Revert cat-output-for-cat-caller propsal

Author: h-vetinari

doc/source/whatsnew/v0.23.0.txt

@@ -325,9 +325,7 @@ In v.0.23 `join` will default to None (meaning no alignment), but this default w
     s.str.cat(t)
     s.str.cat(t, join='left', na_rep='-')
 
-Furthermore:
-- meth:`Series.str.cat` now works as well for ``CategoricalIndex`` as well (previously raised a ``ValueError``; see :issue:`20842`)
-- If concatenating with something (i.e. `others is not None`) the resulting ``Series``/``Index`` will now remain categorical if the calling ``Series``/``Index`` is categorical (see :issue:`20843`)
+Furthermore, meth:`Series.str.cat` now works for ``CategoricalIndex`` as well (previously raised a ``ValueError``; see :issue:`20842`).
 
 .. _whatsnew_0230.enhancements.astype_category:
 

pandas/core/strings.py

@@ -2217,11 +2217,10 @@ def cat(self, others=None, sep=None, na_rep=None, join=None):
         # str_cat discards index
         res = str_cat(data, others=others, sep=sep, na_rep=na_rep)
 
-        dtype = 'category' if self._is_categorical else None
         if isinstance(self._orig, Index):
-            res = Index(res, dtype=dtype)
+            res = Index(res)
         else:  # Series
-            res = Series(res, index=data.index, dtype=dtype)
+            res = Series(res, index=data.index)
         return res
 
     @copy(str_split)

pandas/tests/series/test_api.py

@@ -608,6 +608,7 @@ def test_str_accessor_api_for_categorical(self):
 
         # str functions, which need special arguments
         special_func_defs = [
+            ('cat', (list("zyxw"),), {"sep": ","}),
             ('center', (10,), {}),
             ('contains', ("a",), {}),
             ('count', ("a",), {}),
@@ -643,12 +644,11 @@ def test_str_accessor_api_for_categorical(self):
         ]
         _special_func_names = [f[0] for f in special_func_defs]
 
-        # * cat tested extensively with categorical data in test_strings.py
         # * get, join: they need a individual elements of type lists, but
         #   we can't make a categorical with lists as individual categories.
         #   -> `s.str.split(" ").astype("category")` will error!
         # * `translate` has different interfaces for py2 vs. py3
-        _ignore_names = ["cat", "get", "join", "translate"]
+        _ignore_names = ["get", "join", "translate"]
 
         str_func_names = [f for f in dir(s.str) if not (
             f.startswith("_") or

pandas/tests/test_strings.py

@@ -228,7 +228,7 @@ def test_str_cat_categorical(self, series_or_index,
             s = Series(s)
         t = Index(['b', 'a', 'b', 'c'], dtype=dtype_target)
 
-        exp = Index(['ab', 'aa', 'bb', 'ac'], dtype=dtype_caller)
+        exp = Index(['ab', 'aa', 'bb', 'ac'])
         if series_or_index == 'series':
             exp = Series(exp)