Some test cases in Lib/test/mapping_tests.py are problematic for users
wishing to test their own implementations of the mapping protocol: 

 - TestHashMappingProtocol.test_repr() requires the user implementations
to look like a dict when repr() is applied. It is unclear why this is
required of conforming mapping protocol implementations.
 - TestMappingProtocol.test_fromkeys() cannot pass for any
implementation that uses its constructor in fromkeys(), because baddict1
defines a constructor accepting no arguments. It should accept *args,
**kwargs to be sane for user implementations that handle passing data
sources to the constructor.
 - TestHashMappingProtocol.test_mutatingiteration(), for some faulty
implementations, makes the iteration degrade into an infinite loop.
Making the test more strict (eg. keeping an explicit iteration count and
failing if it goes >1) would be more helpful to buggy implementations.

These all seem like trivial issues. If it is agreed that the repr_test
should be removed from the ABC tests, I can provide a patch implementing
these three corrections.

Walter, this is your code.  Care to incorporate any of David's ideas?

David, for my own code, the usual technique for something like
test_repr() is to subclass TestHashMappingProtocol and override the test
method with a customized test_repr() method.

Any custom mapping class should have a repr test anyway, so IMHO it
doesn't matter whether the base test has a repr test or not.

The suggested fixes for TestMappingProtocol.test_fromkeys() and
TestHashMappingProtocol.test_mutatingiteration() sound OK however.

David, can you provide a patch?