testTmpExists fails if TMPDIR != /tmp on Linux

[mgorny]

Describe the bug
When TMPDIR is not /tmp (or a subdirectory of /tmp), the following test failure occurs:

======================================================================
FAIL: testTmpExists (pyfakefs.tests.fake_filesystem_unittest_test.TestTempDirCreation)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/tmp/pyfakefs/pyfakefs/tests/fake_filesystem_unittest_test.py", line 666, in testTmpExists
    self.assertTrue(os.path.exists("/tmp"))
AssertionError: False is not true

I think this is because https://github.com/pytest-dev/pyfakefs/blob/main/pyfakefs/fake_filesystem_unittest.py#L894-L903 only creates /tmp symlink on Darwin (and only TMPDIR on Linux) but https://github.com/pytest-dev/pyfakefs/blob/main/pyfakefs/tests/fake_filesystem_unittest_test.py#L663-L666 assumes /tmp on all non-win32 platforms.

How To Reproduce

export TMPDIR=/var/tmp
tox

Your environment
Please run the following in the environment where the problem happened and
paste the output.

python -c "import platform; print(platform.platform())"
python -c "import sys; print('Python', sys.version)"
python -c "from pyfakefs.fake_filesystem import __version__; print('pyfakefs', __version__)"
python -c "import pytest; print('pytest', pytest.__version__)"

Linux-6.2.10-gentoo-dist-x86_64-AMD_Ryzen_5_3600_6-Core_Processor-with-glibc2.37
Python 3.11.3 (main, Apr  6 2023, 04:25:37) [GCC 12.2.1 20230304]
Traceback (most recent call last):
  File "<string>", line 1, in <module>
ImportError: cannot import name '__version__' from 'pyfakefs.fake_filesystem' (/tmp/pyfakefs/pyfakefs/fake_filesystem.py)
pytest 7.3.0

(git 1906fde)

[mrbean-bremen]

Thanks! Are you aware if there is always a /tmp dir or link under linux, so that we have do the same as under darwin? Otherwise, we just just would have to adapt the test.

[mgorny]

According to FHS, /tmp must exist — and I think it'd be reasonable to assume it will exist on any Unix system, as too many scripts assume that one does exist.

(of course, using TMPDIR over /tmp [i.e. what gettempdir() returns] is preferable)

[mrbean-bremen]

Ok, thanks - in this case I will just add that link also under linux. Will probably do that tonight...

[mgorny]

Thanks!

[mgorny]

Thank you!

[mrbean-bremen]

Thanks for the report! I will make a new patch release in a moment (because of a regression), so this will also be in the release.

[davidlbaird]

I think that adding new behaviors like this should be introduced through a feature flag first. There are existing unit tests and fake filesystem libraries that subclass pyfakefs that assume an empty fake file system.