Example using the new pandas categorical
type in pandas 0.15+
http://pandas.pydata.org/pandas-docs/version/0.16.2/categorical.html
In [553]: x = pd.Series(['a', 'a', 'a', 'b', 'b', 'c']).astype('category')
In [554]: x
Out[554]:
0 a
1 a
2 a
3 b
4 b
5 c
dtype: category
Categories (3, object): [
a
, b
, c]
In [555]: x.cat.codes
Out[555]:
0 0
1 0
2 0
3 1
4 1
5 2
dtype: int8