row_number() in pandas

one of the best code snippets i used to use all the time in my notebooks is row_number(), but in pandas:

dice_rolls['rn'] = dice_rolls.sort_values(['user_id', 'timestamp'], ascending=[True,True]) \
                .groupby(['user_id']) \
                .cumcount() + 1

honestly, it's strange there is no built-in function for this yet