Pandas dataframe apply to datetime column : does not work

Question

I have a dataframe with a datetime column. I want to apply a function to set value as None in this column if the date is inferior to an other date. But the applied fonction set all my values to None. Could you help me ?

Here my code :

dateused = datetime.datetime.strptime('202004', '%Y%m')
df['date_pack'] =df['date_pack'].apply(lambda x: None if x < dateused else x)

The dtype of df['date_pack'] is datetime64[ns]. After this, all my values in my column 'date_pack' are None.

Thanks

Answer 1

I think you need Series.mask with set values to NaT for misisng values for datetimes:

df = pd.DataFrame({'date_pack': pd.to_datetime(['2020-08-10','2002-02-09'])})

dateused = datetime.datetime.strptime('202004', '%Y%m')
df['date_pack'] = df['date_pack'].mask(df['date_pack'] < dateused)
print (df)
   date_pack
0 2020-08-10
1        NaT