Keeping 'key' column when using groupby with transform in pandas

Another way to achieve something similiar to what Pepacz suggested:

df.loc[:, df.columns.drop('a')] = df.groupby('a').transform(lambda x: x)

that is bizzare!

I tricked it like this

df.groupby(df.a.values).transform(lambda x: x)

enter image description here

The issue is discussed also here.

The returned object has the same indices as the original df, therefore you can do

pd.concat([
    df['a'],
    df.groupby('a').transform(lambda x: x)
], axis=1)

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