Auto row increment code

[codesearcher]

Time being I have used Autofill was working with Range function however am not able to retrieve previous month value..

import pandas as pd

idx = pd.date_range('1/1/2020', '6/1/2020')

s = pd.Series({'1/1/2020': 1,
               '2/1/2020': 5,
               '4/1/2020': 10,
               '6/1/2020': 15})
s.index = pd.DatetimeIndex(s.index)

s = s.reindex(idx, fill_value=0)
print(s)