Review (WillAyd)

pandas/core/strings.py

@@ -54,9 +54,6 @@ def cat_core(list_of_columns, sep):
     nd.array
         The concatenation of list_of_columns with sep
     """
-    if sep == '':
-        # no need to add empty strings
-        return np.sum(list_of_columns, axis=0)
     list_with_sep = [sep] * (2 * len(list_of_columns) - 1)
     list_with_sep[::2] = list_of_columns
     return np.sum(list_with_sep, axis=0)

pandas/tests/test_strings.py

@@ -367,6 +367,12 @@ def test_str_cat_align_mixed_inputs(self, join):
         with tm.assert_raises_regex(ValueError, rgx):
             s.str.cat([t, z], join=join)
 
+    def test_str_cat_raises(self):
+        # non-strings hiding behind object dtype
+        s = Series([1, 2, 3, 4], dtype='object')
+        with tm.assert_raises_regex(TypeError, "unsupported operand type.*"):
+            s.str.cat(s)
+
     def test_str_cat_special_cases(self):
         s = Series(['a', 'b', 'c', 'd'])
         t = Series(['d', 'a', 'e', 'b'], index=[3, 0, 4, 1])
@@ -3089,7 +3095,7 @@ def test_method_on_bytes(self):
         lhs = Series(np.array(list('abc'), 'S1').astype(object))
         rhs = Series(np.array(list('def'), 'S1').astype(object))
         if compat.PY3:
-            pytest.raises(TypeError, lhs.str.cat, rhs, sep=',')
+            pytest.raises(TypeError, lhs.str.cat, rhs)
         else:
             result = lhs.str.cat(rhs)
             expected = Series(np.array(