Pandas sum two columns, skipping NaN

with fillna()

frame['c'] = frame.fillna(0)['a'] + frame.fillna(0)['b']

or as suggested :

frame['c'] = frame.a.fillna(0) + frame.b.fillna(0)

giving :

    a   b  c
0   1   3  4
1   2 NaN  2
2 NaN   4  4