Conditional Python Polars cum_sum over a group, is there a better way?

I want to create a column that is a cumulative sum over a group column but the cumulative sum only happens when the 'days' column meets a certain condition. I have come up with what I regard as a "duct tape" solution, there must be a more elegant way.

import polars as pl# Create a DataFrame with literal valuesdf = pl.DataFrame({"days": [0, 1, 2, 3, 4, 0, 1, 2, 3, 4, 6, 7, 1],"amount": [100, 200, 150, 300, 250, 180, 220, 280, 210, 320,21,456,111],"group": ["A", "B", "A", "C", "B", "A", "C", "B", "C", "A","C","B","B"]})# Display the DataFrameprint(df)#My duct tape solutiondf = (    df    .with_columns(        pl.when(pl.col("days") > 2)        .then(pl.col("amount"))        .otherwise(0).alias("3+days_amount")                  )    .with_columns(        pl.col("3+days_amount").cum_sum().over("group").alias("group_cumsum")    ))print(df)shape: (13, 3)┌──────┬────────┬───────┐│ days ┆ amount ┆ group ││ ---  ┆ ---    ┆ ---   ││ i64  ┆ i64    ┆ str   │╞══════╪════════╪═══════╡│ 0    ┆ 100    ┆ A     ││ 1    ┆ 200    ┆ B     ││ 2    ┆ 150    ┆ A     ││ 3    ┆ 300    ┆ C     ││ 4    ┆ 250    ┆ B     ││…┆…┆…││ 3    ┆ 210    ┆ C     ││ 4    ┆ 320    ┆ A     ││ 6    ┆ 21     ┆ C     ││ 7    ┆ 456    ┆ B     ││ 1    ┆ 111    ┆ B     │└──────┴────────┴───────┘shape: (13, 5)┌──────┬────────┬───────┬───────────────┬──────────────┐│ days ┆ amount ┆ group ┆ 3+days_amount ┆ group_cumsum ││ ---  ┆ ---    ┆ ---   ┆ ---           ┆ ---          ││ i64  ┆ i64    ┆ str   ┆ i64           ┆ i64          │╞══════╪════════╪═══════╪═══════════════╪══════════════╡│ 0    ┆ 100    ┆ A     ┆ 0             ┆ 0            ││ 1    ┆ 200    ┆ B     ┆ 0             ┆ 0            ││ 2    ┆ 150    ┆ A     ┆ 0             ┆ 0            ││ 3    ┆ 300    ┆ C     ┆ 300           ┆ 300          ││ 4    ┆ 250    ┆ B     ┆ 250           ┆ 250          ││…┆…┆…┆…┆…││ 3    ┆ 210    ┆ C     ┆ 210           ┆ 510          ││ 4    ┆ 320    ┆ A     ┆ 320           ┆ 320          ││ 6    ┆ 21     ┆ C     ┆ 21            ┆ 531          ││ 7    ┆ 456    ┆ B     ┆ 456           ┆ 706          ││ 1    ┆ 111    ┆ B     ┆ 0             ┆ 706          │└──────┴────────┴───────┴───────────────┴──────────────┘

polars expressions seem very elegant generally, hoping there's something I am missing.