BUG: DataFrame.where does not handle Series slice correctly (#10218)

doc/source/whatsnew/v0.17.0.txt

@@ -785,3 +785,4 @@ Bug Fixes
 - Bug in ``read_msgpack`` where encoding is not respected (:issue:`10580`)
 - Bug preventing access to the first index when using ``iloc`` with a list containing the appropriate negative integer (:issue:`10547`, :issue:`10779`)
 - Bug in ``TimedeltaIndex`` formatter causing error while trying to save ``DataFrame`` with ``TimedeltaIndex`` using ``to_csv`` (:issue:`10833`)
+- BUG in ``DataFrame.where`` when handling Series slicing (:issue:`10218`, :issue:`9558`)

pandas/core/frame.py

@@ -2596,6 +2596,14 @@ def _reindex_multi(self, axes, copy, fill_value):
                                                copy=copy,
                                                fill_value=fill_value)
 
+    @Appender(_shared_docs['align'] % _shared_doc_kwargs)
+    def align(self, other, join='outer', axis=None, level=None, copy=True,
+              fill_value=None, method=None, limit=None, fill_axis=0,
+              broadcast_axis=None):
+        return super(DataFrame, self).align(other, join=join, axis=axis, level=level, copy=copy,
+                                            fill_value=fill_value, method=method, limit=limit,
+                                            fill_axis=fill_axis, broadcast_axis=broadcast_axis)
+
     @Appender(_shared_docs['reindex'] % _shared_doc_kwargs)
     def reindex(self, index=None, columns=None, **kwargs):
         return super(DataFrame, self).reindex(index=index, columns=columns,

pandas/core/generic.py

@@ -3447,8 +3447,7 @@ def last(self, offset):
         start = self.index.searchsorted(start_date, side='right')
         return self.ix[start:]
 
-    def align(self, other, join='outer', axis=None, level=None, copy=True,
-              fill_value=None, method=None, limit=None, fill_axis=0):
+    _shared_docs['align'] = (
         """
         Align two object on their axes with the
         specified join method for each axis Index
@@ -3470,17 +3469,46 @@ def align(self, other, join='outer', axis=None, level=None, copy=True,
             "compatible" value
         method : str, default None
         limit : int, default None
-        fill_axis : {0, 1}, default 0
+        fill_axis : %(axes_single_arg)s, default 0
             Filling axis, method and limit
+        broadcast_axis : %(axes_single_arg)s, default None
+            Broadcast values along this axis, if aligning two objects of
+            different dimensions
+
+            .. versionadded:: 0.17.0
 
         Returns
         -------
-        (left, right) : (type of input, type of other)
+        (left, right) : (%(klass)s, type of other)
             Aligned objects
         """
+    )
+
+    @Appender(_shared_docs['align'] % _shared_doc_kwargs)
+    def align(self, other, join='outer', axis=None, level=None, copy=True,
+              fill_value=None, method=None, limit=None, fill_axis=0,
+              broadcast_axis=None):
         from pandas import DataFrame, Series
         method = com._clean_fill_method(method)
 
+        if broadcast_axis == 1 and self.ndim != other.ndim:
+            if isinstance(self, Series):
+                # this means other is a DataFrame, and we need to broadcast self
+                df = DataFrame(dict((c, self) for c in other.columns),
+                               **other._construct_axes_dict())
+                return df._align_frame(other, join=join, axis=axis, level=level,
+                                       copy=copy, fill_value=fill_value,
+                                       method=method, limit=limit,
+                                       fill_axis=fill_axis)
+            elif isinstance(other, Series):
+                # this means self is a DataFrame, and we need to broadcast other
+                df = DataFrame(dict((c, other) for c in self.columns),
+                               **self._construct_axes_dict())
+                return self._align_frame(df, join=join, axis=axis, level=level,
+                                         copy=copy, fill_value=fill_value,
+                                         method=method, limit=limit,
+                                         fill_axis=fill_axis)
+
         if axis is not None:
             axis = self._get_axis_number(axis)
         if isinstance(other, DataFrame):
@@ -3516,11 +3544,11 @@ def _align_frame(self, other, join='outer', axis=None, level=None,
                     self.columns.join(other.columns, how=join, level=level,
                                       return_indexers=True)
 
-        left = self._reindex_with_indexers({0: [join_index,   ilidx],
+        left = self._reindex_with_indexers({0: [join_index, ilidx],
                                             1: [join_columns, clidx]},
                                            copy=copy, fill_value=fill_value,
                                            allow_dups=True)
-        right = other._reindex_with_indexers({0: [join_index,   iridx],
+        right = other._reindex_with_indexers({0: [join_index, iridx],
                                               1: [join_columns, cridx]},
                                              copy=copy, fill_value=fill_value,
                                              allow_dups=True)
@@ -3624,7 +3652,7 @@ def where(self, cond, other=np.nan, inplace=False, axis=None, level=None,
               try_cast=False, raise_on_error=True):
 
         if isinstance(cond, NDFrame):
-            cond = cond.reindex(**self._construct_axes_dict())
+            cond, _ = cond.align(self, join='right', broadcast_axis=1)
         else:
             if not hasattr(cond, 'shape'):
                 raise ValueError('where requires an ndarray like object for '

pandas/core/panel.py

@@ -628,6 +628,9 @@ def _needs_reindex_multi(self, axes, method, level):
         """ don't allow a multi reindex on Panel or above ndim """
         return False
 
+    def align(self, other, **kwargs):
+        raise NotImplementedError
+
     def dropna(self, axis=0, how='any', inplace=False):
         """
         Drop 2D from panel, holding passed axis constant

pandas/core/series.py

@@ -2164,6 +2164,14 @@ def _needs_reindex_multi(self, axes, method, level):
         """
         return False
 
+    @Appender(generic._shared_docs['align'] % _shared_doc_kwargs)
+    def align(self, other, join='outer', axis=None, level=None, copy=True,
+              fill_value=None, method=None, limit=None, fill_axis=0,
+              broadcast_axis=None):
+        return super(Series, self).align(other, join=join, axis=axis, level=level, copy=copy,
+                                         fill_value=fill_value, method=method, limit=limit,
+                                         fill_axis=fill_axis, broadcast_axis=broadcast_axis)
+
     @Appender(generic._shared_docs['rename'] % _shared_doc_kwargs)
     def rename(self, index=None, **kwargs):
         return super(Series, self).rename(index=index, **kwargs)

pandas/tests/test_frame.py

@@ -10065,6 +10065,34 @@ def test_align(self):
         self.assertRaises(ValueError, self.frame.align, af.ix[0, :3],
                           join='inner', axis=2)
 
+        # align dataframe to series with broadcast or not
+        idx = self.frame.index
+        s = Series(range(len(idx)), index=idx)
+
+        left, right = self.frame.align(s, axis=0)
+        tm.assert_index_equal(left.index, self.frame.index)
+        tm.assert_index_equal(right.index, self.frame.index)
+        self.assertTrue(isinstance(right, Series))
+
+        left, right = self.frame.align(s, broadcast_axis=1)
+        tm.assert_index_equal(left.index, self.frame.index)
+        expected = {}
+        for c in self.frame.columns:
+            expected[c] = s
+        expected = DataFrame(expected, index=self.frame.index,
+                             columns=self.frame.columns)
+        assert_frame_equal(right, expected)
+
+        # GH 9558
+        df = DataFrame({'a':[1,2,3], 'b':[4,5,6]})
+        result = df[df['a'] == 2]
+        expected = DataFrame([[2, 5]], index=[1], columns=['a', 'b'])
+        assert_frame_equal(result, expected)
+
+        result = df.where(df['a'] == 2, 0)
+        expected = DataFrame({'a':[0, 2, 0], 'b':[0, 5, 0]})
+        assert_frame_equal(result, expected)
+
     def _check_align(self, a, b, axis, fill_axis, how, method, limit=None):
         aa, ab = a.align(b, axis=axis, join=how, method=method, limit=limit,
                          fill_axis=fill_axis)
@@ -10310,6 +10338,13 @@ 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