you can use directly fillna and assigning the result to the column 'bar'

df['bar'].fillna(df['foo'], inplace=True) del df['foo']

general example:

import pandas as pd #creating the table with two missing values df1 = pd.DataFrame({'a':[1,2],'b':[3,4]}, index = [1,2]) df2 = pd.DataFrame({'b':[5,6]}, index = [3,4]) dftot = pd.concat((df1, df2)) print dftot #creating the dataframe to fill the missing values filldf = pd.DataFrame({'a':[7,7,7,7]}) #filling print dftot.fillna(filldf) 

you can use directly fillna and assigning the result to the column 'bar'

df['bar'].fillna(df['foo'], inplace=True) del df['foo'] 

general example:

import pandas as pd #creating the table with two missing values df1 = pd.DataFrame({'a':[1,2],'b':[3,4]}, index = [1,2]) df2 = pd.DataFrame({'b':[5,6]}, index = [3,4]) dftot = pd.concat((df1, df2)) print dftot #creating the dataframe to fill the missing values filldf = pd.DataFrame({'a':[7,7,7,7]}) #filling print dftot.fillna(filldf)