python – Pandas – 基于Boollean DataFrame替换DataFrame中的值

我正在使用Pandas v0.20.2并且我有DataFrame,如下所示：

df = pd.DataFrame(dict(a=[0,1], b=[3,4], c=[6,7]), 
              index=['spam', 'ham'])
#       a  b  c
# spam  0  3  6
# ham   1  4  7

我有另一个掩码的DataFrame：

mask = pd.DataFrame(dict(a=[True,False], b=[True,True]), 
                index=['spam', 'ham'])
#           a     b
# spam   True  True
# ham   False  True

我想将df中的值设置为等于999,其中掩码中为True.

我认为以下内容可行：

df[mask] = 999

但事实并非如此.我收到以下错误：

ValueError                                Traceback (most recent call last)
<ipython-input-65-503f937859ab> in <module>()
----> 1 df[mask] = 999

/home/gbra/anaconda3/envs/outer_disk/lib/python2.7/site-packages/pandas/core/frame.pyc in __setitem__(self, key, value)
   2326             self._setitem_array(key, value)
   2327         elif isinstance(key, DataFrame):
-> 2328             self._setitem_frame(key, value)
   2329         else:
   2330             # set column

/home/gbra/anaconda3/envs/outer_disk/lib/python2.7/site-packages/pandas/core/frame.pyc in _setitem_frame(self, key, value)
   2364         self._check_inplace_setting(value)
   2365         self._check_setitem_copy()
-> 2366         self._where(-key, value, inplace=True)
   2367 
   2368     def _ensure_valid_index(self, value):

/home/gbra/anaconda3/envs/outer_disk/lib/python2.7/site-packages/pandas/core/generic.pyc in _where(self, cond, other, inplace, axis, level, try_cast, raise_on_error)
   5096             for dt in cond.dtypes:
   5097                 if not is_bool_dtype(dt):
-> 5098                     raise ValueError(msg.format(dtype=dt))
   5099 
   5100         cond = cond.astype(bool, copy=False)

ValueError: Boolean array expected for the condition, not float64

我将不胜感激任何帮助.

解决方法:

您可以重新索引蒙版以使其具有与df相同的形状,然后使用df.mask：

df.mask(mask.reindex(df.index, df.columns, fill_value=False), 999)
Out: 
        a    b  c
spam  999  999  6
ham     1  999  7

此时,常规索引也应该有效：

df[mask.reindex(df.index, df.columns, fill_value=False)] = 999