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ABCs, even though they are almost always at the bottom of an inheritance hierarchy, should still do the right thing in the face of being in the middle of an MRO. That means that they should call super() as appropriate. So for methods that return a value, blindly call super(). For methods that do not necessarily return anything (e.g. invalidate_caches()), check if super() as the method and it is callable and if that is true then make the super() call.
This is not backwards-compatible as it is new semantics people will rely on, but neither is it a bug but a bad design decision on my part.

Blog post I wrote explaining what I plan to do: http://sayspy.blogspot.ca/2013/02/remember-to-use-super-in-your-abcs.html 

OK, rewrote that blog post as Thomas pointed out my thinking was worrying about stuff I shouldn't be: http://sayspy.blogspot.ca/2013/02/remember-to-use-super-in-your-abcs.html 

And as part of this I need to update the docstrings to mention default reactions.

New changeset 8e733e30edf6 by Brett Cannon in branch 'default':
Issue #17093,17566,17567: Methods from classes in importlib.abc now raise/return
http://hg.python.org/cpython/rev/8e733e30edf6
New changeset 7e91dc7221d5 by Brett Cannon in branch 'default':
What's new entry for issue #17093
http://hg.python.org/cpython/rev/7e91dc7221d5