Change test for 22721 and adapt whatsnew

Author: h-vetinari

doc/source/whatsnew/v0.24.0.txt

@@ -544,7 +544,7 @@ Other API Changes
 - :class:`Index` subtraction will attempt to operate element-wise instead of raising ``TypeError`` (:issue:`19369`)
 - :class:`pandas.io.formats.style.Styler` supports a ``number-format`` property when using :meth:`~pandas.io.formats.style.Styler.to_excel` (:issue:`22015`)
 - :meth:`DataFrame.corr` and :meth:`Series.corr` now raise a ``ValueError`` along with a helpful error message instead of a ``KeyError`` when supplied with an invalid method (:issue:`22298`)
-- :meth:`Series.str.cat` now also works for binary data in Python 3 (:issue:`22721`)
+- :meth:`Series.str.cat` now also works more consistently for binary data in Python 3, although use of the `.str`-accessor for binary data is not recommended (:issue:`22721`)
 - :meth:`shift` will now always return a copy, instead of the previous behaviour of returning self when shifting by 0 (:issue:`22397`)
 
 .. _whatsnew_0240.deprecations:

pandas/tests/test_strings.py

@@ -3088,7 +3088,10 @@ def test_str_accessor_no_new_attributes(self):
     def test_method_on_bytes(self):
         lhs = Series(np.array(list('abc'), 'S1').astype(object))
         rhs = Series(np.array(list('def'), 'S1').astype(object))
-
-        result = lhs.str.cat(rhs)
-        expected = Series(np.array(['ad', 'be', 'cf'], 'S2').astype(object))
-        tm.assert_series_equal(result, expected)
+        if compat.PY3:
+            pytest.raises(TypeError, lhs.str.cat, rhs, sep=',')
+        else:
+            result = lhs.str.cat(rhs)
+            expected = Series(np.array(
+                ['ad', 'be', 'cf'], 'S2').astype(object))
+            tm.assert_series_equal(result, expected)