Terry Reedy wrote: [...]
Since unittests have been brought up, I have a idea and question. Can this work? Split the current test suite for a concrete class that implements one of the ABCs into concrete-specific and ABC-general portions, with the abstract part parameterized by concrete class.
FWIW, bzr's test suite has this facility, and bzr plugins that implement various bzr interfaces will have tests for those interfaces automatically applied. (Being a Python 2.4+ project, bzr doesn't actually use the ABCs feature, but we certainly use the concept of "interface with many implemenations".)
E.g. if you define a new Transport (in bzr terms, a thing like FTP, HTTP, etc) you probably want to make sure it complies with bzrlib's expectations for Transports. So you can include a get_test_permutations function in your module that returns a list of (transport_class, server_class) pairs. [Unsurprisingly you need a test server to run against, although for transports like LocalTransport (local filesystem access) they can be very simple.]
It works very well, and is very useful both for bzrlib itself and plugins. We have “per-implementation” tests for: branch, bzrdir, repository, interrepository, merger, transport, tree, workingtree, uifactory, and more. Look for bzrlib/tests/per_*.
It's not necessarily easy to write all those tests. The more complex an interface, the more likely it is you'll have many tests for that interface that don't really apply to all implementations — for instance some Transports are read-only, or don't support list_dir, etc. So tests that involve those need to specifically check for that capability and raise NotApplicable, and finding the exact right way to do that can be tricky. It's often easier to say “if isinstance(thing, ParticularImplementation): ...”, but that quickly erodes the applicability of those tests for new implementations.
Also tricky is when the setup or even assertions for some tests needs to vary considerably by implementation: how complicated is your parameterisation interface going to have to be?
bzr has found it worthwhile, so I do encourage trying it. I'd use Robert Collins' http://launchpad.net/testscenarios library if I were providing this infrastructure in a suite that doesn't already have this approach; it's basically a distillation of the infrastructure developed in bzrlib.tests.