Better TypeError for wrong dtype in str.cat

Author: h-vetinari

pandas/core/strings.py

@@ -2280,6 +2280,14 @@ def cat(self, others=None, sep=None, na_rep=None, join=None):
                                  'must all be of the same length as the '
                                  'calling Series/Index.')
 
+        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):
+            # data has already been checked by str-accessor
+            raise TypeError('Can only concatenate list-likes containing only '
+                            'strings (or missing values)!')
+
         if join is None and warn:
             warnings.warn("A future version of pandas will perform index "
                           "alignment when `others` is a Series/Index/"

pandas/tests/test_strings.py

@@ -420,6 +420,15 @@ def test_str_cat_categorical(self, box, dtype_caller, dtype_target, sep):
             result = s.str.cat(t, sep=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):
+        s = Series(['a', 'b', 'c', 'd'])
+        t = box([1, 2, 3, 4])
+
+        msg = 'Can only concatenate list-likes containing only strings.*'
+        with tm.assert_raises_regex(TypeError, msg):
+            s.str.cat(t)
+
     @pytest.mark.parametrize('box', [Series, Index])
     def test_str_cat_mixed_inputs(self, box):
         s = Index(['a', 'b', 'c', 'd'])