Extracting the first day of month of a datetime type column in pandas

Question

I have the following dataframe:

user_id    purchase_date 
  1        2015-01-23 14:05:21
  2        2015-02-05 05:07:30
  3        2015-02-18 17:08:51
  4            


        

Answer 1

We can use date offset in conjunction with Series.dt.normalize:

In [60]: df['month'] = df['purchase_date'].dt.normalize() - pd.offsets.MonthBegin(1)

In [61]: df
Out[61]:
   user_id       purchase_date      month
0        1 2015-01-23 14:05:21 2015-01-01
1        2 2015-02-05 05:07:30 2015-02-01
2        3 2015-02-18 17:08:51 2015-02-01
3        4 2015-03-21 17:07:30 2015-03-01
4        5 2015-03-11 18:32:56 2015-03-01
5        6 2015-03-03 11:02:30 2015-03-01

Or much nicer solution from @BradSolomon

In [95]: df['month'] = df['purchase_date'] - pd.offsets.MonthBegin(1, normalize=True)

In [96]: df
Out[96]:
   user_id       purchase_date      month
0        1 2015-01-23 14:05:21 2015-01-01
1        2 2015-02-05 05:07:30 2015-02-01
2        3 2015-02-18 17:08:51 2015-02-01
3        4 2015-03-21 17:07:30 2015-03-01
4        5 2015-03-11 18:32:56 2015-03-01
5        6 2015-03-03 11:02:30 2015-03-01