.0  ┆ 7.0  ┆ 3.666667 │
│ 9.0  ┆ 9.0  ┆ 6.333333 │
│ 10.0 ┆ null ┆ 9.0      │
└──────┴──────┴──────────┘
join(
other: LazyFrame,
on: str | Expr | Sequence[str | Expr] | None = None,
how: JoinStrategy = 'inner',
*,
left_on: str | Expr | Sequence[str | Expr] | None = None,
right_on: str | Expr | Sequence[str | Expr] | None = None,
suffix: str = '_right',
validate: JoinValidation = 'm:m',
join_nulls: bool = False,
coalesce: bool | None = None,
allow_parallel: bool = True,
force_parallel: bool = False,
) LazyFrame[source]

Add a join operation to the Logical Plan.

Parameters:
other

Lazy DataFrame to join with.

on

Join column of both DataFrames. If set, left_on and right_on should be None.

how{‘inner’, ‘left’, ‘full’, ‘semi’, ‘anti’, ‘cross’}

Join strategy.

  • inner

    Returns rows that have matching values in both tables

  • left

    Returns all rows from the left table, and the matched rows from the right table

  • full

    Returns all rows when there is a match in either left or right table

  • cross

    Returns the Cartesian product of rows from both tables

  • semi

    Filter rows that have a match in the right table.

  • anti

    Filter rows that not have a match in the right table.

Note

A left join preserves the row order of the left DataFrame.

left_on

Join column of the left DataFrame.

right_on

Join column of the right DataFrame.

suffix

Suffix to append to columns with a duplicate name.

validate: {‘m:m’, ‘m:1’, ‘1:m’, ‘1:1’}

Checks if join is of specified type.

  • many_to_many

    “m:m”: default, does not result in checks

  • one_to_one

    “1:1”: check if join keys are unique in both left and right datasets

  • one_to_many

    “1:m”: check if join keys are unique in left dataset

  • many_to_one

    “m:1”: check if join keys are unique in right dataset

Note

This is currently not supported by the streaming engine.

join_nulls

Join on null values. By default null values will never produce matches.

coalesce

Coalescing behavior (merging of join columns).

  • None: -> join specific.

  • True: -> Always coalesce join columns.

  • False: -> Never coalesce join columns.

Note that joining on any other expressions than col will turn off coalescing.

allow_parallel

Allow the physical plan to optionally evaluate the computation of both DataFrames up to the join in parallel.

force_parallel

Force the physical plan to evaluate the computation of both DataFrames up to the join in parallel.

See also

join_asof

Examples

>>> lf = pl.LazyFrame(
...     {
...         "foo": [1, 2, 3],
...         "bar": [6.0, 7.0, 8.0],
...         "ham": ["a", "b", "c"],
...     }
... )
>>> other_lf = pl.LazyFrame(
...     {
...         "apple": ["x", "y", "z"],
...         "ham": ["a", "b", "d"],
...     }
... )
>>> lf.join(other_lf, on="ham").collect()
shape: (2, 4)
┌─────┬─────┬─────┬───────┐
│ foo ┆ bar ┆ ham ┆ apple │
│ --- ┆ --- ┆ --- ┆ ---   │
│ i64 ┆ f64 ┆ str ┆ str   │
╞═════╪═════╪═════╪═══════╡
│ 1   ┆ 6.0 ┆ a   ┆ x     │
│ 2   ┆ 7.0 ┆ b   ┆ y     │
└─────┴─────┴─────┴───────┘
>>> lf.join(other_lf, on="ham", how="full").collect()
shape: (4, 5)
┌──────┬──────┬──────┬───────┬───────────┐
│ foo  ┆ bar  ┆ ham  ┆ apple ┆ ham_right │
│ ---  ┆ ---  ┆ ---  ┆ ---   ┆ ---       │
│ i64  ┆ f64  ┆ str  ┆ str   ┆ str       │
╞══════╪══════╪══════╪═══════╪═══════════╡
│ 1    ┆ 6.0  ┆ a    ┆ x     ┆ a         │
│ 2    ┆ 7.0  ┆ b    ┆ y     ┆ b         │
│ null ┆ null ┆ null ┆ z     ┆ d         │
│ 3    ┆ 8.0  ┆ c    ┆ null  ┆ null      │
└──────┴──────┴──────┴───────┴───────────┘
>>> lf.join(other_lf, on="ham", how="left", coalesce=True).collect()
shape: (3, 4)
┌─────┬─────┬─────┬───────┐
│ foo ┆ bar ┆ ham ┆ apple │
│ --- ┆ --- ┆ --- ┆ ---   │
│ i64 ┆ f64 ┆ str ┆ str   │
╞═════╪═════╪═════╪═══════╡
│ 1   ┆ 6.0 ┆ a   ┆ x     │
│ 2   ┆ 7.0 ┆ b   ┆ y     │
│ 3   ┆ 8.0 ┆ c   ┆ null  │
└─────┴─────┴─────┴───────┘
>>> lf.join(other_lf, on="ham", how="semi").collect()
shape: (2, 3)
┌─────┬─────┬─────┐
│ foo ┆ bar ┆ ham │
│ --- ┆ --- ┆ --- │
│ i64 ┆ f64 ┆ str │
╞═════╪═════╪═════╡
│ 1   ┆ 6.0 ┆ a   │
│ 2   ┆ 7.0 ┆ b   │
└─────┴─────┴─────┘
>>> lf.join(other_lf, on="ham", how="anti").collect()
shape: (1, 3)
┌─────┬─────┬─────┐
│ foo ┆ bar ┆ ham │
│ --- ┆ --- ┆ --- │
│ i64 ┆ f64 ┆ str │
╞═════╪═════╪═════╡
│ 3   ┆ 8.0 ┆ c   │
└─────┴─────┴─────┘
join_asof(
other: LazyFrame,
*,
left_on: str | None | Expr = None,
right_on: str | None | Expr = None,
on: str | None | Expr = None,
by_left: str | Sequence[str] | None = None,
by_right: str | Sequence[str] | None = None,
by: str | Sequence[str] | None = None,
strategy: AsofJoinStrategy = 'backward',
suffix: str = '_right',
tolerance: str | int | float | timedelta | None = None,
allow_parallel: bool = True,
force_parallel: bool = False,
coalesce: bool | None = None,
) LazyFrame[source]

Perform an asof join.

This is similar to a left-join except that we match on nearest key rather than equal keys.

Both DataFrames must be sorted by the join_asof key.

For each row in the left DataFrame:

  • A “backward” search selects the last row in the right DataFrame whose ‘on’ key is less than or equal to the left’s key.

  • A “forward” search selects the first row in the right DataFrame whose ‘on’ key is greater than or equal to the left’s key.

    A “nearest” search selects the last row in the right DataFrame whose value is nearest to the left’s key. String keys are not currently supported for a nearest search.

The default is “backward”.

Parameters:
other

Lazy DataFrame to join with.

left_on

Join column of the left DataFrame.

right_on

Join column of the right DataFrame.

on

Join column of both DataFrames. If set, left_on and right_on should be None.

by

Join on these columns before doing asof join.

by_left

Join on these columns before doing asof join.

by_right

Join on these columns before doing asof join.

strategy{‘backward’, ‘forward’, ‘nearest’}

Join strategy.

suffix

Suffix to append to columns with a duplicate name.

tolerance

Numeric tolerance. By setting this the join will only be done if the near keys are within this distance. If an asof join is done on columns of dtype “Date”, “Datetime”, “Duration” or “Time”, use either a datetime.timedelta object or the following string language:

  • 1ns (1 nanosecond)

  • 1us (1 microsecond)

  • 1ms (1 millisecond)

  • 1s (1 second)

  • 1m (1 minute)

  • 1h (1 hour)

  • 1d (1 calendar day)

  • 1w (1 calendar week)

  • 1mo (1 calendar month)

  • 1q (1 calendar quarter)

  • 1y (1 calendar year)

Or combine them: “3d12h4m25s” # 3 days, 12 hours, 4 minutes, and 25 seconds

By “calendar day”, we mean the corresponding time on the next day (which may not be 24 hours, due to daylight savings). Similarly for “calendar week”, “calendar month”, “calendar quarter”, and “calendar year”.

allow_parallel

Allow the physical plan to optionally evaluate the computation of both DataFrames up to the join in parallel.

force_parallel

Force the physical plan to evaluate the computation of both DataFrames up to the join in parallel.

coalesce

Coalescing behavior (merging of join columns).

  • None: -> join specific.

  • True: -> Always coalesce join columns.

  • False: -> Never coalesce join columns.

Note that joining on any other expressions than col will turn off coalescing.

Examples

>>> from datetime import datetime
>>> gdp = pl.LazyFrame(
...     {
...         "date": [
...             datetime(2016, 1, 1),
...             datetime(2017, 1, 1),
...             datetime(2018, 1, 1),
...             datetime(2019, 1, 1),
...         ],  # note record date: Jan 1st (sorted!)
...         "gdp": [4164, 4411, 4566, 4696],
...     }
... ).set_sorted("date")
>>> population = pl.LazyFrame(
...     {
...         "date": [
...             datetime(2016, 5, 12),
...             datetime(2017, 5, 12),
...             datetime(2018, 5, 12),
...             datetime(2019, 5, 12),
...         ],  # note record date: May 12th (sorted!)
...         "population": [82.19, 82.66, 83.12, 83.52],
...     }
... ).set_sorted("date")
>>> population.join_asof(gdp, on="date", strategy="backward").collect()
shape: (4, 3)
┌─────────────────────┬────────────┬──────┐
│ date                ┆ population ┆ gdp  │
│ ---                 ┆ ---        ┆ ---  │
│ datetime[μs]        ┆ f64        ┆ i64  │
╞═════════════════════╪════════════╪══════╡
│ 2016-05-12 00:00:00 ┆ 82.19      ┆ 4164 │
│ 2017-05-12 00:00:00 ┆ 82.66      ┆ 4411 │
│ 2018-05-12 00:00:00 ┆ 83.12      ┆ 4566 │
│ 2019-05-12 00:00:00 ┆ 83.52      ┆ 4696 │
└─────────────────────┴────────────┴──────┘
last() LazyFrame[source]

Get the last row of the DataFrame.

Examples

>>> lf = pl.LazyFrame(
...     {
...         "a": [1, 3, 5],
...         "b": [2, 4, 6],
...     }
... )
>>> lf.last().collect()
shape: (1, 2)
┌─────┬─────┐
│ a   ┆ b   │
│ --- ┆ --- │
│ i64 ┆ i64 │
╞═════╪═════╡
│ 5   ┆ 6   │
└─────┴─────┘
lazy() LazyFrame[source]

Return lazy representation, i.e. itself.

Useful for writing code that expects either a DataFrame or LazyFrame.

Returns:
LazyFrame

Examples

>>> lf = pl.LazyFrame(
...     {
...         "a": [None, 2, 3, 4],
...         "b": [0.5, None, 2.5, 13],
...         "c": [True, True, False, None],
...     }
... )
>>> lf.lazy()  
<LazyFrame at ...>
limit(n: int = 5) LazyFrame[source]

Get the first n rows.

Alias for LazyFrame.head().

Parameters:
n

Number of rows to return.

Notes

Consider using the fetch() operation if you only want to test your query. The fetch() operation will load the first n rows at the scan level, whereas the head()/limit() are applied at the end.

Examples

>>> lf = pl.LazyFrame(
...     {
...         "a": [1, 2, 3, 4, 5, 6],
...         "b": [7, 8, 9, 10, 11, 12],
...     }
... )
>>> lf.limit().collect()
shape: (5, 2)
┌─────┬─────┐
│ a   ┆ b   │
│ --- ┆ --- │
│ i64 ┆ i64 │
╞═════╪═════╡
│ 1   ┆ 7   │
│ 2   ┆ 8   │
│ 3   ┆ 9   │
│ 4   ┆ 10  │
│ 5   ┆ 11  │
└─────┴─────┘
>>> lf.limit(2).collect()
shape: (2, 2)
┌─────┬─────┐
│ a   ┆ b   │
│ --- ┆ --- │
│ i64 ┆ i64 │
╞═════╪═════╡
│ 1   ┆ 7   │
│ 2   ┆ 8   │
└─────┴─────┘
map_batches(
function: Callable[[DataFrame], DataFrame],
*,
predicate_pushdown: bool = True,
projection_pushdown: bool = True,
slice_pushdown: bool = True,
no_optimizations: bool = False,
schema: None | SchemaDict = None,
validate_output_schema: bool = True,
streamable: bool = False,
) LazyFrame[source]

Apply a custom function.

It is important that the function returns a Polars DataFrame.

Parameters:
function

Lambda/ function to apply.

predicate_pushdown

Allow predicate pushdown optimization to pass this node.

projection_pushdown

Allow projection pushdown optimization to pass this node.

slice_pushdown

Allow slice pushdown optimization to pass this node.

no_optimizations

Turn off all optimizations past this point.

schema

Output schema of the function, if set to None we assume that the schema will remain unchanged by the applied function.

validate_output_schema

It is paramount that polars’ schema is correct. This flag will ensure that the output schema of this function will be checked with the expected schema. Setting this to False will not do this check, but may lead to hard to debug bugs.

streamable

Whether the function that is given is eligible to be running with the streaming engine. That means that the function must produce the same result when it is executed in batches or when it is be executed on the full dataset.

Warning

The schema of a LazyFrame must always be correct. It is up to the caller of this function to ensure that this invariant is upheld.

It is important that the optimization flags are correct. If the custom function for instance does an aggregation of a column, predicate_pushdown should not be allowed, as this prunes rows and will influence your aggregation results.

Examples

>>> lf = (  
...     pl.LazyFrame(
...         {
...             "a": pl.int_range(-100_000, 0, eager=True),
...             "b": pl.int_range(0, 100_000, eager=True),
...         }
...     )
...     .map_batches(lambda x: 2 * x, streamable=True)
...     .collect(streaming=True)
... )
shape: (100_000, 2)
┌─────────┬────────┐
│ a       ┆ b      │
│ ---     ┆ ---    │
│ i64     ┆ i64    │
╞═════════╪════════╡
│ -200000 ┆ 0      │
│ -199998 ┆ 2      │
│ -199996 ┆ 4      │
│ -199994 ┆ 6      │
│ …       ┆ …      │
│ -8      ┆ 199992 │
│ -6      ┆ 199994 │
│ -4      ┆ 199996 │
│ -2      ┆ 199998 │
└─────────┴────────┘
max() LazyFrame[source]

Aggregate the columns in the LazyFrame to their maximum value.

Examples

>>> lf = pl.LazyFrame(
...     {
...         "a": [1, 2, 3, 4],
...         "b": [1, 2, 1, 1],
...     }
... )
>>> lf.max().collect()
shape: (1, 2)
┌─────┬─────┐
│ a   ┆ b   │
│ --- ┆ --- │
│ i64 ┆ i64 │
╞═════╪═════╡
│ 4   ┆ 2   │
└─────┴─────┘
mean() LazyFrame[source]

Aggregate the columns in the LazyFrame to their mean value.

Examples

>>> lf = pl.LazyFrame(
...     {
...         "a": [1, 2, 3, 4],
...         "b": [1, 2, 1, 1],
...     }
... )
>>> lf.mean().collect()
shape: (1, 2)
┌─────┬──────┐
│ a   ┆ b    │
│ --- ┆ ---  │
│ f64 ┆ f64  │
╞═════╪══════╡
│ 2.5 ┆ 1.25 │
└─────┴──────┘
median() LazyFrame[source]

Aggregate the columns in the LazyFrame to their median value.

Examples

>>> lf = pl.LazyFrame(
...     {
...         "a": [1, 2, 3, 4],
...         "b": [1, 2, 1, 1],
...     }
... )
>>> lf.median().collect()
shape: (1, 2)
┌─────┬─────┐
│ a   ┆ b   │
│ --- ┆ --- │
│ f64 ┆ f64 │
╞═════╪═════╡
│ 2.5 ┆ 1.0 │
└─────┴─────┘
melt(
id_vars: ColumnNameOrSelector | Sequence[ColumnNameOrSelector] | None = None,
value_vars: ColumnNameOrSelector | Sequence[ColumnNameOrSelector] | None = None,
variable_name: str | None = None,
value_name: str | None = None,
*,
streamable: bool = True,
) LazyFrame[source]

Unpivot a DataFrame from wide to long format.

Optionally leaves identifiers set.

This function is useful to massage a DataFrame into a format where one or more columns are identifier variables (id_vars) while all other columns, considered measured variables (value_vars), are “unpivoted” to the row axis leaving just two non-identifier columns, ‘variable’ and ‘value’.

Parameters:
id_vars

Column(s) or selector(s) to use as identifier variables.

value_vars

Column(s) or selector(s) to use as values variables; if value_vars is empty all columns that are not in id_vars will be used.

variable_name

Name to give to the variable column. Defaults to “variable”

value_name

Name to give to the value column. Defaults to “value”

streamable

Allow this node to run in the streaming engine. If this runs in streaming, the output of the unpivot operation will not have a stable ordering.

merge_sorted(
other: LazyFrame,
key: str,
) LazyFrame[source]

Take two sorted DataFrames and merge them by the sorted key.

The output of this operation will also be sorted. It is the callers responsibility that the frames are sorted by that key otherwise the output will not make sense.

The schemas of both LazyFrames must be equal.

Parameters:
other

Other DataFrame that must be merged

key

Key that is sorted.

Examples

>>> df0 = pl.LazyFrame(
...     {"name": ["steve", "elise", "bob"], "age": [42, 44, 18]}
... ).sort("age")
>>> df0.collect()
shape: (3, 2)
┌───────┬─────┐
│ name  ┆ age │
│ ---   ┆ --- │
│ str   ┆ i64 │
╞═══════╪═════╡
│ bob   ┆ 18  │
│ steve ┆ 42  │
│ elise ┆ 44  │
└───────┴─────┘
>>> df1 = pl.LazyFrame(
...     {"name": ["anna", "megan", "steve", "thomas"], "age": [21, 33, 42, 20]}
... ).sort("age")
>>> df1.collect()
shape: (4, 2)
┌────────┬─────┐
│ name   ┆ age │
│ ---    ┆ --- │
│ str    ┆ i64 │
╞════════╪═════╡
│ thomas ┆ 20  │
│ anna   ┆ 21  │
│ megan  ┆ 33  │
│ steve  ┆ 42  │
└────────┴─────┘
>>> df0.merge_sorted(df1, key="age").collect()
shape: (7, 2)
┌────────┬─────┐
│ name   ┆ age │
│ ---    ┆ --- │
│ str    ┆ i64 │
╞════════╪═════╡
│ bob    ┆ 18  │
│ thomas ┆ 20  │
│ anna   ┆ 21  │
│ megan  ┆ 33  │
│ steve  ┆ 42  │
│ steve  ┆ 42  │
│ elise  ┆ 44  │
└────────┴─────┘
min() LazyFrame[source]

Aggregate the columns in the LazyFrame to their minimum value.

Examples

>>> lf = pl.LazyFrame(
...     {
...         "a": [1, 2, 3, 4],
...         "b": [1, 2, 1, 1],
...     }
... )
>>> lf.min().collect()
shape: (1, 2)
┌─────┬─────┐
│ a   ┆ b   │
│ --- ┆ --- │
│ i64 ┆ i64 │
╞═════╪═════╡
│ 1   ┆ 1   │
└─────┴─────┘
null_count() LazyFrame[source]

Aggregate the columns in the LazyFrame as the sum of their null value count.

Examples

>>> lf = pl.LazyFrame(
...     {
...         "foo": [1, None, 3],
...         "bar": [6, 7, None],
...         "ham": ["a", "b", "c"],
...     }
... )
>>> lf.null_count().collect()
shape: (1, 3)
┌─────┬─────┬─────┐
│ foo ┆ bar ┆ ham │
│ --- ┆ --- ┆ --- │
│ u32 ┆ u32 ┆ u32 │
╞═════╪═════╪═════╡
│ 1   ┆ 1   ┆ 0   │
└─────┴─────┴─────┘
pipe(
function: Callable[Concatenate[LazyFrame, P], T],
*args: P.args,
**kwargs: P.kwargs,
) T[source]

Offers a structured way to apply a sequence of user-defined functions (UDFs).

Parameters:
function

Callable; will receive the frame as the first parameter, followed by any given args/kwargs.

*args

Arguments to pass to the UDF.

**kwargs

Keyword arguments to pass to the UDF.

Examples

>>> def cast_str_to_int(lf: pl.LazyFrame, col_name: str) -> pl.LazyFrame:
...     return lf.with_columns(pl.col(col_name).cast(pl.Int64))
>>> lf = pl.LazyFrame(
...     {
...         "a": [1, 2, 3, 4],
...         "b": ["10", "20", "30", "40"],
...     }
... )
>>> lf.pipe(cast_str_to_int, col_name="b").collect()
shape: (4, 2)
┌─────┬─────┐
│ a   ┆ b   │
│ --- ┆ --- │
│ i64 ┆ i64 │
╞═════╪═════╡
│ 1   ┆ 10  │
│ 2   ┆ 20  │
│ 3   ┆ 30  │
│ 4   ┆ 40  │
└─────┴─────┘
>>> lf = pl.LazyFrame(
...     {
...         "b": [1, 2],
...         "a": [3, 4],
...     }
... )
>>> lf.collect()
shape: (2, 2)
┌─────┬─────┐
│ b   ┆ a   │
│ --- ┆ --- │
│ i64 ┆ i64 │
╞═════╪═════╡
│ 1   ┆ 3   │
│ 2   ┆ 4   │
└─────┴─────┘
>>> lf.pipe(lambda lf: lf.select(sorted(lf.collect_schema()))).collect()
shape: (2, 2)
┌─────┬─────┐
│ a   ┆ b   │
│ --- ┆ --- │
│ i64 ┆ i64 │
╞═════╪═════╡
│ 3   ┆ 1   │
│ 4   ┆ 2   │
└─────┴─────┘
profile(
*,
type_coercion: bool = True,
predicate_pushdown: bool = True,
projection_pushdown: bool = True,
simplify_expression: bool = True,
no_optimization: bool = False,
slice_pushdown: bool = True,
comm_subplan_elim: bool = True,
comm_subexpr_elim: bool = True,
cluster_with_columns: bool = True,
show_plot: bool = False,
truncate_nodes: int = 0,
figsize: tuple[int, int] = (18, 8),
streaming: bool = False,
) tuple[DataFrame, DataFrame][source]

Profile a LazyFrame.

This will run the query and return a tuple containing the materialized DataFrame and a DataFrame that contains profiling information of each node that is executed.

The units of the timings are microseconds.

Parameters:
type_coercion

Do type coercion optimization.

predicate_pushdown

Do predicate pushdown optimization.

projection_pushdown

Do projection pushdown optimization.

simplify_expression

Run simplify expressions optimization.

no_optimization

Turn off (certain) optimizations.

slice_pushdown

Slice pushdown optimization.

comm_subplan_elim

Will try to cache branching subplans that occur on self-joins or unions.

comm_subexpr_elim

Common subexpressions will be cached and reused.

cluster_with_columns

Combine sequential independent calls to with_columns

show_plot

Show a gantt chart of the profiling result

truncate_nodes

Truncate the label lengths in the gantt chart to this number of characters.

figsize

matplotlib figsize of the profiling plot

streaming

Run parts of the query in a streaming fashion (this is in an alpha state)

Examples

>>> lf = pl.LazyFrame(
...     {
...         "a": ["a", "b", "a", "b", "b", "c"],
...         "b": [1, 2, 3, 4, 5, 6],
...         "c": [6, 5, 4, 3, 2, 1],
...     }
... )
>>> lf.group_by("a", maintain_order=True).agg(pl.all().sum()).sort(
...     "a"
... ).profile()  
(shape: (3, 3)
 ┌─────┬─────┬─────┐
 │ a   ┆ b   ┆ c   │
 │ --- ┆ --- ┆ --- │
 │ str ┆ i64 ┆ i64 │
 ╞═════╪═════╪═════╡
 │ a   ┆ 4   ┆ 10  │
 │ b   ┆ 11  ┆ 10  │
 │ c   ┆ 6   ┆ 1   │
 └─────┴─────┴─────┘,
 shape: (3, 3)
 ┌─────────────────────────┬───────┬──────┐
 │ node                    ┆ start ┆ end  │
 │ ---                     ┆ ---   ┆ ---  │
 │ str                     ┆ u64   ┆ u64  │
 ╞═════════════════════════╪═══════╪══════╡
 │ optimization            ┆ 0     ┆ 5    │
 │ group_by_partitioned(a) ┆ 5     ┆ 470  │
 │ sort(a)                 ┆ 475   ┆ 1964 │
 └─────────────────────────┴───────┴──────┘)
quantile(
quantile: float | Expr,
interpolation: RollingInterpolationMethod = 'nearest',
) LazyFrame[source]

Aggregate the columns in the LazyFrame to their quantile value.

Parameters:
quantile

Quantile between 0.0 and 1.0.

interpolation{‘nearest’, ‘higher’, ‘lower’, ‘midpoint’, ‘linear’}

Interpolation method.

Examples

>>> lf = pl.LazyFrame(
...     {
...         "a": [1, 2, 3, 4],
...         "b": [1, 2, 1, 1],
...     }
... )
>>> lf.quantile(0.7).collect()
shape: (1, 2)
┌─────┬─────┐
│ a   ┆ b   │
│ --- ┆ --- │
│ f64 ┆ f64 │
╞═════╪═════╡
│ 3.0 ┆ 1.0 │
└─────┴─────┘
rename(
mapping: dict[str, str] | Callable[[str], str],
) LazyFrame[source]

Rename column names.

Parameters:
mapping

Key value pairs that map from old name to new name, or a function that takes the old name as input and returns the new name.

Notes

If existing names are swapped (e.g. ‘A’ points to ‘B’ and ‘B’ points to ‘A’), polars will block projection and predicate pushdowns at this node.

Examples

>>> lf = pl.LazyFrame(
...     {
...         "foo": [1, 2, 3],
...         "bar": [6, 7, 8],
...         "ham": ["a", "b", "c"],
...     }
... )
>>> lf.rename({"foo": "apple"}).collect()
shape: (3, 3)
┌───────┬─────┬─────┐
│ apple ┆ bar ┆ ham │
│ ---   ┆ --- ┆ --- │
│ i64   ┆ i64 ┆ str │
╞═══════╪═════╪═════╡
│ 1     ┆ 6   ┆ a   │
│ 2     ┆ 7   ┆ b   │
│ 3     ┆ 8   ┆ c   │
└───────┴─────┴─────┘
>>> lf.rename(lambda column_name: "c" + column_name[1:]).collect()
shape: (3, 3)
┌─────┬─────┬─────┐
│ coo ┆ car ┆ cam │
│ --- ┆ --- ┆ --- │
│ i64 ┆ i64 ┆ str │
╞═════╪═════╪═════╡
│ 1   ┆ 6   ┆ a   │
│ 2   ┆ 7   ┆ b   │
│ 3   ┆ 8   ┆ c   │
└─────┴─────┴─────┘
reverse() LazyFrame[source]

Reverse the DataFrame.

Examples

>>> lf = pl.LazyFrame(
...     {
...         "key": ["a", "b", "c"],
...         "val": [1, 2, 3],
...     }
... )
>>> lf.reverse().collect()
shape: (3, 2)
┌─────┬─────┐
│ key ┆ val │
│ --- ┆ --- │
│ str ┆ i64 │
╞═════╪═════╡
│ c   ┆ 3   │
│ b   ┆ 2   │
│ a   ┆ 1   │
└─────┴─────┘
rolling(
index_column: IntoExpr,
*,
period: str | timedelta,
offset: str | timedelta | None = None,
closed: ClosedInterval = 'right',
group_by: IntoExpr | Iterable[IntoExpr] | None = None,
) LazyGroupBy[source]

Create rolling groups based on a temporal or integer column.

Different from a group_by_dynamic the windows are now determined by the individual values and are not of constant intervals. For constant intervals use LazyFrame.group_by_dynamic().

If you have a time series <t_0, t_1, ..., t_n>, then by default the windows created will be

  • (t_0 - period, t_0]

  • (t_1 - period, t_1]

  • (t_n - period, t_n]

whereas if you pass a non-default offset, then the windows will be

  • (t_0 + offset, t_0 + offset + period]

  • (t_1 + offset, t_1 + offset + period]

  • (t_n + offset, t_n + offset + period]

The period and offset arguments are created either from a timedelta, or by using the following string language:

  • 1ns (1 nanosecond)

  • 1us (1 microsecond)

  • 1ms (1 millisecond)

  • 1s (1 second)

  • 1m (1 minute)

  • 1h (1 hour)

  • 1d (1 calendar day)

  • 1w (1 calendar week)

  • 1mo (1 calendar month)

  • 1q (1 calendar quarter)

  • 1y (1 calendar year)

  • 1i (1 index count)

Or combine them: “3d12h4m25s” # 3 days, 12 hours, 4 minutes, and 25 seconds

By “calendar day”, we mean the corresponding time on the next day (which may not be 24 hours, due to daylight savings). Similarly for “calendar week”, “calendar month”, “calendar quarter”, and “calendar year”.

Parameters:
index_column

Column used to group based on the time window. Often of type Date/Datetime. This column must be sorted in ascending order (or, if group_by is specified, then it must be sorted in ascending order within each group).

In case of a rolling group by on indices, dtype needs to be one of {UInt32, UInt64, Int32, Int64}. Note that the first three get temporarily cast to Int64, so if performance matters use an Int64 column.

period

Length of the window - must be non-negative.

offset

Offset of the window. Default is -period.

closed{‘right’, ‘left’, ‘both’, ‘none’}

Define which sides of the temporal interval are closed (inclusive).

group_by

Also group by this column/these columns

Returns:
LazyGroupBy

Object you can call .agg on to aggregate by groups, the result of which will be sorted by index_column (but note that if group_by columns are passed, it will only be sorted within each group).

See also

group_by_dynamic

Examples

>>> dates = [
...     "2020-01-01 13:45:48",
...     "2020-01-01 16:42:13",
...     "2020-01-01 16:45:09",
...     "2020-01-02 18:12:48",
...     "2020-01-03 19:45:32",
...     "2020-01-08 23:16:43",
... ]
>>> df = pl.LazyFrame({"dt": dates, "a": [3, 7, 5, 9, 2, 1]}).with_columns(
...     pl.col("dt").str.strptime(pl.Datetime).set_sorted()
... )
>>> out = (
...     df.rolling(index_column="dt", period="2d")
...     .agg(
...         pl.sum("a").alias("sum_a"),
...         pl.min("a").alias("min_a"),
...         pl.max("a").alias("max_a"),
...     )
...     .collect()
... )
>>> out
shape: (6, 4)
┌─────────────────────┬───────┬───────┬───────┐
│ dt                  ┆ sum_a ┆ min_a ┆ max_a │
│ ---                 ┆ ---   ┆ ---   ┆ ---   │
│ datetime[μs]        ┆ i64   ┆ i64   ┆ i64   │
╞═════════════════════╪═══════╪═══════╪═══════╡
│ 2020-01-01 13:45:48 ┆ 3     ┆ 3     ┆ 3     │
│ 2020-01-01 16:42:13 ┆ 10    ┆ 3     ┆ 7     │
│ 2020-01-01 16:45:09 ┆ 15    ┆ 3     ┆ 7     │
│ 2020-01-02 18:12:48 ┆ 24    ┆ 3     ┆ 9     │
│ 2020-01-03 19:45:32 ┆ 11    ┆ 2     ┆ 9     │
│ 2020-01-08 23:16:43 ┆ 1     ┆ 1     ┆ 1     │
└─────────────────────┴───────┴───────┴───────┘
property schema: Schema[source]

Get an ordered mapping of column names to their data type.

Warning

Resolving the schema of a LazyFrame is a potentially expensive operation. Using collect_schema() is the idiomatic way to resolve the schema. This property exists only for symmetry with the DataFrame class.

See also

collect_schema
Schema

Examples

>>> lf = pl.LazyFrame(
...     {
...         "foo": [1, 2, 3],
...         "bar": [6.0, 7.0, 8.0],
...         "ham": ["a", "b", "c"],
...     }
... )
>>> lf.schema  
Schema({'foo': Int64, 'bar': Float64, 'ham': String})
select(
*exprs: IntoExpr | Iterable[IntoExpr],
**named_exprs: IntoExpr,
) LazyFrame[source]

Select columns from this LazyFrame.

Parameters:
*exprs

Column(s) to select, specified as positional arguments. Accepts expression input. Strings are parsed as column names, other non-expression inputs are parsed as literals.

**named_exprs

Additional columns to select, specified as keyword arguments. The columns will be renamed to the keyword used.

Examples

Pass the name of a column to select that column.

>>> lf = pl.LazyFrame(
...     {
...         "foo": [1, 2, 3],
...         "bar": [6, 7, 8],
...         "ham": ["a", "b", "c"],
...     }
... )
>>> lf.select("foo").collect()
shape: (3, 1)
┌─────┐
│ foo │
│ --- │
│ i64 │
╞═════╡
│ 1   │
│ 2   │
│ 3   │
└─────┘

Multiple columns can be selected by passing a list of column names.

>>> lf.select(["foo", "bar"]).collect()
shape: (3, 2)
┌─────┬─────┐
│ foo ┆ bar │
│ --- ┆ --- │
│ i64 ┆ i64 │
╞═════╪═════╡
│ 1   ┆ 6   │
│ 2   ┆ 7   │
│ 3   ┆ 8   │
└─────┴─────┘

Multiple columns can also be selected using positional arguments instead of a list. Expressions are also accepted.

>>> lf.select(pl.col("foo"), pl.col("bar") + 1).collect()
shape: (3, 2)
┌─────┬─────┐
│ foo ┆ bar │
│ --- ┆ --- │
│ i64 ┆ i64 │
╞═════╪═════╡
│ 1   ┆ 7   │
│ 2   ┆ 8   │
│ 3   ┆ 9   │
└─────┴─────┘

Use keyword arguments to easily name your expression inputs.

>>> lf.select(
...     threshold=pl.when(pl.col("foo") > 2).then(10).otherwise(0)
... ).collect()
shape: (3, 1)
┌───────────┐
│ threshold │
│ ---       │
│ i32       │
╞═══════════╡
│ 0         │
│ 0         │
│ 10        │
└───────────┘

Expressions with multiple outputs can be automatically instantiated as Structs by enabling the setting Config.set_auto_structify(True):

>>> with pl.Config(auto_structify=True):
...     lf.select(
...         is_odd=(pl.col(pl.Int64) % 2 == 1).name.suffix("_is_odd"),
...     ).collect()
shape: (3, 1)
┌──────────────┐
│ is_odd       │
│ ---          │
│ struct[2]    │
╞══════════════╡
│ {true,false} │
│ {false,true} │
│ {true,false} │
└──────────────┘
select_seq(
*exprs: IntoExpr | Iterable[IntoExpr],
**named_exprs: IntoExpr,
) LazyFrame[source]

Select columns from this LazyFrame.

This will run all expression sequentially instead of in parallel. Use this when the work per expression is cheap.

Parameters:
*exprs

Column(s) to select, specified as positional arguments. Accepts expression input. Strings are parsed as column names, other non-expression inputs are parsed as literals.

**named_exprs

Additional columns to select, specified as keyword arguments. The columns will be renamed to the keyword used.

See also

select
serialize(
file: IOBase | str | Path | None = None,
*,
format: SerializationFormat = 'binary',
) bytes | str | None[source]

Serialize the logical plan of this LazyFrame to a file or string in JSON format.

Parameters:
file

File path to which the result should be written. If set to None (default), the output is returned as a string instead.

format

The format in which to serialize. Options:

  • "binary": Serialize to binary format (bytes). This is the default.

  • "json": Serialize to JSON format (string).

Notes

Serialization is not stable across Polars versions: a LazyFrame serialized in one Polars version may not be deserializable in another Polars version.

Examples

Serialize the logical plan into a binary representation.

>>> lf = pl.LazyFrame({"a": [1, 2, 3]}).sum()
>>> bytes = lf.serialize()
>>> bytes  
b'\xa1kMapFunction\xa2einput\xa1mDataFrameScan\xa4bdf\xa1gcolumns\x81\xa4d...'

The bytes can later be deserialized back into a LazyFrame.

>>> import io
>>> pl.LazyFrame.deserialize(io.BytesIO(bytes)).collect()
shape: (1, 1)
┌─────┐
│ a   │
│ --- │
│ i64 │
╞═════╡
│ 6   │
└─────┘
set_sorted(
column: str,
*,
descending: bool = False,
) LazyFrame[source]

Indicate that one or multiple columns are sorted.

This can speed up future operations.

Parameters:
column

Columns that are sorted

descending

Whether the columns are sorted in descending order.

Warning

This can lead to incorrect results if the data is NOT sorted!! Use with care!

shift(
n: int | IntoExprColumn = 1,
*,
fill_value: IntoExpr | None = None,
) LazyFrame[source]

Shift values by the given number of indices.

Parameters:
n

Number of indices to shift forward. If a negative value is passed, values are shifted in the opposite direction instead.

fill_value

Fill the resulting null values with this value. Accepts expression input. Non-expression inputs are parsed as literals.

Notes

This method is similar to the LAG operation in SQL when the value for n is positive. With a negative value for n, it is similar to LEAD.

Examples

By default, values are shifted forward by one index.

>>> lf = pl.LazyFrame(
...     {
...         "a": [1, 2, 3, 4],
...         "b": [5, 6, 7, 8],
...     }
... )
>>> lf.shift().collect()
shape: (4, 2)
┌──────┬──────┐
│ a    ┆ b    │
│ ---  ┆ ---  │
│ i64  ┆ i64  │
╞══════╪══════╡
│ null ┆ null │
│ 1    ┆ 5    │
│ 2    ┆ 6    │
│ 3    ┆ 7    │
└──────┴──────┘

Pass a negative value to shift in the opposite direction instead.

>>> lf.shift(-2).collect()
shape: (4, 2)
┌──────┬──────┐
│ a    ┆ b    │
│ ---  ┆ ---  │
│ i64  ┆ i64  │
╞══════╪══════╡
│ 3    ┆ 7    │
│ 4    ┆ 8    │
│ null ┆ null │
│ null ┆ null │
└──────┴──────┘

Specify fill_value to fill the resulting null values.

>>> lf.shift(-2, fill_value=100).collect()
shape: (4, 2)
┌─────┬─────┐
│ a   ┆ b   │
│ --- ┆ --- │
│ i64 ┆ i64 │
╞═════╪═════╡
│ 3   ┆ 7   │
│ 4   ┆ 8   │
│ 100 ┆ 100 │
│ 100 ┆ 100 │
└─────┴─────┘
show_graph(
*,
optimized: bool = True,
show: bool = True,
output_path: str | Path | None = None,
raw_output: bool = False,
figsize: tuple[float, float] = (16.0, 12.0),
type_coercion: bool = True,
predicate_pushdown: bool = True,
projection_pushdown: bool = True,
simplify_expression: bool = True,
slice_pushdown: bool = True,
comm_subplan_elim: bool = True,
comm_subexpr_elim: bool = True,
cluster_with_columns: bool = True,
streaming: bool = False,
) str | None[source]

Show a plot of the query plan.

Note that graphviz must be installed to render the visualization (if not already present you can download it here: <https://graphviz.org/download>`_).

Parameters:
optimized

Optimize the query plan.

show

Show the figure.

output_path

Write the figure to disk.

raw_output

Return dot syntax. This cannot be combined with show and/or output_path.

figsize

Passed to matplotlib if show == True.

type_coercion

Do type coercion optimization.

predicate_pushdown

Do predicate pushdown optimization.

projection_pushdown

Do projection pushdown optimization.

simplify_expression

Run simplify expressions optimization.

slice_pushdown

Slice pushdown optimization.

comm_subplan_elim

Will try to cache branching subplans that occur on self-joins or unions.

comm_subexpr_elim

Common subexpressions will be cached and reused.

cluster_with_columns

Combine sequential independent calls to with_columns

streaming

Run parts of the query in a streaming fashion (this is in an alpha state)

Examples

>>> lf = pl.LazyFrame(
...     {
...         "a": ["a", "b", "a", "b", "b", "c"],
...         "b": [1, 2, 3, 4, 5, 6],
...         "c": [6, 5, 4, 3, 2, 1],
...     }
... )
>>> lf.group_by("a", maintain_order=True).agg(pl.all().sum()).sort(
...     "a"
... ).show_graph()  
sink_csv(
path: str | Path,
*,
include_bom: bool = False,
include_header: bool = True,
separator: str = ',',
line_terminator: str = '\n',
quote_char: str = '"',
batch_size: int = 1024,
datetime_format: str | None = None,
date_format: str | None = None,
time_format: str | None = None,
float_scientific: bool | None = None,
float_precision: int | None = None,
null_value: str | None = None,
quote_style: CsvQuoteStyle | None = None,
maintain_order: bool = True,
type_coercion: bool = True,
predicate_pushdown: bool = True,
projection_pushdown: bool = True,
simplify_expression: bool = True,
slice_pushdown: bool = True,
no_optimization: bool = False,
) DataFrame[source]

Evaluate the query in streaming mode and write to a CSV file.

Warning

Streaming mode is considered unstable. It may be changed at any point without it being considered a breaking change.

This allows streaming results that are larger than RAM to be written to disk.

Parameters:
path

File path to which the file should be written.

include_bom

Whether to include UTF-8 BOM in the CSV output.

include_header

Whether to include header in the CSV output.

separator

Separate CSV fields with this symbol.

line_terminator

String used to end each row.

quote_char

Byte to use as quoting character.

batch_size

Number of rows that will be processed per thread.

datetime_format

A format string, with the specifiers defined by the chrono Rust crate. If no format specified, the default fractional-second precision is inferred from the maximum timeunit found in the frame’s Datetime cols (if any).

date_format

A format string, with the specifiers defined by the chrono Rust crate.

time_format

A format string, with the specifiers defined by the chrono Rust crate.

float_scientific

Whether to use scientific form always (true), never (false), or automatically (None) for Float32 and Float64 datatypes.

float_precision

Number of decimal places to write, applied to both Float32 and Float64 datatypes.

null_value

A string representing null values (defaulting to the empty string).

quote_style{‘necessary’, ‘always’, ‘non_numeric’, ‘never’}

Determines the quoting strategy used.

  • necessary (default): This puts quotes around fields only when necessary. They are necessary when fields contain a quote, delimiter or record terminator. Quotes are also necessary when writing an empty record (which is indistinguishable from a record with one empty field). This is the default.

  • always: This puts quotes around every field. Always.

  • never: This never puts quotes around fields, even if that results in invalid CSV data (e.g.: by not quoting strings containing the separator).

  • non_numeric: This puts quotes around all fields that are non-numeric. Namely, when writing a field that does not parse as a valid float or integer, then quotes will be used even if they aren`t strictly necessary.

maintain_order

Maintain the order in which data is processed. Setting this to False will be slightly faster.

type_coercion

Do type coercion optimization.

predicate_pushdown

Do predicate pushdown optimization.

projection_pushdown

Do projection pushdown optimization.

simplify_expression

Run simplify expressions optimization.

slice_pushdown

Slice pushdown optimization.

no_optimization

Turn off (certain) optimizations.

Returns:
DataFrame

Examples

>>> lf = pl.scan_csv("/path/to/my_larger_than_ram_file.csv")  
>>> lf.sink_csv("out.csv")  
sink_ipc(
path: str | Path,
*,
compression: str | None = 'zstd',
maintain_order: bool = True,
type_coercion: bool = True,
predicate_pushdown: bool = True,
projection_pushdown: bool = True,
simplify_expression: bool = True,
slice_pushdown: bool = True,
no_optimization: bool = False,
) DataFrame[source]

Evaluate the query in streaming mode and write to an IPC file.

Warning

Streaming mode is considered unstable. It may be changed at any point without it being considered a breaking change.

This allows streaming results that are larger than RAM to be written to disk.

Parameters:
path

File path to which the file should be written.

compression{‘lz4’, ‘zstd’}

Choose “zstd” for good compression performance. Choose “lz4” for fast compression/decompression.

maintain_order

Maintain the order in which data is processed. Setting this to False will be slightly faster.

type_coercion

Do type coercion optimization.

predicate_pushdown

Do predicate pushdown optimization.

projection_pushdown

Do projection pushdown optimization.

simplify_expression

Run simplify expressions optimization.

slice_pushdown

Slice pushdown optimization.

no_optimization

Turn off (certain) optimizations.

Returns:
DataFrame

Examples

>>> lf = pl.scan_csv("/path/to/my_larger_than_ram_file.csv")  
>>> lf.sink_ipc("out.arrow")  
sink_ndjson(
path: str | Path,
*,
maintain_order: bool = True,
type_coercion: bool = True,
predicate_pushdown: bool = True,
projection_pushdown: bool = True,
simplify_expression: bool = True,
slice_pushdown: bool = True,
no_optimization: bool = False,
) DataFrame[source]

Evaluate the query in streaming mode and write to an NDJSON file.

Warning

Streaming mode is considered unstable. It may be changed at any point without it being considered a breaking change.

This allows streaming results that are larger than RAM to be written to disk.

Parameters:
path

File path to which the file should be written.

maintain_order

Maintain the order in which data is processed. Setting this to False will be slightly faster.

type_coercion

Do type coercion optimization.

predicate_pushdown

Do predicate pushdown optimization.

projection_pushdown

Do projection pushdown optimization.

simplify_expression

Run simplify expressions optimization.

slice_pushdown

Slice pushdown optimization.

no_optimization

Turn off (certain) optimizations.

Returns:
DataFrame

Examples

>>> lf = pl.scan_csv("/path/to/my_larger_than_ram_file.csv")  
>>> lf.sink_ndjson("out.ndjson")  
sink_parquet(
path: str | Path,
*,
compression: str = 'zstd',
compression_level: int | None = None,
statistics: bool | str | dict[str, bool] = True,
row_group_size: int | None = None,
data_pagesize_limit: int | None = None,
maintain_order: bool = True,
type_coercion: bool = True,
predicate_pushdown: bool = True,
projection_pushdown: bool = True,
simplify_expression: bool = True,
slice_pushdown: bool = True,
no_optimization: bool = False,
) None[source]

Evaluate the query in streaming mode and write to a Parquet file.

Warning

Streaming mode is considered unstable. It may be changed at any point without it being considered a breaking change.

This allows streaming results that are larger than RAM to be written to disk.

Parameters:
path

File path to which the file should be written.

compression{‘lz4’, ‘uncompressed’, ‘snappy’, ‘gzip’, ‘lzo’, ‘brotli’, ‘zstd’}

Choose “zstd” for good compression performance. Choose “lz4” for fast compression/decompression. Choose “snappy” for more backwards compatibility guarantees when you deal with older parquet readers.

compression_level

The level of compression to use. Higher compression means smaller files on disk.

  • “gzip” : min-level: 0, max-level: 10.

  • “brotli” : min-level: 0, max-level: 11.

  • “zstd” : min-level: 1, max-level: 22.

statistics

Write statistics to the parquet headers. This is the default behavior.

Possible values:

  • True: enable default set of statistics (default)

  • False: disable all statistics

  • “full”: calculate and write all available statistics. Cannot be combined with use_pyarrow.

  • { "statistic-key": True / False, ... }. Cannot be combined with use_pyarrow. Available keys:

    • “min”: column minimum value (default: True)

    • “max”: column maximum value (default: True)

    • “distinct_count”: number of unique column values (default: False)

    • “null_count”: number of null values in column (default: True)

row_group_size

Size of the row groups in number of rows. If None (default), the chunks of the DataFrame are used. Writing in smaller chunks may reduce memory pressure and improve writing speeds.

data_pagesize_limit

Size limit of individual data pages. If not set defaults to 1024 * 1024 bytes

maintain_order

Maintain the order in which data is processed. Setting this to False will be slightly faster.

type_coercion

Do type coercion optimization.

predicate_pushdown

Do predicate pushdown optimization.

projection_pushdown

Do projection pushdown optimization.

simplify_expression

Run simplify expressions optimization.

slice_pushdown

Slice pushdown optimization.

no_optimization

Turn off (certain) optimizations.

Returns:
DataFrame

Examples

>>> lf = pl.scan_csv("/path/to/my_larger_than_ram_file.csv")  
>>> lf.sink_parquet("out.parquet")  
slice(
offset: int,
length: int | None = None,
) LazyFrame[source]

Get a slice of this DataFrame.

Parameters:
offset

Start index. Negative indexing is supported.

length

Length of the slice. If set to None, all rows starting at the offset will be selected.

Examples

>>> lf = pl.LazyFrame(
...     {
...         "a": ["x", "y", "z"],
...         "b": [1, 3, 5],
...         "c": [2, 4, 6],
...     }
... )
>>> lf.slice(1, 2).collect()
shape: (2, 3)
┌─────┬─────┬─────┐
│ a   ┆ b   ┆ c   │
│ --- ┆ --- ┆ --- │
│ str ┆ i64 ┆ i64 │
╞═════╪═════╪═════╡
│ y   ┆ 3   ┆ 4   │
│ z   ┆ 5   ┆ 6   │
└─────┴─────┴─────┘
sort(
by: IntoExpr | Iterable[IntoExpr],
*more_by: IntoExpr,
descending: bool | Sequence[bool] = False,
nulls_last: bool | Sequence[bool] = False,
maintain_order: bool = False,
multithreaded: bool = True,
) LazyFrame[source]

Sort the LazyFrame by the given columns.

Parameters:
by

Column(s) to sort by. Accepts expression input. Strings are parsed as column names.

*more_by

Additional columns to sort by, specified as positional arguments.

descending

Sort in descending order. When sorting by multiple columns, can be specified per column by passing a sequence of booleans.

nulls_last

Place null values last; can specify a single boolean applying to all columns or a sequence of booleans for per-column control.

maintain_order

Whether the order should be maintained if elements are equal. Note that if true streaming is not possible and performance might be worse since this requires a stable search.

multithreaded

Sort using multiple threads.

Examples

Pass a single column name to sort by that column.

>>> lf = pl.LazyFrame(
...     {
...         "a": [1, 2, None],
...         "b": [6.0, 5.0, 4.0],
...         "c": ["a", "c", "b"],
...     }
... )
>>> lf.sort("a").collect()
shape: (3, 3)
┌──────┬─────┬─────┐
│ a    ┆ b   ┆ c   │
│ ---  ┆ --- ┆ --- │
│ i64  ┆ f64 ┆ str │
╞══════╪═════╪═════╡
│ null ┆ 4.0 ┆ b   │
│ 1    ┆ 6.0 ┆ a   │
│ 2    ┆ 5.0 ┆ c   │
└──────┴─────┴─────┘

Sorting by expressions is also supported.

>>> lf.sort(pl.col("a") + pl.col("b") * 2, nulls_last=True).collect()
shape: (3, 3)
┌──────┬─────┬─────┐
│ a    ┆ b   ┆ c   │
│ ---  ┆ --- ┆ --- │
│ i64  ┆ f64 ┆ str │
╞══════╪═════╪═════╡
│ 2    ┆ 5.0 ┆ c   │
│ 1    ┆ 6.0 ┆ a   │
│ null ┆ 4.0 ┆ b   │
└──────┴─────┴─────┘

Sort by multiple columns by passing a list of columns.

>>> lf.sort(["c", "a"], descending=True).collect()
shape: (3, 3)
┌──────┬─────┬─────┐
│ a    ┆ b   ┆ c   │
│ ---  ┆ --- ┆ --- │
│ i64  ┆ f64 ┆ str │
╞══════╪═════╪═════╡
│ 2    ┆ 5.0 ┆ c   │
│ null ┆ 4.0 ┆ b   │
│ 1    ┆ 6.0 ┆ a   │
└──────┴─────┴─────┘

Or use positional arguments to sort by multiple columns in the same way.

>>> lf.sort("c", "a", descending=[False, True]).collect()
shape: (3, 3)
┌──────┬─────┬─────┐
│ a    ┆ b   ┆ c   │
│ ---  ┆ --- ┆ --- │
│ i64  ┆ f64 ┆ str │
╞══════╪═════╪═════╡
│ 1    ┆ 6.0 ┆ a   │
│ null ┆ 4.0 ┆ b   │
│ 2    ┆ 5.0 ┆ c   │
└──────┴─────┴─────┘
sql(
query: str,
*,
table_name: str = 'self',
) LazyFrame[source]

Execute a SQL query against the LazyFrame.

Added in version 0.20.23.

Warning

This functionality is considered unstable, although it is close to being considered stable. It may be changed at any point without it being considered a breaking change.

Parameters:
query

SQL query to execute.

table_name

Optionally provide an explicit name for the table that represents the calling frame (defaults to “self”).

See also

SQLContext

Notes

  • The calling frame is automatically registered as a table in the SQL context under the name “self”. If you want access to the DataFrames and LazyFrames found in the current globals, use the top-level pl.sql.

  • More control over registration and execution behaviour is available by using the SQLContext object.

Examples

>>> lf1 = pl.LazyFrame({"a": [1, 2, 3], "b": [6, 7, 8], "c": ["z", "y", "x"]})
>>> lf2 = pl.LazyFrame({"a": [3, 2, 1], "d": [125, -654, 888]})

Query the LazyFrame using SQL:

>>> lf1.sql("SELECT c, b FROM self WHERE a > 1").collect()
shape: (2, 2)
┌─────┬─────┐
│ c   ┆ b   │
│ --- ┆ --- │
│ str ┆ i64 │
╞═════╪═════╡
│ y   ┆ 7   │
│ x   ┆ 8   │
└─────┴─────┘

Apply SQL transforms (aliasing “self” to “frame”) then filter natively (you can freely mix SQL and native operations):

>>> lf1.sql(
...     query='''
...         SELECT
...             a,
...             (a % 2 == 0) AS a_is_even,
...             (b::float4 / 2) AS "b/2",
...             CONCAT_WS(':', c, c, c) AS c_c_c
...         FROM frame
...         ORDER BY a
...     ''',
...     table_name="frame",
... ).filter(~pl.col("c_c_c").str.starts_with("x")).collect()
shape: (2, 4)
┌─────┬───────────┬─────┬───────┐
│ a   ┆ a_is_even ┆ b/2 ┆ c_c_c │
│ --- ┆ ---       ┆ --- ┆ ---   │
│ i64 ┆ bool      ┆ f32 ┆ str   │
╞═════╪═══════════╪═════╪═══════╡
│ 1   ┆ false     ┆ 3.0 ┆ z:z:z │
│ 2   ┆ true      ┆ 3.5 ┆ y:y:y │
└─────┴───────────┴─────┴───────┘
std(ddof: int = 1) LazyFrame[source]

Aggregate the columns in the LazyFrame to their standard deviation value.

Parameters:
ddof

“Delta Degrees of Freedom”: the divisor used in the calculation is N - ddof, where N represents the number of elements. By default ddof is 1.

Examples

>>> lf = pl.LazyFrame(
...     {
...         "a": [1, 2, 3, 4],
...         "b": [1, 2, 1, 1],
...     }
... )
>>> lf.std().collect()
shape: (1, 2)
┌──────────┬─────┐
│ a        ┆ b   │
│ ---      ┆ --- │
│ f64      ┆ f64 │
╞══════════╪═════╡
│ 1.290994 ┆ 0.5 │
└──────────┴─────┘
>>> lf.std(ddof=0).collect()
shape: (1, 2)
┌──────────┬──────────┐
│ a        ┆ b        │
│ ---      ┆ ---      │
│ f64      ┆ f64      │
╞══════════╪══════════╡
│ 1.118034 ┆ 0.433013 │
└──────────┴──────────┘
sum() LazyFrame[source]

Aggregate the columns in the LazyFrame to their sum value.

Examples

>>> lf = pl.LazyFrame(
...     {
...         "a": [1, 2, 3, 4],
...         "b": [1, 2, 1, 1],
...     }
... )
>>> lf.sum().collect()
shape: (1, 2)
┌─────┬─────┐
│ a   ┆ b   │
│ --- ┆ --- │
│ i64 ┆ i64 │
╞═════╪═════╡
│ 10  ┆ 5   │
└─────┴─────┘
tail(n: int = 5) LazyFrame[source]

Get the last n rows.

Parameters:
n

Number of rows to return.

Examples

>>> lf = pl.LazyFrame(
...     {
...         "a": [1, 2, 3, 4, 5, 6],
...         "b": [7, 8, 9, 10, 11, 12],
...     }
... )
>>> lf.tail().collect()
shape: (5, 2)
┌─────┬─────┐
│ a   ┆ b   │
│ --- ┆ --- │
│ i64 ┆ i64 │
╞═════╪═════╡
│ 2   ┆ 8   │
│ 3   ┆ 9   │
│ 4   ┆ 10  │
│ 5   ┆ 11  │
│ 6   ┆ 12  │
└─────┴─────┘
>>> lf.tail(2).collect()
shape: (2, 2)
┌─────┬─────┐
│ a   ┆ b   │
│ --- ┆ --- │
│ i64 ┆ i64 │
╞═════╪═════╡
│ 5   ┆ 11  │
│ 6   ┆ 12  │
└─────┴─────┘
top_k(
k: int,
*,
by: IntoExpr | Iterable[IntoExpr],
reverse: bool | Sequence[bool] = False,
) LazyFrame[source]

Return the k largest rows.

Non-null elements are always preferred over null elements, regardless of the value of reverse. The output is not guaranteed to be in any particular order, call sort() after this function if you wish the output to be sorted.

Parameters:
k

Number of rows to return.

by

Column(s) used to determine the top rows. Accepts expression input. Strings are parsed as column names.

reverse

Consider the k smallest elements of the by column(s) (instead of the k largest). This can be specified per column by passing a sequence of booleans.

See also

bottom_k

Examples

>>> lf = pl.LazyFrame(
...     {
...         "a": ["a", "b", "a", "b", "b", "c"],
...         "b": [2, 1, 1, 3, 2, 1],
...     }
... )

Get the rows which contain the 4 largest values in column b.

>>> lf.top_k(4, by="b").collect()
shape: (4, 2)
┌─────┬─────┐
│ a   ┆ b   │
│ --- ┆ --- │
│ str ┆ i64 │
╞═════╪═════╡
│ b   ┆ 3   │
│ a   ┆ 2   │
│ b   ┆ 2   │
│ b   ┆ 1   │
└─────┴─────┘

Get the rows which contain the 4 largest values when sorting on column b and a.

>>> lf.top_k(4, by=["b", "a"]).collect()
shape: (4, 2)
┌─────┬─────┐
│ a   ┆ b   │
│ --- ┆ --- │
│ str ┆ i64 │
╞═════╪═════╡
│ b   ┆ 3   │
│ b   ┆ 2   │
│ a   ┆ 2   │
│ c   ┆ 1   │
└─────┴─────┘
unique(
subset: ColumnNameOrSelector | Collection[ColumnNameOrSelector] | None = None,
*,
keep: UniqueKeepStrategy = 'any',
maintain_order: bool = False,
) LazyFrame[source]

Drop duplicate rows from this DataFrame.

Parameters:
subset

Column name(s) or selector(s), to consider when identifying duplicate rows. If set to None (default), use all columns.

keep{‘first’, ‘last’, ‘any’, ‘none’}

Which of the duplicate rows to keep.

  • ‘any’: Does not give any guarantee of which row is kept.

    This allows more optimizations.

  • ‘none’: Don’t keep duplicate rows.

  • ‘first’: Keep first unique row.

  • ‘last’: Keep last unique row.

maintain_order

Keep the same order as the original DataFrame. This is more expensive to compute. Settings this to True blocks the possibility to run on the streaming engine.

Returns:
LazyFrame

LazyFrame with unique rows.

Warning

This method will fail if there is a column of type List in the DataFrame or subset.

Notes

If you’re coming from pandas, this is similar to pandas.DataFrame.drop_duplicates.

Examples

>>> lf = pl.LazyFrame(
...     {
...         "foo": [1, 2, 3, 1],
...         "bar": ["a", "a", "a", "a"],
...         "ham": ["b", "b", "b", "b"],
...     }
... )
>>> lf.unique(maintain_order=True).collect()
shape: (3, 3)
┌─────┬─────┬─────┐
│ foo ┆ bar ┆ ham │
│ --- ┆ --- ┆ --- │
│ i64 ┆ str ┆ str │
╞═════╪═════╪═════╡
│ 1   ┆ a   ┆ b   │
│ 2   ┆ a   ┆ b   │
│ 3   ┆ a   ┆ b   │
└─────┴─────┴─────┘
>>> lf.unique(subset=["bar", "ham"], maintain_order=True).collect()
shape: (1, 3)
┌─────┬─────┬─────┐
│ foo ┆ bar ┆ ham │
│ --- ┆ --- ┆ --- │
│ i64 ┆ str ┆ str │
╞═════╪═════╪═════╡
│ 1   ┆ a   ┆ b   │
└─────┴─────┴─────┘
>>> lf.unique(keep="last", maintain_order=True).collect()
shape: (3, 3)
┌─────┬─────┬─────┐
│ foo ┆ bar ┆ ham │
│ --- ┆ --- ┆ --- │
│ i64 ┆ str ┆ str │
╞═════╪═════╪═════╡
│ 2   ┆ a   ┆ b   │
│ 3   ┆ a   ┆ b   │
│ 1   ┆ a   ┆ b   │
└─────┴─────┴─────┘
unnest(
columns: ColumnNameOrSelector | Collection[ColumnNameOrSelector],
*more_columns: ColumnNameOrSelector,
) LazyFrame[source]

Decompose struct columns into separate columns for each of their fields.

The new columns will be inserted into the DataFrame at the location of the struct column.

Parameters:
columns

Name of the struct column(s) that should be unnested.

*more_columns

Additional columns to unnest, specified as positional arguments.

Examples

>>> df = pl.LazyFrame(
...     {
...         "before": ["foo", "bar"],
...         "t_a": [1, 2],
...         "t_b": ["a", "b"],
...         "t_c": [True, None],
...         "t_d": [[1, 2], [3]],
...         "after": ["baz", "womp"],
...     }
... ).select("before", pl.struct(pl.col("^t_.$")).alias("t_struct"), "after")
>>> df.collect()
shape: (2, 3)
┌────────┬─────────────────────┬───────┐
│ before ┆ t_struct            ┆ after │
│ ---    ┆ ---                 ┆ ---   │
│ str    ┆ struct[4]           ┆ str   │
╞════════╪═════════════════════╪═══════╡
│ foo    ┆ {1,"a",true,[1, 2]} ┆ baz   │
│ bar    ┆ {2,"b",null,[3]}    ┆ womp  │
└────────┴─────────────────────┴───────┘
>>> df.unnest("t_struct").collect()
shape: (2, 6)
┌────────┬─────┬─────┬──────┬───────────┬───────┐
│ before ┆ t_a ┆ t_b ┆ t_c  ┆ t_d       ┆ after │
│ ---    ┆ --- ┆ --- ┆ ---  ┆ ---       ┆ ---   │
│ str    ┆ i64 ┆ str ┆ bool ┆ list[i64] ┆ str   │
╞════════╪═════╪═════╪══════╪═══════════╪═══════╡
│ foo    ┆ 1   ┆ a   ┆ true ┆ [1, 2]    ┆ baz   │
│ bar    ┆ 2   ┆ b   ┆ null ┆ [3]       ┆ womp  │
└────────┴─────┴─────┴──────┴───────────┴───────┘
unpivot(
on: ColumnNameOrSelector | Sequence[ColumnNameOrSelector] | None = None,
*,
index: ColumnNameOrSelector | Sequence[ColumnNameOrSelector] | None = None,
variable_name: str | None = None,
value_name: str | None = None,
streamable: bool = True,
) LazyFrame[source]

Unpivot a DataFrame from wide to long format.

Optionally leaves identifiers set.

This function is useful to massage a DataFrame into a format where one or more columns are identifier variables (index) while all other columns, considered measured variables (on), are “unpivoted” to the row axis leaving just two non-identifier columns, ‘variable’ and ‘value’.

Parameters:
on

Column(s) or selector(s) to use as values variables; if on is empty all columns that are not in index will be used.

index

Column(s) or selector(s) to use as identifier variables.

variable_name

Name to give to the variable column. Defaults to “variable”

value_name

Name to give to the value column. Defaults to “value”

streamable

Allow this node to run in the streaming engine. If this runs in streaming, the output of the unpivot operation will not have a stable ordering.

Notes

If you’re coming from pandas, this is similar to pandas.DataFrame.melt, but with index replacing id_vars and on replacing value_vars. In other frameworks, you might know this operation as pivot_longer.

Examples

>>> lf = pl.LazyFrame(
...     {
...         "a": ["x", "y", "z"],
...         "b": [1, 3, 5],
...         "c": [2, 4, 6],
...     }
... )
>>> import polars.selectors as cs
>>> lf.unpivot(cs.numeric(), index="a").collect()
shape: (6, 3)
┌─────┬──────────┬───────┐
│ a   ┆ variable ┆ value │
│ --- ┆ ---      ┆ ---   │
│ str ┆ str      ┆ i64   │
╞═════╪══════════╪═══════╡
│ x   ┆ b        ┆ 1     │
│ y   ┆ b        ┆ 3     │
│ z   ┆ b        ┆ 5     │
│ x   ┆ c        ┆ 2     │
│ y   ┆ c        ┆ 4     │
│ z   ┆ c        ┆ 6     │
└─────┴──────────┴───────┘
update(
other: LazyFrame,
on: str | Sequence[str] | None = None,
how: Literal['left', 'inner', 'full'] = 'left',
*,
left_on: str | Sequence[str] | None = None,
right_on: str | Sequence[str] | None = None,
include_nulls: bool = False,
) LazyFrame[source]

Update the values in this LazyFrame with the values in other.

Warning

This functionality is considered unstable. It may be changed at any point without it being considered a breaking change.

Parameters:
other

LazyFrame that will be used to update the values

on

Column names that will be joined on. If set to None (default), the implicit row index of each frame is used as a join key.

how{‘left’, ‘inner’, ‘full’}
  • ‘left’ will keep all rows from the left table; rows may be duplicated if multiple rows in the right frame match the left row’s key.

  • ‘inner’ keeps only those rows where the key exists in both frames.

  • ‘full’ will update existing rows where the key matches while also adding any new rows contained in the given frame.

left_on

Join column(s) of the left DataFrame.

right_on

Join column(s) of the right DataFrame.

include_nulls

Overwrite values in the left frame with null values from the right frame. If set to False (default), null values in the right frame are ignored.

Notes

This is syntactic sugar for a left/inner join, with an optional coalesce when include_nulls = False.

Examples

>>> lf = pl.LazyFrame(
...     {
...         "A": [1, 2, 3, 4],
...         "B": [400, 500, 600, 700],
...     }
... )
>>> lf.collect()
shape: (4, 2)
┌─────┬─────┐
│ A   ┆ B   │
│ --- ┆ --- │
│ i64 ┆ i64 │
╞═════╪═════╡
│ 1   ┆ 400 │
│ 2   ┆ 500 │
│ 3   ┆ 600 │
│ 4   ┆ 700 │
└─────┴─────┘
>>> new_lf = pl.LazyFrame(
...     {
...         "B": [-66, None, -99],
...         "C": [5, 3, 1],
...     }
... )

Update df values with the non-null values in new_df, by row index:

>>> lf.update(new_lf).collect()
shape: (4, 2)
┌─────┬─────┐
│ A   ┆ B   │
│ --- ┆ --- │
│ i64 ┆ i64 │
╞═════╪═════╡
│ 1   ┆ -66 │
│ 2   ┆ 500 │
│ 3   ┆ -99 │
│ 4   ┆ 700 │
└─────┴─────┘

Update df values with the non-null values in new_df, by row index, but only keeping those rows that are common to both frames:

>>> lf.update(new_lf, how="inner").collect()
shape: (3, 2)
┌─────┬─────┐
│ A   ┆ B   │
│ --- ┆ --- │
│ i64 ┆ i64 │
╞═════╪═════╡
│ 1   ┆ -66 │
│ 2   ┆ 500 │
│ 3   ┆ -99 │
└─────┴─────┘

Update df values with the non-null values in new_df, using a full outer join strategy that defines explicit join columns in each frame:

>>> lf.update(new_lf, left_on=["A"], right_on=["C"], how="full").collect()
shape: (5, 2)
┌─────┬─────┐
│ A   ┆ B   │
│ --- ┆ --- │
│ i64 ┆ i64 │
╞═════╪═════╡
│ 1   ┆ -99 │
│ 2   ┆ 500 │
│ 3   ┆ 600 │
│ 4   ┆ 700 │
│ 5   ┆ -66 │
└─────┴─────┘

Update df values including null values in new_df, using a full outer join strategy that defines explicit join columns in each frame:

>>> lf.update(
...     new_lf, left_on="A", right_on="C", how="full", include_nulls=True
... ).collect()
shape: (5, 2)
┌─────┬──────┐
│ A   ┆ B    │
│ --- ┆ ---  │
│ i64 ┆ i64  │
╞═════╪══════╡
│ 1   ┆ -99  │
│ 2   ┆ 500  │
│ 3   ┆ null │
│ 4   ┆ 700  │
│ 5   ┆ -66  │
└─────┴──────┘
var(ddof: int = 1) LazyFrame[source]

Aggregate the columns in the LazyFrame to their variance value.

Parameters:
ddof

“Delta Degrees of Freedom”: the divisor used in the calculation is N - ddof, where N represents the number of elements. By default ddof is 1.

Examples

>>> lf = pl.LazyFrame(
...     {
...         "a": [1, 2, 3, 4],
...         "b": [1, 2, 1, 1],
...     }
... )
>>> lf.var().collect()
shape: (1, 2)
┌──────────┬──────┐
│ a        ┆ b    │
│ ---      ┆ ---  │
│ f64      ┆ f64  │
╞══════════╪══════╡
│ 1.666667 ┆ 0.25 │
└──────────┴──────┘
>>> lf.var(ddof=0).collect()
shape: (1, 2)
┌──────┬────────┐
│ a    ┆ b      │
│ ---  ┆ ---    │
│ f64  ┆ f64    │
╞══════╪════════╡
│ 1.25 ┆ 0.1875 │
└──────┴────────┘
property width: int[source]

Get the number of columns.

Returns:
int

Warning

Determining the width of a LazyFrame requires resolving its schema, which is a potentially expensive operation. Using collect_schema() is the idiomatic way to resolve the schema. This property exists only for symmetry with the DataFrame class.

See also

collect_schema
Schema.len

Examples

>>> lf = pl.LazyFrame(
...     {
...         "foo": [1, 2, 3],
...         "bar": [4, 5, 6],
...     }
... )
>>> lf.width  
2
with_columns(
*exprs: IntoExpr | Iterable[IntoExpr],
**named_exprs: IntoExpr,
) LazyFrame[source]

Add columns to this LazyFrame.

Added columns will replace existing columns with the same name.

Parameters:
*exprs

Column(s) to add, specified as positional arguments. Accepts expression input. Strings are parsed as column names, other non-expression inputs are parsed as literals.

**named_exprs

Additional columns to add, specified as keyword arguments. The columns will be renamed to the keyword used.

Returns:
LazyFrame

A new LazyFrame with the columns added.

Notes

Creating a new LazyFrame using this method does not create a new copy of existing data.

Examples

Pass an expression to add it as a new column.

>>> lf = pl.LazyFrame(
...     {
...         "a": [1, 2, 3, 4],
...         "b": [0.5, 4, 10, 13],
...         "c": [True, True, False, True],
...     }
... )
>>> lf.with_columns((pl.col("a") ** 2).alias("a^2")).collect()
shape: (4, 4)
┌─────┬──────┬───────┬─────┐
│ a   ┆ b    ┆ c     ┆ a^2 │
│ --- ┆ ---  ┆ ---   ┆ --- │
│ i64 ┆ f64  ┆ bool  ┆ i64 │
╞═════╪══════╪═══════╪═════╡
│ 1   ┆ 0.5  ┆ true  ┆ 1   │
│ 2   ┆ 4.0  ┆ true  ┆ 4   │
│ 3   ┆ 10.0 ┆ false ┆ 9   │
│ 4   ┆ 13.0 ┆ true  ┆ 16  │
└─────┴──────┴───────┴─────┘

Added columns will replace existing columns with the same name.

>>> lf.with_columns(pl.col("a").cast(pl.Float64)).collect()
shape: (4, 3)
┌─────┬──────┬───────┐
│ a   ┆ b    ┆ c     │
│ --- ┆ ---  ┆ ---   │
│ f64 ┆ f64  ┆ bool  │
╞═════╪══════╪═══════╡
│ 1.0 ┆ 0.5  ┆ true  │
│ 2.0 ┆ 4.0  ┆ true  │
│ 3.0 ┆ 10.0 ┆ false │
│ 4.0 ┆ 13.0 ┆ true  │
└─────┴──────┴───────┘

Multiple columns can be added using positional arguments.

>>> lf.with_columns(
...     (pl.col("a") ** 2).alias("a^2"),
...     (pl.col("b") / 2).alias("b/2"),
...     (pl.col("c").not_()).alias("not c"),
... ).collect()
shape: (4, 6)
┌─────┬──────┬───────┬─────┬──────┬───────┐
│ a   ┆ b    ┆ c     ┆ a^2 ┆ b/2  ┆ not c │
│ --- ┆ ---  ┆ ---   ┆ --- ┆ ---  ┆ ---   │
│ i64 ┆ f64  ┆ bool  ┆ i64 ┆ f64  ┆ bool  │
╞═════╪══════╪═══════╪═════╪══════╪═══════╡
│ 1   ┆ 0.5  ┆ true  ┆ 1   ┆ 0.25 ┆ false │
│ 2   ┆ 4.0  ┆ true  ┆ 4   ┆ 2.0  ┆ false │
│ 3   ┆ 10.0 ┆ false ┆ 9   ┆ 5.0  ┆ true  │
│ 4   ┆ 13.0 ┆ true  ┆ 16  ┆ 6.5  ┆ false │
└─────┴──────┴───────┴─────┴──────┴───────┘

Multiple columns can also be added by passing a list of expressions.

>>> lf.with_columns(
...     [
...         (pl.col("a") ** 2).alias("a^2"),
...         (pl.col("b") / 2).alias("b/2"),
...         (pl.col("c").not_()).alias("not c"),
...     ]
... ).collect()
shape: (4, 6)
┌─────┬──────┬───────┬─────┬──────┬───────┐
│ a   ┆ b    ┆ c     ┆ a^2 ┆ b/2  ┆ not c │
│ --- ┆ ---  ┆ ---   ┆ --- ┆ ---  ┆ ---   │
│ i64 ┆ f64  ┆ bool  ┆ i64 ┆ f64  ┆ bool  │
╞═════╪══════╪═══════╪═════╪══════╪═══════╡
│ 1   ┆ 0.5  ┆ true  ┆ 1   ┆ 0.25 ┆ false │
│ 2   ┆ 4.0  ┆ true  ┆ 4   ┆ 2.0  ┆ false │
│ 3   ┆ 10.0 ┆ false ┆ 9   ┆ 5.0  ┆ true  │
│ 4   ┆ 13.0 ┆ true  ┆ 16  ┆ 6.5  ┆ false │
└─────┴──────┴───────┴─────┴──────┴───────┘

Use keyword arguments to easily name your expression inputs.

>>> lf.with_columns(
...     ab=pl.col("a") * pl.col("b"),
...     not_c=pl.col("c").not_(),
... ).collect()
shape: (4, 5)
┌─────┬──────┬───────┬──────┬───────┐
│ a   ┆ b    ┆ c     ┆ ab   ┆ not_c │
│ --- ┆ ---  ┆ ---   ┆ ---  ┆ ---   │
│ i64 ┆ f64  ┆ bool  ┆ f64  ┆ bool  │
╞═════╪══════╪═══════╪══════╪═══════╡
│ 1   ┆ 0.5  ┆ true  ┆ 0.5  ┆ false │
│ 2   ┆ 4.0  ┆ true  ┆ 8.0  ┆ false │
│ 3   ┆ 10.0 ┆ false ┆ 30.0 ┆ true  │
│ 4   ┆ 13.0 ┆ true  ┆ 52.0 ┆ false │
└─────┴──────┴───────┴──────┴───────┘

Expressions with multiple outputs can automatically be instantiated as Structs by enabling the experimental setting Config.set_auto_structify(True):

>>> with pl.Config(auto_structify=True):
...     lf.drop("c").with_columns(
...         diffs=pl.col(["a", "b"]).diff().name.suffix("_diff"),
...     ).collect()
shape: (4, 3)
┌─────┬──────┬─────────────┐
│ a   ┆ b    ┆ diffs       │
│ --- ┆ ---  ┆ ---         │
│ i64 ┆ f64  ┆ struct[2]   │
╞═════╪══════╪═════════════╡
│ 1   ┆ 0.5  ┆ {null,null} │
│ 2   ┆ 4.0  ┆ {1,3.5}     │
│ 3   ┆ 10.0 ┆ {1,6.0}     │
│ 4   ┆ 13.0 ┆ {1,3.0}     │
└─────┴──────┴─────────────┘
with_columns_seq(
*exprs: IntoExpr | Iterable[IntoExpr],
**named_exprs: IntoExpr,
) LazyFrame[source]

Add columns to this LazyFrame.

Added columns will replace existing columns with the same name.

This will run all expression sequentially instead of in parallel. Use this when the work per expression is cheap.

Parameters:
*exprs

Column(s) to add, specified as positional arguments. Accepts expression input. Strings are parsed as column names, other non-expression inputs are parsed as literals.

**named_exprs

Additional columns to add, specified as keyword arguments. The columns will be renamed to the keyword used.

Returns:
LazyFrame

A new LazyFrame with the columns added.

See also

with_columns
with_context(other: Self | list[Self]) LazyFrame[source]

Add an external context to the computation graph.

Deprecated since version 1.0.0: Use concat() instead with how='horizontal'

This allows expressions to also access columns from DataFrames that are not part of this one.

Parameters:
other

Lazy DataFrame to join with.

Examples

>>> lf = pl.LazyFrame({"a": [1, 2, 3], "b": ["a", "c", None]})
>>> lf_other = pl.LazyFrame({"c": ["foo", "ham"]})
>>> lf.with_context(lf_other).select(  
...     pl.col("b") + pl.col("c").first()
... ).collect()
shape: (3, 1)
┌──────┐
│ b    │
│ ---  │
│ str  │
╞══════╡
│ afoo │
│ cfoo │
│ null │
└──────┘

Fill nulls with the median from another DataFrame:

>>> train_lf = pl.LazyFrame(
...     {"feature_0": [-1.0, 0, 1], "feature_1": [-1.0, 0, 1]}
... )
>>> test_lf = pl.LazyFrame(
...     {"feature_0": [-1.0, None, 1], "feature_1": [-1.0, 0, 1]}
... )
>>> test_lf.with_context(  
...     train_lf.select(pl.all().name.suffix("_train"))
... ).select(
...     pl.col("feature_0").fill_null(pl.col("feature_0_train").median())
... ).collect()
shape: (3, 1)
┌───────────┐
│ feature_0 │
│ ---       │
│ f64       │
╞═══════════╡
│ -1.0      │
│ 0.0       │
│ 1.0       │
└───────────┘
with_row_count(
name: str = 'row_nr',
offset: int = 0,
) LazyFrame[source]

Add a column at index 0 that counts the rows.

Deprecated since version 0.20.4: Use with_row_index() instead. Note that the default column name has changed from ‘row_nr’ to ‘index’.

Parameters:
name

Name of the column to add.

offset

Start the row count at this offset.

Warning

This can have a negative effect on query performance. This may, for instance, block predicate pushdown optimization.

Examples

>>> lf = pl.LazyFrame(
...     {
...         "a": [1, 3, 5],
...         "b": [2, 4, 6],
...     }
... )
>>> lf.with_row_count().collect()  
shape: (3, 3)
┌────────┬─────┬─────┐
│ row_nr ┆ a   ┆ b   │
│ ---    ┆ --- ┆ --- │
│ u32    ┆ i64 ┆ i64 │
╞════════╪═════╪═════╡
│ 0      ┆ 1   ┆ 2   │
│ 1      ┆ 3   ┆ 4   │
│ 2      ┆ 5   ┆ 6   │
└────────┴─────┴─────┘
with_row_index(
name: str = 'index',
offset: int = 0,
) LazyFrame[source]

Add a row index as the first column in the LazyFrame.

Parameters:
name

Name of the index column.

offset

Start the index at this offset. Cannot be negative.

Warning

Using this function can have a negative effect on query performance. This may, for instance, block predicate pushdown optimization.

Notes

The resulting column does not have any special properties. It is a regular column of type UInt32 (or UInt64 in polars-u64-idx).

Examples

>>> lf = pl.LazyFrame(
...     {
...         "a": [1, 3, 5],
...         "b": [2, 4, 6],
...     }
... )
>>> lf.with_row_index().collect()
shape: (3, 3)
┌───────┬─────┬─────┐
│ index ┆ a   ┆ b   │
│ ---   ┆ --- ┆ --- │
│ u32   ┆ i64 ┆ i64 │
╞═══════╪═════╪═════╡
│ 0     ┆ 1   ┆ 2   │
│ 1     ┆ 3   ┆ 4   │
│ 2     ┆ 5   ┆ 6   │
└───────┴─────┴─────┘
>>> lf.with_row_index("id", offset=1000).collect()
shape: (3, 3)
┌──────┬─────┬─────┐
│ id   ┆ a   ┆ b   │
│ ---  ┆ --- ┆ --- │
│ u32  ┆ i64 ┆ i64 │
╞══════╪═════╪═════╡
│ 1000 ┆ 1   ┆ 2   │
│ 1001 ┆ 3   ┆ 4   │
│ 1002 ┆ 5   ┆ 6   │
└──────┴─────┴─────┘

An index column can also be created using the expressions int_range() and len().

>>> lf.select(
...     pl.int_range(pl.len(), dtype=pl.UInt32).alias("index"),
...     pl.all(),
... ).collect()
shape: (3, 3)
┌───────┬─────┬─────┐
│ index ┆ a   ┆ b   │
│ ---   ┆ --- ┆ --- │
│ u32   ┆ i64 ┆ i64 │
╞═══════╪═════╪═════╡
│ 0     ┆ 1   ┆ 2   │
│ 1     ┆ 3   ┆ 4   │
│ 2     ┆ 5   ┆ 6   │
└───────┴─────┴─────┘