Fix alignment in the case DataFrame vs DataFrame case. Add tests

Author: evanpw

pandas/core/generic.py

@@ -3411,10 +3411,11 @@ def where(self, cond, other=np.nan, inplace=False, axis=None, level=None,
 
         if axis is None:
             axis = 0
+
+        if self.ndim == getattr(other, 'ndim', 0):
             align = True
         else:
-            axis = self._get_axis_number(axis)
-            align = (axis == 1)
+            align = (self._get_axis_number(axis) == 1)
 
         block_axis = self._get_block_manager_axis(axis)
 

pandas/tests/test_frame.py

@@ -9947,8 +9947,36 @@ def test_where_axis(self):
         result.where(mask, s2, axis='index', inplace=True)
         assert_frame_equal(result, expected)
 
+        # DataFrame vs DataFrame
+        d1 = df.copy().drop(1, axis=0)
+        expected = df.copy()
+        expected.loc[1, :] = np.nan
 
+        result = df.where(mask, d1)
+        assert_frame_equal(result, expected)
+        result = df.where(mask, d1, axis='index')
+        assert_frame_equal(result, expected)
+        result = df.copy()
+        result.where(mask, d1, inplace=True)
+        assert_frame_equal(result, expected)
+        result = df.copy()
+        result.where(mask, d1, inplace=True, axis='index')
+        assert_frame_equal(result, expected)
 
+        d2 = df.copy().drop(1, axis=1)
+        expected = df.copy()
+        expected.loc[:, 1] = np.nan
+
+        result = df.where(mask, d2)
+        assert_frame_equal(result, expected)
+        result = df.where(mask, d2, axis='columns')
+        assert_frame_equal(result, expected)
+        result = df.copy()
+        result.where(mask, d2, inplace=True)
+        assert_frame_equal(result, expected)
+        result = df.copy()
+        result.where(mask, d2, inplace=True, axis='columns')
+        assert_frame_equal(result, expected)
 
     def test_mask(self):
         df = DataFrame(np.random.randn(5, 3))