GvR asked me to add this to the bug tracker.

If you do this:

    class A(ABCMeta): pass

    A.register(list)

you get this error message:

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
RuntimeError: maximum recursion depth exceeded in __instancecheck__

This is not very helpful.

You probably meant to write:

    class A(metaclass=ABCMeta): pass

Perhaps a warning message like this would be better:

"register() should not be called on an ABCMeta subclass; maybe you
forgot the metaclass keyword argument when declaring the class?"

This would be easier to fix if we didn't have unbound methods.  I'm
going to ask the py3k list if anybody really cares about having those.

Are you fine with the error message or do you have a better one?

Index: Lib/abc.py
===================================================================
--- Lib/abc.py  (Revision 59228)
+++ Lib/abc.py  (Arbeitskopie)
@@ -137,8 +137,12 @@
         cls._abc_negative_cache_version = ABCMeta._abc_invalidation_counter
         return cls

-    def register(cls, subclass):
+    def register(cls, subclass=None):
         """Register a virtual subclass of an ABC."""
+        if subclass is None:
+            raise TypeError("register() cannot be called on an ABCMeta "
+                "subclass. Maybe you forgot the metaclass keyword
argument "
+                "when declaring the class?")
         if not isinstance(cls, type):
             raise TypeError("Can only register classes")
         if issubclass(subclass, cls):

Fixed in r59233. Please adjust the error message if you don't like it.

TypeError: register() cannot be called on an ABCMeta subclass, use class
Example(metaclass=abc.ABCMeta) instead.

Please roll this back.  The error message you added is inappropriate
when the parameter to a legitimate register() call is omitted, e.g.

collections.Sequence.register()

Since we got rid of unbound methods, the infinite recursion is gone;
that's a good enough fix, there are tons of other situations where
confusion between class and instance (or between metaclass and class)
causes confusing error messages.

On 2007-11-30, Christian Heimes wrote:
> Christian Heimes added the comment:
>
> Fixed in r59233. Please adjust the error message if you don't like it.
>
> TypeError: register() cannot be called on an ABCMeta subclass, use class
> Example(metaclass=abc.ABCMeta) instead.

I think it is fine---but seems that GvR doesn't want it!

I've reverted the changes.