narwhals.Expr.list

len

len() -> ExprT

Return the number of elements in each list.

Null values count towards the total.

Returns:

Type	Description
ExprT	A new expression.

Examples:

>>> import polars as pl
>>> import narwhals as nw
>>> df_native = pl.DataFrame({"a": [[1, 2], [3, 4, None], None, []]})
>>> df = nw.from_native(df_native)
>>> df.with_columns(a_len=nw.col("a").list.len())
┌────────────────────────┐
|   Narwhals DataFrame   |
|------------------------|
|shape: (4, 2)           |
|┌──────────────┬───────┐|
|│ a            ┆ a_len │|
|│ ---          ┆ ---   │|
|│ list[i64]    ┆ u32   │|
|╞══════════════╪═══════╡|
|│ [1, 2]       ┆ 2     │|
|│ [3, 4, null] ┆ 3     │|
|│ null         ┆ null  │|
|│ []           ┆ 0     │|
|└──────────────┴───────┘|
└────────────────────────┘