Fail with a meaningful error msg on old-style classes

attr/_make.py

@@ -160,6 +160,8 @@ def attributes(maybe_cl=None, these=None,
     :type no_init: bool
     """
     def wrap(cl):
+        if getattr(cl, "__class__", None) is None:
+            raise TypeError("attrs only works with new-style classes.")
         _transform_attrs(cl, these)
         if not no_repr:
             cl = _add_repr(cl)
@@ -172,12 +174,11 @@ def attributes(maybe_cl=None, these=None,
         return cl
 
     # attrs_or class type depends on the usage of the decorator.  It's a class
-    # if it's used as `@attributes` but ``None`` (or a value passed) if used
-    # as `@attributes()`.
-    if isinstance(maybe_cl, type):
-        return wrap(maybe_cl)
-    else:
+    # if it's used as `@attributes` but ``None`` if used # as `@attributes()`.
+    if maybe_cl is None:
         return wrap
+    else:
+        return wrap(maybe_cl)
 
 
 def _attrs_to_tuple(obj, attrs):

tests/test_make.py

@@ -9,6 +9,7 @@ from __future__ import absolute_import, division, print_function
 import pytest
 
 from . import simple_attr
+from attr._compat import PY3
 from attr._make import (
     Attribute,
     NOTHING,
@@ -117,6 +118,17 @@ class TestAttributes(object):
     """
     Tests for the `attributes` class decorator.
     """
+    @pytest.mark.skipif(PY3, reason="No old-style classes in Py3")
+    def test_catches_old_style(self):
+        """
+        Raises TypeError on old-style classes.
+        """
+        with pytest.raises(TypeError) as e:
+            @attributes
+            class C:
+                pass
+        assert ("attrs only works with new-style classes.",) == e.value.args
+
     def test_sets_attrs(self):
         """
         Sets the `__attrs_attrs__` class attribute with a list of `Attribute`s.