While upgrading pandas from 0.7.2 to 0.9.1 we have found that groupby().max() operation now removes non-numeric columns. This broke our code in several places. Workaround is to use groupby().aggregate(np.max).
Here is an example demonstrating the problem:
aa=DataFrame({'nn':[11,11,22,22],'ii':[1,2,3,4],'ss':4*['mama']})
aa.groupby('nn').max()
output on pandas 0.7.2
ii nn ss
nn
11 2 11 mama
22 4 22 mama
output on pandas 0.9.1
ii
nn
11 2
22 4
As you see, object column 'ss' is dropped in new version !!!
This was very un-intuitive.
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