python pandas groupby() result

Here’s one way (though it feels this should work in one go with an apply, I can’t get it).

In [11]: g = df.groupby(['A', 'B'])

In [12]: df1 = df.set_index(['A', 'B'])

The size groupby function is the one you want, we have to match it to the ‘A’ and ‘B’ as the index:

In [13]: df1['D'] = g.size()  # unfortunately this doesn't play nice with as_index=False
# Same would work with g['C'].sum()

In [14]: df1.reset_index()
Out[14]:
    A  B  C  D
0   1  5  1  2
1   1  5  1  2
2   1  6  1  1
3   1  7  1  1
4   2  5  1  1
5   2  6  1  2
6   2  6  1  2
7   3  7  1  2
8   3  7  1  2
9   4  6  1  1
10  4  7  1  2
11  4  7  1  2