Pandas .fillna() should handle "inf"
Division by 0 in pandas will give the value "inf". But the .fillna() method doesn't recognize that. We should make .fillna() handle "inf" the same way it handles "NaN'. (for reference, the numpy.isfinite() method treats NaN and Inf interchangably -- pandas should do the same).

import pandas
vals = [ 1.1, 1.1, 1.1]
p =  pandas.DataFrame( { 'first' : vals }, columns=['first'])
p['first'] = p['first'] / 0 #Creates a bunch of "inf" values
p['first'].fillna(0, inplace=True) #Has no effect
