How to keep original index of a DataFrame after groupby 2 columns?

Question

Is there any way I can retain the original index of my large dataframe after I perform a groupby? The reason I need to this is because I need to do an inner merge back to my ori

Answer 1

I think you are are looking for transform in this situation:

df['count'] = df.groupby(['col1', 'col2'])['col3'].transform('count')