groupby for pandas Series not working

if you need to groupby series' values:

grouped = a.groupby(a)

or

grouped = a.groupby(lambda x: a[x])

You need to pass a mapping of some kind (could be a dict/function/index)

In [6]: a
Out[6]: 
4    1
3    2
2    3
1    4
dtype: int64

In [7]: a.groupby(a.index).sum()
Out[7]: 
1    4
2    3
3    2
4    1
dtype: int64

In [3]: a.groupby(lambda x: x % 2 == 0).sum()
Out[3]: 
False    6
True     4
dtype: int64