Extending the API#

Providing new functionality#

These functions allow you to register custom functionality in a dedicated namespace on the underlying Polars classes without requiring subclassing or mixins. Expr, DataFrame, LazyFrame, and Series are all supported targets.

This feature is primarily intended for use by library authors providing domain-specific capabilities which may not exist (or belong) in the core library.

Available registrations#

register_expr_namespace(name)

Decorator for registering custom functionality with a Polars Expr.

register_dataframe_namespace(name)

Decorator for registering custom functionality with a Polars DataFrame.

register_lazyframe_namespace(name)

Decorator for registering custom functionality with a Polars LazyFrame.

register_series_namespace(name)

Decorator for registering custom functionality with a polars Series.

Note

You cannot override existing Polars namespaces (such as .str or .dt), and attempting to do so will raise an AttributeError. However, you can override other custom namespaces (which will only generate a UserWarning).

Examples#

@pl.api.register_expr_namespace("greetings")
class Greetings:
    def __init__(self, expr: pl.Expr) -> None:
        self._expr = expr

    def hello(self) -> pl.Expr:
        return (pl.lit("Hello ") + self._expr).alias("hi there")

    def goodbye(self) -> pl.Expr:
        return (pl.lit("Sayōnara ") + self._expr).alias("bye")


pl.DataFrame(data=["world", "world!", "world!!"]).select(
    [
        pl.all().greetings.hello(),
        pl.all().greetings.goodbye(),
    ]
)

# shape: (3, 1)   shape: (3, 2)
# ┌──────────┐    ┌───────────────┬──────────────────┐
# │ column_0 │    │ hi there      ┆ bye              │
# │ ---      │    │ ---           ┆ ---              │
# │ str      │    │ str           ┆ str              │
# ╞══════════╡ >> ╞═══════════════╪══════════════════╡
# │ world    │    │ Hello world   ┆ Sayōnara world   │
# │ world!   │    │ Hello world!  ┆ Sayōnara world!  │
# │ world!!  │    │ Hello world!! ┆ Sayōnara world!! │
# └──────────┘    └───────────────┴──────────────────┘

@pl.api.register_dataframe_namespace("split")
class SplitFrame:
    def __init__(self, df: pl.DataFrame) -> None:
        self._df = df

    def by_alternate_rows(self) -> list[pl.DataFrame]:
        df = self._df.with_row_index(name="n")
        return [
            df.filter((pl.col("n") % 2) == 0).drop("n"),
            df.filter((pl.col("n") % 2) != 0).drop("n"),
        ]


pl.DataFrame(
    data=["aaa", "bbb", "ccc", "ddd", "eee", "fff"],
    schema=[("txt", pl.String)],
).split.by_alternate_rows()

# [┌─────┐  ┌─────┐
#  │ txt │  │ txt │
#  │ --- │  │ --- │
#  │ str │  │ str │
#  ╞═════╡  ╞═════╡
#  │ aaa │  │ bbb │
#  │ ccc │  │ ddd │
#  │ eee │  │ fff │
#  └─────┘, └─────┘]

@pl.api.register_lazyframe_namespace("types")
class DTypeOperations:
    def __init__(self, ldf: pl.LazyFrame) -> None:
        self._ldf = ldf

    def upcast_integer_types(self) -> pl.LazyFrame:
        return self._ldf.with_columns(
            pl.col(tp).cast(pl.Int64)
            for tp in (pl.Int8, pl.Int16, pl.Int32)
        )


ldf = pl.DataFrame(
    data={"a": [1, 2], "b": [3, 4], "c": [5.6, 6.7]},
    schema=[("a", pl.Int16), ("b", pl.Int32), ("c", pl.Float32)],
).lazy()

ldf.types.upcast_integer_types()

# shape: (2, 3)          shape: (2, 3)
# ┌─────┬─────┬─────┐    ┌─────┬─────┬─────┐
# │ a   ┆ b   ┆ c   │    │ a   ┆ b   ┆ c   │
# │ --- ┆ --- ┆ --- │    │ --- ┆ --- ┆ --- │
# │ i16 ┆ i32 ┆ f32 │ >> │ i64 ┆ i64 ┆ f32 │
# ╞═════╪═════╪═════╡    ╞═════╪═════╪═════╡
# │ 1   ┆ 3   ┆ 5.6 │    │ 1   ┆ 3   ┆ 5.6 │
# │ 2   ┆ 4   ┆ 6.7 │    │ 2   ┆ 4   ┆ 6.7 │
# └─────┴─────┴─────┘    └─────┴─────┴─────┘

@pl.api.register_series_namespace("math")
class MathShortcuts:
    def __init__(self, s: pl.Series) -> None:
        self._s = s

    def square(self) -> pl.Series:
        return self._s * self._s

    def cube(self) -> pl.Series:
        return self._s * self._s * self._s


s = pl.Series("n", [1, 2, 3, 4, 5])

s2 = s.math.square().rename("n2")
s3 = s.math.cube().rename("n3")

# shape: (5,)          shape: (5,)           shape: (5,)
# Series: 'n' [i64]    Series: 'n2' [i64]    Series: 'n3' [i64]
# [                    [                     [
#     1                    1                      1
#     2                    4                      8
#     3                    9                      27
#     4                    16                     64
#     5                    25                    125
# ]                    ]                    ]