A.
>>> month = [item[5:7] for item in quotesKOdf.index] >>> quotesKOdf.groupby(month).volume.sum()
B.
>>> month = [item[5:7] for item in quotesKOdf.index] >>> sum(quotesKOdf.groupby(month).volume)
C.
>>> month = [item[5:7] for item in quotesKOdf.index] >>> quotesKOdf.groupby(month).volume.apply(sum())
D.
>>> month = [item[5:7] for item in quotesKOdf.index] >>> quotesKOdf.groupby(month).volume.apply(sum)