polars.Expr.filter

Expr.filter(predicate: Expr) → Self

Filter a single column.

Mostly useful in an aggregation context. If you want to filter on a DataFrame level, use LazyFrame.filter.

Parameters:

predicate

Boolean expression.

Examples

>>> df = pl.DataFrame(
...     {
...         "group_col": ["g1", "g1", "g2"],
...         "b": [1, 2, 3],
...     }
... )
>>> df.groupby("group_col").agg(
...     [
...         pl.col("b").filter(pl.col("b") < 2).sum().alias("lt"),
...         pl.col("b").filter(pl.col("b") >= 2).sum().alias("gte"),
...     ]
... ).sort("group_col")
shape: (2, 3)
┌───────────┬─────┬─────┐
│ group_col ┆ lt  ┆ gte │
│ ---       ┆ --- ┆ --- │
│ str       ┆ i64 ┆ i64 │
╞═══════════╪═════╪═════╡
│ g1        ┆ 1   ┆ 2   │
│ g2        ┆ 0   ┆ 3   │
└───────────┴─────┴─────┘