Restore concat casting cat -> object

pandas-dev · TomAugspurger · Feb 2, 2018 · Jan 15, 2018 · Jan 18, 2018 · Jan 18, 2018
commit 87a70e3958d603868a0a5dff13b04f8779290965
diff --git a/pandas/core/arrays/categorical.py b/pandas/core/arrays/categorical.py
@@ -2141,8 +2141,9 @@ def _can_hold_na(self):
 
     @classmethod
     def _concat_same_type(self, to_concat):
-        from pandas.types.concat import union_categoricals
-        return union_categoricals(to_concat)
+        from pandas.core.dtypes.concat import _concat_categorical
+
+        return _concat_categorical(to_concat)
 
     def _formatting_values(self):
         return self

diff --git a/pandas/core/internals.py b/pandas/core/internals.py
@@ -2539,6 +2539,26 @@ def to_native_types(self, slicer=None, na_rep='', quoting=None, **kwargs):
         # we are expected to return a 2-d ndarray
         return values.reshape(1, len(values))
 
+    def concat_same_type(self, to_concat, placement=None):
+        """
+        Concatenate list of single blocks of the same type.
+
+        Note that this CategoricalBlock._concat_same_type *may* not
+        return a CategoricalBlock. When the categories in `to_concat`
+        differ, this will return an object ndarray.
+
+        If / when we decide we don't like that behavior:
+
+        1. Change Categorical._concat_same_type to use union_categoricals
+        2. Delete this method.
+        """
+        values = self._concatenator([blk.values for blk in to_concat],
+                                    axis=self.ndim - 1)
+        # not using self.make_block_same_class as values can be object dtype
+        return make_block(
+            values, placement=placement or slice(0, len(values), 1),
+            ndim=self.ndim)
+
 
 class DatetimeBlock(DatetimeLikeBlockMixin, Block):
     __slots__ = ()