narwhals.Series.list

len()

Return the number of elements in each list.

Null values count towards the total.

Returns:

Type	Description
SeriesT	A new series.

Examples:

>>> import narwhals as nw
>>> from narwhals.typing import IntoSeriesT
>>> import pandas as pd
>>> import polars as pl
>>> import pyarrow as pa
>>> data = [[1, 2], [3, 4, None], None, []]

Let's define a dataframe-agnostic function:

>>> def agnostic_list_len(s_native: IntoSeriesT) -> IntoSeriesT:
...     s = nw.from_native(s_native, series_only=True)
...     return s.list.len().to_native()

We can then pass pandas / PyArrow / Polars / any other supported library:

>>> agnostic_list_len(
...     pd.Series(data, dtype=pd.ArrowDtype(pa.list_(pa.int64())))
... )
0       2
1       3
2    <NA>
3       0
dtype: int32[pyarrow]

>>> agnostic_list_len(pl.Series(data))
shape: (4,)
Series: '' [u32]
[
   2
   3
   null
   0
]

>>> agnostic_list_len(pa.chunked_array([data]))
<pyarrow.lib.ChunkedArray object at ...>
[
  [
    2,
    3,
    null,
    0
  ]
]