[Python-Dev] Looking for an "import" expert

[Tim Peters]

Fred Drake opened an irritating <wink> bug report (#112436).

Cut to the chase:

regrtest.py imports test_support.
test_support.verbose is 1 after that.
regrtest's main() reaches into test_support and
    stomps on test_support.verbose, usually setting to 0.

Now in my build directory, if I run

   python ..\lib\test\regrtest.py test_getopt

the test passes.  However, it *shouldn't* (and the crux of Fred's bug report
is that it does fail when he runs regrtest in an old & deprecated way).

What happens is that test_getopt.py has this near the top:

    from test.test_support import verbose

and this is causing *another* copy of the test_support module to get loaded,
and *its* verbose attr is 1.

So when we run test_getopt "normally" via regrtest, it incorrectly believes
that verbose is 1, and the "expected result" file (which I generated via
regrtest -g) is in fact verbose-mode output.

If I change the import at the top of test_getopt.py to

    from test import test_support
    from test_support import verbose

then test_getopt.py sees the 0 it's supposed to see.

The story is exactly the same, btw, if I run regrtest while *in* the test
directory (so this has nothing to do with that I usually run regrtest from
my build directory).

Now what *Fred* does is equivalent to getting into a Python shell and typing

>>> from test import regrtest
>>> regrtest.main()

and in *that* case (the original) test_getopt sees the 0 it's supposed to
see.

I confess I lost track how fancy Python imports work a long time ago.  Can
anyone make sense of these symptoms?  Why is a 2nd version of test_support
getting loaded, and why only sometimes?