python - Sum across all NaNs in pandas returns zero? -

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i'm trying sum across columns of pandas dataframe, , when have nans in every column i'm getting sum = zero; i'd expected sum = nan based on docs. here's i've got:

in [136]: df = pd.dataframe()  in [137]: df['a'] = [1,2,np.nan,3]  in [138]: df['b'] = [4,5,np.nan,6]  in [139]: df out[139]:        b 0   1   4 1   2   5 2 nan nan 3   3   6  in [140]: df['total'] = df.sum(axis=1)  in [141]: df out[141]:        b  total 0   1   4      5 1   2   5      7 2 nan nan      0 3   3   6      9

the pandas.dataframe.sum docs "if entire row/column na, result na", don't understand why "total" = 0 , not nan index 2. missing?

a solution select cases rows all-nan, set sum nan: 

df['total'] = df.sum(axis=1)     df.loc[df['a'].isnull() & df['b'].isnull(),'total']=np.nan

or 

df['total'] = df.sum(axis=1)     df.loc[df[['a','b']].isnull().all(1),'total']=np.nan

the latter option more practical, because can create list of columns ['a','b', ... , 'z'] may want sum.


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