Incorrect result with cum_sum in dynamic_group_by ternary expression #24566
Description
Checks
- I have checked that this issue has not already been reported.
- I have confirmed this bug exists on the latest version of Polars.
Reproducible example
import polars as pl
df = pl.select(pl.date_range(pl.date(2023, 1, 1), pl.date(2023, 1, 5))).with_row_index()
out = df.group_by_dynamic(
index_column="date",
period="3d",
every="1d",
).agg(
[
pl.col("index"),
pl.when(pl.col("date") >= pl.col("date"))
.then(pl.col("index").cast(pl.Int64).cum_sum())
.last()
.alias("cum_sum_last"),
],
)
print(out)Log output
shape: (5, 3)
┌────────────┬───────────┬──────────────┐
│ date ┆ index ┆ cum_sum_last │
│ --- ┆ --- ┆ --- │
│ date ┆ list[u32] ┆ i64 │
╞════════════╪═══════════╪══════════════╡
│ 2023-01-01 ┆ [0, 1, 2] ┆ 3 │
│ 2023-01-02 ┆ [1, 2, 3] ┆ 1 │
│ 2023-01-03 ┆ [2, 3, 4] ┆ 3 │
│ 2023-01-04 ┆ [3, 4] ┆ 3 │
│ 2023-01-05 ┆ [4] ┆ 3 │
└────────────┴───────────┴──────────────┘Issue description
The cum_sum_last result is incorrect. Internal note: the issue is related to how the groups are not updated correctly throughout the evaluation of the expression (see unroll() path).
Expected behavior
Expected result:
shape: (5, 3)
┌────────────┬───────────┬──────────────┐
│ date ┆ index ┆ cum_sum_last │
│ --- ┆ --- ┆ --- │
│ date ┆ list[u32] ┆ i64 │
╞════════════╪═══════════╪══════════════╡
│ 2023-01-01 ┆ [0, 1, 2] ┆ 3 │
│ 2023-01-02 ┆ [1, 2, 3] ┆ 6 │
│ 2023-01-03 ┆ [2, 3, 4] ┆ 9 │
│ 2023-01-04 ┆ [3, 4] ┆ 7 │
│ 2023-01-05 ┆ [4] ┆ 4 │
└────────────┴───────────┴──────────────┘
Which is what we get with the following expression (note the change in the when predicate):
out = df.group_by_dynamic(
index_column="date",
period="3d",
every="1d",
).agg(
[
pl.col("index"),
pl.when([True])
.then(pl.col("index").cast(pl.Int64).cum_sum())
.last()
.alias("cum_sum_last"),
],
)Installed versions
--------Version info---------
Polars: 1.33.1
Index type: UInt32
Platform: Linux-6.6.87.2-microsoft-standard-WSL2-x86_64-with-glibc2.39
Python: 3.13.5 (main, Jul 18 2025, 09:47:32) [GCC 13.3.0]
LTS CPU: False