Add an environment variable to keep temporary directories when testing

[pfmoore]

This is a bit of a hack, but cleanup of temporary directories when trying to test locally is a real pain, and this gives a way out without needing to modify conftest.py.

@pypa/pip-committers Does this seem like a reasonable addition, or is it too much of a hack?

[dstufft]

It seems fine to me. Keeping the last N runs of temp files is actually the default behavior of py.test... we just override it because I think a full test run is something like 12GB of temporary files for us.

[pfmoore]

Yes, ideally it should be some sort of command line flag (so people don't get surprised by forgetting to delete the env var) but IMO it's too niche a need to be worth that extra complexity.

I never run the full test suite locally anyway, it takes way too long. But running one test to iterate on fixes for a CI failure does save time.

[dstufft]

If you want to add a CLI option, it's not very hard, you just add:

def pytest_addoption(parser):
    parser.addoption(
        "--cmdopt", action="store", default="type1", help="my option: type1 or type2"
    )

to conftest.py, then you can access it in the fixture by adding a request parameter to the function and accessing it like so:

@pytest.fixture
def cmdopt(request):
    return request.config.getoption("--cmdopt")

See more details at https://docs.pytest.org/en/latest/example/simple.html#pass-different-values-to-a-test-function-depending-on-command-line-options.

[pradyunsg]

I do run the entire test suite locally regularly (it takes less time than Travis, due to more cores/threads).

This seems reasonable to me.

[pfmoore]

@dstufft Thanks for that - it is indeed nice & easy to add a command line option, so I've done that.

[pradyunsg]

That said, @dstufft's suggestion of making this a CLI flag would also work.

Personally, I doubt I'd use this much and even when I do, both would work just fine so I don't feel strongly here.

[pradyunsg]

@pfmoore Nice timing. :P

[pfmoore]

Pinging appveyor

[lock]

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