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

Most proposed solutions don't work for the first day of the month.

Following solution works for any day of the month:

df['month'] = df['purchase_date'] + pd.offsets.MonthEnd(0) - pd.offsets.MonthBegin(normalize=True)

[EDIT]

Another, more readable, solution is:

from pandas.tseries.offsets import MonthBegin
df['month'] = df['purchase_date'].dt.normalize().map(MonthBegin().rollback)

Be aware not to use:

df['month'] = df['purchase_date'].map(MonthBegin(normalize=True).rollback)

because that gives incorrect results for the first day due to a bug: https://github.com/pandas-dev/pandas/issues/32616