Review (simonjayhawkins)

Author: h-vetinari

pandas/core/strings.py

@@ -2284,10 +2284,13 @@ def cat(self, others=None, sep=None, na_rep=None, join=None):
         # but others could still have Series of dtypes (e.g. integers) which
         # will necessarily fail in concatenation. To avoid deep and confusing
         # traces, we raise here for anything that's not object or all-NA float.
-        if any(not (x.dtype == 'O' or (is_categorical_dtype(x)
-                                       and x.cat.categories.dtype == 'O')
-                    or (x.dtype == 'float' and x.isna().all()))
-               for x in others):
+        def _legal_dtype(series):
+            # unify dtype handling between categorical/non-categorical
+            dtype = (series.dtype if not is_categorical_dtype(series)
+                     else series.cat.categories.dtype)
+            legal = dtype == 'O' or (dtype == 'float' and series.isna().all())
+            return legal
+        if any(not _legal_dtype(x) for x in others):
             raise TypeError('Can only concatenate list-likes containing only '
                             'strings (or missing values)')
 

pandas/tests/test_strings.py

@@ -421,7 +421,7 @@ def test_str_cat_categorical(self, box, dtype_caller, dtype_target, sep):
             assert_series_or_index_equal(result, expected)
 
     @pytest.mark.parametrize('box', [Series, Index, np.array, list])
-    def test_str_cat_raise_wrong_dtype(self, box):
+    def test_str_cat_wrong_dtype_raises(self, box):
         # GH 22722
         s = Series(['a', 'b', 'c', 'd'])
         t = box([1, 2, 3, 4])