I noticed this weird behavior in Python 3.6:

Apparently import package.module as _alias fails in a threaded environment, sometimes raising an ImportError, seemingly at random.

Everything seems to be working perfectly fine:

• On 3.5 and lower
• When not defining an alias, eg. import package.module
• When using the from package import module as alias style

See this repo with code for reproducing the issue:

https://github.com/rshk/python36-import-alias-concurrency-bug

I tested this on the system Python 3.6.1 shipped with Archlinux (both inside and outside of a virtualenv), and in the official docker image for 3.6.

This all started here psycopg/psycopg2#550 as others were experiencing this issue too.

See also bpo-23203.

This was fixed in master in bpo-30024. Maybe it is worth to backport that change to maintained versions?

On other side, bpo-23203 fixed just a symptom. The issue is reproduced in master when replace "import foobar.submodule as foo" with "import foobar.submodule; foo = foobar.submodule".

The regression may be related to bpo-22557. Either the optimization introduced a bug, or it exposed an existing bug by changing timings.

My guess is the import of package has completed but package.submodule hasn't (I don't know what state it would be in, but obviously before the module is assigned as an attribute on the package), a context switch occurs, and then the assignment fails due to package.submodule not being set on package yet.

I wonder if package.submodule is in sys.modules when the AttributeError is triggered? I don't remember when the assignment of a submodule to an attriute on a package occurs, but this suggests it's rather late and that's why this is failing. Maybe the assignment is outside of the per-module lock being released and that is what's causing this?

Hey, at least importing in a thread doesn't deadlock anymore like it used to. ;)

Documenting explicitly what I believe the expected order of module lock acquisition would be in this case:

Thread A acquires package.subpackage lock
Thread B blocks on package.subpackage lock
Thread A acquires package lock
Thread A releases package lock
Thread A releases package.subpackage lock
Thread B acquires & immediately releases package.subpackage lock

Looking at https://hg.python.org/cpython/rev/64f195790a3a#l4.367 (the commit for bpo-22557), there's a potentially suspect change in the scope of a "_PyImport_AcquireLock/_PyImport_ReleaseLock" pair inside PyImport_ImportModuleLevelObject

Specifically, I think we may now have a race condition at https://github.com/python/cpython/blob/master/Python/import.c#L1534, where two threads can *both* end up trying to initialize the same module, since we're no longer holding the global import lock around that "mod = PyDict_GetItem(interp->modules, abs_name);" call and the associated state updates where the first thread indicates that it is already initializing that module so the second thread should just wait for it to finish doing so.

I have came to the same conclusion.

PR 2580 adds a double check in _bootstrap._find_and_load() (and also moves the locking into it). It also simplifies the C code by removing the fast path for the case sys.module[name] is None.

New changeset b4baace by Serhiy Storchaka in branch 'master':
bpo-30814: Fixed a race condition when import a submodule from a package. (bpo-2580)
b4baace

New changeset 03b0e83 by Serhiy Storchaka in branch '3.6':
[3.6] bpo-30814: Fixed a race condition when import a submodule from a package. (GH-2580). (bpo-2598)
03b0e83

I just wanted to say thanks to everyone who helped to fix this bug. The locking situation in import is probably the trickiest part of it and has also been tweaked the most as of late and so solving these kinds of issues is tricky.

test_concurrency() of test_import fails randomly on Windows: see bpo-30891.

It is worth to backport the test to older versions. Just to be sure that this is a 3.6 regression.

When running tests from installed location, test_import now fails on master and 3.6 with:

======================================================================
ERROR: test_concurrency (test.test_import.ImportTests)
----------------------------------------------------------------------

Traceback (most recent call last):
  File "/Users/nad/Projects/PyDev/active/dev/3x/root/uxd/lib/python3.7/test/test_import/__init__.py", line 408, in test_concurrency
    raise exc
  File "/Users/nad/Projects/PyDev/active/dev/3x/root/uxd/lib/python3.7/test/test_import/__init__.py", line 393, in run
    import package
ModuleNotFoundError: No module named 'package'

When running tests from installed location, test_import now fails on master and 3.6 with:

Which Git revisions are you testing? On master, do you have my latest commit e72b135 ?

Which Git revisions are you testing? On master, do you have my latest commit e72b135 ?

Yes, this is with current top of trunk of both branches. Perhaps I wasn't clear about "installed location". The test does not fail if you run it directly from the build directory like with "make test". It fails when you do a "make install" and then run the test using the newly installed Python. Tests need to work both ways.

Serhiy on PR 2851:

I forget to include directories with new test files in the list of the library directories. This caused that these test files were not installed.

Oh, too bad that Zach's "x86 Gentoo Installed with X 3.6" buildbot is offline.

I also opened python/devguide#241 : "Document how to add a new file or a new directory". It's not the first time that we make such mistake.

New changeset d5ed47d by Serhiy Storchaka in branch 'master':
bpo-30814, bpo-30876: Add new import test files to projects. (bpo-2851)
d5ed47d

New changeset 95b16a9 by Serhiy Storchaka in branch '3.6':
[3.6] bpo-30814, bpo-30876: Add new import test files to projects. (GH-2851). (bpo-2912)
95b16a9