Changing certain values in multiple columns of a pandas DataFrame at once

Question

Suppose I have the following DataFrame:

In [1]: df
Out[1]:
  apple banana cherry
0     0      3   good
1     1      4    bad
2     2      5   good
         


        

Answer 1

It's because df[['apple', 'banana']][df.cherry == 'bad'] = np.nan assigning to the copy of DataFrame. Try this:

df.ix[df.cherry == 'bad', ['apple', 'banana']] = np.nan