Stop using the `imp` module

[Aymeric Augustin]

It's deprecated in Python 3.4: ​http://docs.python.org/3.4/library/imp. Django uses it in a handful of files.

Notably, it calls imp.acquire_lock() and imp.release_lock() in AppCache.populate(). These functions don't have a replacement. I'm not sure how to deal with that, maybe call them only on Python < 3.4?

[Aymeric Augustin]

We'll most likely have to write some wrappers in django.utils.importlib to cover all Python versions.

[loic84]

As @aaugustin pointed in a github comment, these functions used to be implemented as normal reentrant threads and that caused deadlocks (#18251).

It seems the behavior of those functions changed in 3.3 as well, dunno if the change affected us:

"Changed in version 3.3: The locking scheme has changed to per-module locks for the most part. A global import lock is kept for some critical tasks, such as initializing the per-module locks."

FWIW the python bug where the deprecation was discussed: ​http://bugs.python.org/issue17177.

[merb]

@aaugustin i think we could just ignore this issue.

Currently I tried: https://code.djangoproject.com/attachment/ticket/18251/threading_lock.tar.gz
On Python 2.7.5+ and on Python 3.3.2+ the issue never occured. Without calling imp.acquire_lock() or imp.release_lock()

Maybe the "test" itself is somehow not correct anymore.

But I think its due to the fact that we now (since the last commit ​https://github.com/django/django/compare/fe1389e911b0...4a56a93cc458) use ​https://pypi.python.org/pypi/importlib a backport of importlib for python 2.7 due to the fact that the next django version won't be compatible to python 2.6. So just remove imp and we are fine. (btw. imp.acquire_lock() does nothing when you are working on master)

[Aymeric Augustin <aymeric.augustin@…>]

In d674fe6dee16735dd2670243153326806b7e6cb0:

Used a regular lock for app registry population.

Since the app registry is always populated before the first request is
processed, the situation described in #18251 for the old app cache
cannot happen any more.

Refs #18251, #21628.

[Aymeric Augustin]

The app registry no longer uses the import lock, but there's still 5 uses of imp.find_module, one of imp.load_module and one of imp.new_module.

[Marc Tamlyn]

It's worth noting that imp.find_module is the only one of these which is used in the source code (4 times), the others are only used in tests. Two of the uses in the main codebase are in a function which needs rewriting as it's about the old sys.path hackery from the old project layout, and we've already decided we are willing to *maybe* break some stuff there. There's already a TODO to this effect.

The others are in module_loading and we will need to write a separate path for py3+

[Alex Gaynor <alex.gaynor@…>]

In 9d487ae2a16d1efd501e65c78e6885c8cdcd4421:

Refs #21628 -- removed one usage of the imp module in the tests. It is deprecated in Python 3.4

[Claude Paroz]

FWIW, the only remaining usage of imp is in django.utils.module_loading.module_has_submodule (+ in the corresponding test module, test_module_loading.py).

[Aymeric Augustin <aymeric.augustin@…>]

In d7f1f316bcb21853597515a25e6e43639b9ed021:

Simplified module_has_submodule on Python >= 3.3.

Stopped using the imp module on Python >= 3.3. Refs #21628.

[Aymeric Augustin]

Removing usage in the tests is quite difficult. I tried to use the replacements from importlib but I keep hitting infinite recursions.

In fact it's a bit difficult to figure out what matters in the test. They were introduced in #13464 but that ticket is short on details.

If I understand Python's deprecation policy correctly, the imp module won't be removed before Python 4. We could simply silence the deprecation warning.

The alternative would be to run these tests only under Python < 3.3. The implementation on Python >= 3.3 is drastically simpler and may not require as many tests.

[Collin Anderson]

I think the one remaining case is in ProxyFinder in tests/utils_tests/test_module_loading.py

[Tim Graham]

Another usage popped up:

db/migrations/loader.py: six.moves.reload_module(module)

This uses imp.reload() on Python 3. It should use importlib.reload() on Python 3.4+ to avoid a warning. ​Created ticket to see if it can be addressed in six.

[Tim Graham]

Issue in previous comment will be fixed in the next version of six. I don't think there are any remaining tasks for this ticket.

[Tim Graham]

Oops, import imp still exists in tests/utils_tests/test_module_loading.py.

[Tim Graham]

The test from #15662 only affected a Python 2 code path. With Python 2 support removed in master, it seems okay to remove it. ​PR

[GitHub <noreply@…>]

In 9e9e7373:

Refs #23919 -- Removed an obsolete test for a Python 2 code path (refs #15662).

Fixed #21628 by removing the last usage of the imp module.