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Jonathan,

> I don't think addTest should check the type of test --
> duck typing is sufficient here. Other testing frameworks 
> should not have to subclass unittest.TestCase in order to be
> able to add their test cases to a unittest.TestSuite.

I added these checks in order to be consistent with the rest
of the module, where inheritance from TestCase is required
(TestLoader.loadTestsFromModule(),
TestLoader.loadTestsFromName(),
TestLoader.loadTestsFromNames()). This policy should be
enforced throughout the module, not piecemeal.

> In loadTestsFromName, it's not strictly necessary to
> return TestSuite([test]). The core idea behind the
> TestLoader methods is to return something that can be
> run(). Also, it's generally not a good idea to change
> long-standing behaviour to match documentation. It should
> be the other way around.

Given that the docs for TestLoader.loadTestsFrom*() have
begun with "Return a suite of all test cases" since r20345
-- that is, for the last _five years_ -- I'd say this is a
long-standing bug in the code, not the documentation.