This command reports an "invalid warning category"
error, but it shouldn't.

./python -W'ignore:Not importing directory:ImportWarning'

I think the change to new-style exceptions exposed this
problem.  warnings.py contains the following code to
trigger this error:

    if (not isinstance(cat, types.ClassType) or
        not issubclass(cat, Warning)):
        raise _OptionError("invalid warning category:
%r" % (category,))

The new-style exceptions mean that the first
isinstance() now returns False.   I think there may be
two fixes needed.  First, I think it should be
"isinstance(cat, types.ClassType) or isinstance(cat,
type)".

Second issue: hould the warnings.py code use "and"
instead of "or"?  Surely if issubclass(cat, Warning) is
True, the category is certainly OK; no?

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Once the problem is fixed, the attached patch adds some
tests for warning parsing.

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Well, the docs for warnings
(http://docs.python.org/dev/lib/warning-categories.html) say
that a category must subclass Warning.  So I removed the
ClassType check entirely, and that causes test_exceptions to
segfault.  Lovely.

I will try to look at this Friday.

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Turns out Armin's r47061 checkin fixed it; for some reason I
had not rebuilt.  Doing a distclean and re-running tests
before committing the change.

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r47072 has just the check for the subclass of Warning and
added your tests.  Thanks, Andrew.