polars.DataFrame.group_by#
- DataFrame.group_by(
- *by: IntoExpr | Iterable[IntoExpr],
- maintain_order: bool = False,
- **named_by: IntoExpr,
Start a group by operation.
- Parameters:
- *by
Column(s) to group by. Accepts expression input. Strings are parsed as column names.
- maintain_order
Ensure that the order of the groups is consistent with the input data. This is slower than a default group by. Settings this to
True
blocks the possibility to run on the streaming engine.Note
Within each group, the order of rows is always preserved, regardless of this argument.
- **named_by
Additional columns to group by, specified as keyword arguments. The columns will be renamed to the keyword used.
- Returns:
- GroupBy
Object which can be used to perform aggregations.
Examples
Group by one column and call
agg
to compute the grouped sum of another column.>>> df = pl.DataFrame( ... { ... "a": ["a", "b", "a", "b", "c"], ... "b": [1, 2, 1, 3, 3], ... "c": [5, 4, 3, 2, 1], ... } ... ) >>> df.group_by("a").agg(pl.col("b").sum()) shape: (3, 2) ┌─────┬─────┐ │ a ┆ b │ │ --- ┆ --- │ │ str ┆ i64 │ ╞═════╪═════╡ │ a ┆ 2 │ │ b ┆ 5 │ │ c ┆ 3 │ └─────┴─────┘
Set
maintain_order=True
to ensure the order of the groups is consistent with the input.>>> df.group_by("a", maintain_order=True).agg(pl.col("c")) shape: (3, 2) ┌─────┬───────────┐ │ a ┆ c │ │ --- ┆ --- │ │ str ┆ list[i64] │ ╞═════╪═══════════╡ │ a ┆ [5, 3] │ │ b ┆ [4, 2] │ │ c ┆ [1] │ └─────┴───────────┘
Group by multiple columns by passing a list of column names.
>>> df.group_by(["a", "b"]).agg(pl.max("c")) shape: (4, 3) ┌─────┬─────┬─────┐ │ a ┆ b ┆ c │ │ --- ┆ --- ┆ --- │ │ str ┆ i64 ┆ i64 │ ╞═════╪═════╪═════╡ │ a ┆ 1 ┆ 5 │ │ b ┆ 2 ┆ 4 │ │ b ┆ 3 ┆ 2 │ │ c ┆ 3 ┆ 1 │ └─────┴─────┴─────┘
Or use positional arguments to group by multiple columns in the same way. Expressions are also accepted.
>>> df.group_by("a", pl.col("b") // 2).agg(pl.col("c").mean()) shape: (3, 3) ┌─────┬─────┬─────┐ │ a ┆ b ┆ c │ │ --- ┆ --- ┆ --- │ │ str ┆ i64 ┆ f64 │ ╞═════╪═════╪═════╡ │ a ┆ 0 ┆ 4.0 │ │ b ┆ 1 ┆ 3.0 │ │ c ┆ 1 ┆ 1.0 │ └─────┴─────┴─────┘
The
GroupBy
object returned by this method is iterable, returning the name and data of each group.>>> for name, data in df.group_by("a"): ... print(name) ... print(data) ('a',) shape: (2, 3) ┌─────┬─────┬─────┐ │ a ┆ b ┆ c │ │ --- ┆ --- ┆ --- │ │ str ┆ i64 ┆ i64 │ ╞═════╪═════╪═════╡ │ a ┆ 1 ┆ 5 │ │ a ┆ 1 ┆ 3 │ └─────┴─────┴─────┘ ('b',) shape: (2, 3) ┌─────┬─────┬─────┐ │ a ┆ b ┆ c │ │ --- ┆ --- ┆ --- │ │ str ┆ i64 ┆ i64 │ ╞═════╪═════╪═════╡ │ b ┆ 2 ┆ 4 │ │ b ┆ 3 ┆ 2 │ └─────┴─────┴─────┘ ('c',) shape: (1, 3) ┌─────┬─────┬─────┐ │ a ┆ b ┆ c │ │ --- ┆ --- ┆ --- │ │ str ┆ i64 ┆ i64 │ ╞═════╪═════╪═════╡ │ c ┆ 3 ┆ 1 │ └─────┴─────┴─────┘