BUG: DataFrame.where does not handle Series slice correctly (fixes pandas-dev#10218)

Author: mortada

pandas/core/generic.py

@@ -3439,6 +3439,8 @@ def where(self, cond, other=np.nan, inplace=False, axis=None, level=None,
               try_cast=False, raise_on_error=True):
 
         if isinstance(cond, NDFrame):
+            if isinstance(cond, pd.Series) and isinstance(self, pd.DataFrame):
+                cond = self._getitem_array(cond)
             cond = cond.reindex(**self._construct_axes_dict())
         else:
             if not hasattr(cond, 'shape'):

pandas/tests/test_frame.py

@@ -9846,6 +9846,13 @@ def _check_set(df, cond, check_dtypes = True):
             cond = (df >= 0)[1:]
             _check_set(df, cond)
 
+        # GH 10218
+        # test DataFrame.where with Series slicing
+        df = DataFrame({'a': range(3), 'b': range(4, 7)})
+        result = df.where(df['a'] == 1)
+        expected = df[df['a'] == 1].reindex(df.index)
+        assert_frame_equal(result, expected)
+
     def test_where_bug(self):
 
         # GH 2793