47 lines
1.7 KiB
Python
47 lines
1.7 KiB
Python
from __future__ import annotations
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from typing import TYPE_CHECKING, Generic, TypeVar
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if TYPE_CHECKING:
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from narwhals.expr import Expr
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ExprT = TypeVar("ExprT", bound="Expr")
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class ExprListNamespace(Generic[ExprT]):
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def __init__(self, expr: ExprT) -> None:
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self._expr = expr
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def len(self) -> ExprT:
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"""Return the number of elements in each list.
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Null values count towards the total.
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Returns:
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A new expression.
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Examples:
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>>> import polars as pl
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>>> import narwhals as nw
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>>> df_native = pl.DataFrame({"a": [[1, 2], [3, 4, None], None, []]})
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>>> df = nw.from_native(df_native)
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>>> df.with_columns(a_len=nw.col("a").list.len())
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┌────────────────────────┐
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| Narwhals DataFrame |
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|------------------------|
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|shape: (4, 2) |
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|┌──────────────┬───────┐|
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|│ a ┆ a_len │|
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|│ --- ┆ --- │|
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|│ list[i64] ┆ u32 │|
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|╞══════════════╪═══════╡|
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|│ [1, 2] ┆ 2 │|
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|│ [3, 4, null] ┆ 3 │|
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|│ null ┆ null │|
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|│ [] ┆ 0 │|
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|└──────────────┴───────┘|
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└────────────────────────┘
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"""
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return self._expr._with_elementwise(
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lambda plx: self._expr._to_compliant_expr(plx).list.len()
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)
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