Group by min and fill NAs with value from another column

I have a sample data frame that looks like this.

df = pd.DataFrame (data = {'uid': [1,1,1,2,2,3], 'pagename':['home', 'blah', 'blah', 'home', 'blah', 'blah'], 'startpage': ['NA', 'NA', 'NA', 'home', 'home', 'blah'], 'date_time': [0,1,2,5,9,1]})

What I want to do is group by the UID and find the min date_time. If the startpage of the min date_time is Null (I put string 'NA' for Null) then I want to use the pagename from that row to populate the startpage column. I also want the startpage to be populated for all rows with the same UID.

This is the ending dataframe that I want.

df = pd.DataFrame (data = {'uid': [1,1,1,2,2,3], 'pagename':['home', 'blah', 'blah', 'home', 'blah', 'blah'], 'startpage': ['home', 'home', 'home', 'home', 'home', 'blah'], 'date_time': [0,1,2,5,9,1]})


fillna with transform

i = df.groupby('uid').date_time.transform('idxmin') df.startpage = df.startpage.fillna(i.map(df.pagename)) print(df) date_time pagename startpage uid 0 0 home home 1 1 1 blah home 1 2 2 blah home 1 3 5 home home 2 4 9 blah home 2 5 1 blah blah 3


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