[Python] Cumulative count per group in PyArrow

Say I have

data = {'a': [1,1,2], 'b': [4,5,6]}


and I'd like to get a cumulative count (1-indexed) per group.

In pandas, I can do:

import pandas as pd

pd.DataFrame(data).groupby('a').cumcount()+1


How can I do this in PyArrow, starting from pa.table(data), without doing any conversion to pandas?

I'm OK with using NumPy functions (e.g. np.arange) to generate the new column, but I can't convert the starting pyarrow.Table to numpy (nor to pandas)

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