Use replace
instead of map
:
>>> s = pd.Series(['one','two','three','four'])
>>> recodes = {'one':'A', 'two':'B', 'three':'C'}
>>> s.map(recodes)
0 A
1 B
2 C
3 NaN
dtype: object
>>> s.replace(recodes)
0 A
1 B
2 C
3 four
dtype: object