Review (jreback)

pandas/core/frame.py

@@ -6507,9 +6507,10 @@ def _join_compat(self, other, on=None, how='left', lsuffix='', rsuffix='',
             # join indexes only using concat
             if can_concat:
                 if how == 'left':
-                    how = 'outer'
-                    return concat(frames, axis=1, join=how,
-                                  verify_integrity=True).reindex(self.index)
+                    res = concat(frames, axis=1, join='outer',
+                                 verify_integrity=True, copy=False)
+                    res = res.reindex(self.index, copy=False)
+                    return res
                 else:
                     return concat(frames, axis=1, join=how,
                                   verify_integrity=True)

pandas/core/generic.py

@@ -8931,7 +8931,8 @@ def describe_1d(data):
                 if name not in names:
                     names.append(name)
 
-        d = pd.concat([x.reindex(names) for x in ldesc], axis=1, sort=False)
+        d = pd.concat([x.reindex(names) for x in ldesc], axis=1,
+                      sort=False, copy=False)
         d.columns = data.columns.copy()
         return d
 

pandas/core/groupby/generic.py

@@ -518,10 +518,11 @@ def _transform_general(self, func, *args, **kwargs):
                 applied.append(res)
 
         concat_index = obj.columns if self.axis == 0 else obj.index
-        other_axis = (self.axis + 1) % 2  # switches from 0 to 1 or from 1 to 0
+        other_axis = 1 if self.axis == 0 else 0  # switches between 0 & 1
         concatenated = concat(applied, axis=self.axis,
-                              verify_integrity=False).reindex(concat_index,
-                                                              axis=other_axis)
+                              verify_integrity=False, copy=False)
+        concatenated = concatenated.reindex(concat_index, axis=other_axis,
+                                            copy=False)
         return self._set_result_index_ordered(concatenated)
 
     @Substitution(klass='DataFrame', selected='')

pandas/core/reshape/concat.py

@@ -223,8 +223,7 @@ def concat(objs, axis=0, join='outer', join_axes=None, ignore_index=False,
     if join_axes is not None:
         warnings.warn('The join_axes-keyword is deprecated. Use .reindex or '
                       '.reindex_like on the result to achieve the same '
-                      'functionality, or apply those methods to the inputs if '
-                      'performance-sensitive.', FutureWarning, stacklevel=2)
+                      'functionality.', FutureWarning, stacklevel=2)
     op = _Concatenator(objs, axis=axis, join_axes=join_axes,
                        ignore_index=ignore_index, join=join,
                        keys=keys, levels=levels, names=names,

pandas/tests/reshape/test_concat.py

@@ -729,7 +729,8 @@ def test_concat_categorical_empty(self):
         tm.assert_series_equal(pd.concat([s2, s1], ignore_index=True), exp)
         tm.assert_series_equal(s2.append(s1, ignore_index=True), exp)
 
-    def test_concat_join_axes(self, axis):
+    def test_concat_join_axes_deprecated(self, axis):
+        # GH21951
         one = pd.DataFrame([[0., 1.], [2., 3.]], columns=list('ab'))
         two = pd.DataFrame([[10., 11.], [12., 13.]], index=[1, 2],
                            columns=list('bc'))