条件を満たす要素をまとめて変える

gistfile1.md

python, pandas

In [1]: df
Out[1]:
  apple banana cherry
0     0      3   good
1     1      4    bad
2     2      5   good
In [2]: df.loc[df.cherry == 'bad', ['apple', 'banana']] = np.nan
In [3]: df
Out[3]: 
   apple  banana cherry
0      0       3   good
1    NaN     NaN    bad
2      2       5   good

http://stackoverflow.com/questions/19867734/changing-certain-values-in-multiple-columns-of-a-pandas-dataframe-at-once