[Python] Polars alternative solution to using map_groups with with_columns

I think after the recent update of Polars (version 0.20.7), it stopped supporting the use of map_groups with with_columns I got a Type Error stating

TypeError: cannot call `map_groups` when grouping by an expression


I am trying to Group By a column called Product Number and fill the null values using backward strategy for this Group By.

The code I wrote was.

df1 = (
df1
.group_by(['Product Number'])
.map_groups(lambda df1: (
df1
.with_columns(
df1['New Date'].fill_null(strategy='backward').alias('New Date1')
)
)
)
)


Is there alternate solution which supports grouping with with_columns ?

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