polars.LazyFrame.bottom_k#

LazyFrame.bottom_k( k: int, *, by: IntoExpr | Iterable[IntoExpr], descending: bool | Sequence[bool] = False, nulls_last: bool = False, maintain_order: bool = False, ) → Self[source]#

Return the k smallest elements.

If ‘descending=True` the largest elements will be given.

Parameters:

k: Number of rows to return.
by: Column(s) included in sort order. Accepts expression input. Strings are parsed as column names.
descending: Return the ‘k’ smallest. Top-k by multiple columns can be specified per column by passing a sequence of booleans.
nulls_last: Place null values last.
maintain_order: Whether the order should be maintained if elements are equal. Note that if true streaming is not possible and performance might be worse since this requires a stable search.

See also

top_k

Examples

>>> lf = pl.LazyFrame(
...     {
...         "a": ["a", "b", "a", "b", "b", "c"],
...         "b": [2, 1, 1, 3, 2, 1],
...     }
... )

Get the rows which contain the 4 smallest values in column b.

>>> lf.bottom_k(4, by="b").collect()
shape: (4, 2)
┌─────┬─────┐
│ a   ┆ b   │
│ --- ┆ --- │
│ str ┆ i64 │
╞═════╪═════╡
│ b   ┆ 1   │
│ a   ┆ 1   │
│ c   ┆ 1   │
│ a   ┆ 2   │
└─────┴─────┘

Get the rows which contain the 4 smallest values when sorting on column a and b.

>>> lf.bottom_k(4, by=["a", "b"]).collect()
shape: (4, 2)
┌─────┬─────┐
│ a   ┆ b   │
│ --- ┆ --- │
│ str ┆ i64 │
╞═════╪═════╡
│ a   ┆ 1   │
│ a   ┆ 2   │
│ b   ┆ 1   │
│ b   ┆ 2   │
└─────┴─────┘