Here's an example test that fails:

def test_isabstract_during_init_subclass(self):
    from abc import ABCMeta, abstractmethod

    isabstract_checks = []

    class AbstractChecker(metaclass=ABCMeta):
        def __init_subclass__(cls):
            abstract_checks.append(inspect.isabstract(cls))

    class AbstractClassExample(AbstractChecker):

        @abstractmethod
        def foo(self):
            pass

    class ClassExample(AbstractClassExample):
        def foo(self):
            pass

    self.assertEqual(isabstract_checks, [True, False])

To run the test, you'll need to be on a version of python where bpo-29581 is fixed (e.g., a cpython branch with https://github.com/python/cpython/pull/527 merged) in order for __init_subclass__ to work with ABCMeta at all in the first place. I have a simple patch to inspect.isabstract that fixes this, and will make a PR shortly.

Serhiy, sorry for a distraction, but it looks like here is one more situation where inspect.isabstract is problematic, similar to what was discussed in http://bugs.python.org/issue29638 recently.

Is this an alternate fix of issue29638?

I didn't know about issue29638, and I'm not sure whether my PR fixes it. Looking at that bug, I don't think that my PR would fix it, because I still trust TPFLAGS_IS_ABSTRACT when __abstractmethods__ exists. That said, I'm not clear on how the cache works, so it's possible that my PR would fix 29638.

My issue appears when one uses py3.6's new __init_subclass__ hook with an ABC. __init_subclass__ is run by type.__new__, which means that, as of py3.6, users can (in a natural/reasonable way) inspect ABCMeta classes before ABCMeta.__new__ finishes executing. I didn't see any reasonable way to have ABCMeta.__new__ finish setting up the ABC before calling super().__new__, so I figured the fix should go into inspect.isabstract. But there may be better solutions I just didn't think of.

LGTM, but would be nice if Yury and Nick take a look.

New changeset fcfe80ec2592fed8b3941c79056a8737abef7d3b by Serhiy Storchaka (Nate) in branch 'master':
bpo-29822: Make inspect.isabstract() work during __init_subclass__. (#678)
https://github.com/python/cpython/commit/fcfe80ec2592fed8b3941c79056a8737abef7d3b

New changeset 09b6c0c71ea944f7e8b46998f3ebaf5b9fbe15f6 by Serhiy Storchaka (Nate) in branch '3.6':
[3.6] bpo-29822: make inspect.isabstract() work during __init_subclass__ (#1979)
https://github.com/python/cpython/commit/09b6c0c71ea944f7e8b46998f3ebaf5b9fbe15f6

Thank you for your contribution Nate.