Retaining A Column With All Strings During Groupby On A Pandas Dataframe

datetime col_A col_B 1/1/2012 125.501 A 1/2/2012 NaN A 1/3/2012 125.501 A 1/4/2013 NaN A 1/5/2013 125.501 B 2/28/2013 125.501 B 2/28/2014 125.

Solution 1:

I think you can add col_A:

df['col_A'] = df.groupby([df.index.month, df.index.day])['col_A'].transform(lambda x: 
                                                                          x.fillna(x.mean()))
print df
              col_A col_B
datetime                 
2012-01-01  125.501     A
2012-01-02  125.501     A
2012-01-03  125.501     A
2013-01-04  125.501     A
2013-01-05  125.501     B
2013-02-28  125.501     B
2014-02-28  125.501     B
2016-01-02  125.501     B
2016-01-04  125.501     B
2016-02-28  125.501     B