polars.LazyFrame.group_by_dynamic#

LazyFrame.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"]           │
└─────────────────┴─────────────────┴─────┴─────────────────┘