Minor inconsistency?
Series.unique returns an numpy array but np.unique(series) returns a series. Not sure if it is intentional but it might be better to keep both the calls consistent.

In [173]: a[:10]
Out[173]: 
0       4
1     210
2     210
3     210
4    3197
5    3197
6    3197
7    3457
8    3713
9    3741

In [174]: a[:10].unique()
Out[174]: array([   4,  210, 3197, 3457, 3713, 3741])

In [176]: np.unique(a[:10])
Out[176]: 
0       4
1     210
4    3197
7    3457
8    3713
9    3741
