Answer by Quang Hoang for Is there a way to replace null values with values from another column in dataframe?

You can use bfill followed by fillna().

df['Home_Zip'] = df.iloc[:, 2:].bfill(1).fillna('Unknown')['Home_Zip']

Or chain several fillna:

df['Home_Zip'] = (df['Home_Zip'].fillna(df['work_zip'])                     .fillna(df['Family_zip'])                     .fillna('Unknown')                 )

Output:

   s/n     Name Home_Zip work_zip Family_zip0    1     John    56729      NaN      836AB1    2     sawn    73836      NaN      738362    3  Michael    839CD    839CD      830933    4   Nathan    93932    78393        N/a4    5     Jack  Unknown      NaN        NaN