In contextlib, there is code which roughly looks like

        def _exit_wrapper(exc_type, exc, tb):
            return cm_exit(cm, exc_type, exc, tb)
        _exit_wrapper.__self__ = cm

This creates a new function _exit_wrapper from a given function cm_exit by prepending the __self__ attribute to *args. Now this is exactly what a method does too.

It would be better to use an actual method for this: it's cleaner, faster and it doesn't abuse a double-underscore attribute. The latter will actually break with PEP 575, as __self__ will become a special name  instead of an arbitrary attribute.

Yury, could you double check the async exit stack change in the PR? I think it's fine since the bound method just passes back the underlying coroutine and the tests all still pass, but a second opinion would be good :)

Yep, I think this is a good fix!

New changeset 23ab5ee667a9b29014f6f7f01797c611f63ff743 by Nick Coghlan (jdemeyer) in branch 'master':
bpo-33265: use an actual method instead of a method-like function in ExitStack (GH-6456)
https://github.com/python/cpython/commit/23ab5ee667a9b29014f6f7f01797c611f63ff743

Classifying this as a minor performance enhancement, since methods are much simpler objects than full Python level closures.

Thanks!