pyfakefs breaks subprocesses

[majuscule]

pyfakefs breaks stdlib subprocess calls as shown below. Running on Ubuntu 16.04.5 LTS.

import subprocess

def test_fake_fs(fs):
    assert subprocess.run(['echo'])

self = <pyfakefs.fake_filesystem.FakeFilesystem object at 0x7fcbfda2acc0>, errno = 9, filename = '9', winerror = None

    def raise_os_error(self, errno, filename=None, winerror=None):
        """Raises OSError.
        The error message is constructed from the given error code and shall
        start with the error string issued in the real system.
        Note: this is not true under Windows if winerror is given - in this
        case a localized message specific to winerror will be shown in the
        real file system.
    
        Args:
            errno: A numeric error code from the C variable errno.
            filename: The name of the affected file, if any.
            winerror: Windows only - the specific Windows error code.
        """
        message = self._error_message(errno)
        if (winerror is not None and sys.platform == 'win32' and
                self.is_windows_fs):
            if IS_PY2:
                raise WindowsError(winerror, message, filename)
            raise OSError(errno, message, filename, winerror)
>       raise OSError(errno, message, filename)
E       OSError: [Errno 9] Bad file descriptor in the fake filesystem: '9'

[majuscule]

On second look, I'm guessing this is due to "pyfakefs will not work with Python libraries that use C libraries to access the file system."

[mrbean-bremen]

Yep - correct. Sorry, out of luck here!

[majuscule]

Is there any way to temporarily escape the fakefs from a test function, in order to run necessary subprocesses, add their output to the fakefs tree, and return? If not, would this be considered if submitted as a feature request? Thanks for the module!

[mrbean-bremen]

Hm, no such thing there at the moment. As I understand, you want to to "pause" using the fake filesystem inside a test, use the real file system for a while, and restore the previous fake file system state later? Can you eloborate a bit on your test setup?
And yes, this can certainly be submitted as a feature request, though I can't promise any fast solution at the moment...

[majuscule]

Yes that is correct. My project is manipulating a git repository and retrieving files from various checkouts. To do this I need a git subprocess, however I desire to test all other functionality (which includes dangerous operations such as file deletions) with the fakefs for safety. The process controlling these checkouts is recursive, which makes it impossible to complete all checkouts before engaging the fakefs. If I was able to "pause" the fakefs, return to the real fs, checkout, add the resulting files to the fakefs, and then resume, it would allow me to continue with this solution. I'll submit a new issue for the feature request. Thank you!

[GozzoMan]

Hello, same problem here.

In my case, it would be sufficient to have subprocess.Popen() do nothing, instead of raising bad file descriptor.

At the moment I'm using the following hack (which satisfy how subprocess is used in the target code):

origSubprocessPopen = subprocess.Popen

def setUpModule():

    # ...

    class mockOut:
        def __enter__(self):
            return []

        def __exit__(self, type, value, traceback):
            pass

    class mockPopen:
        def __init__(self):
            self.stdout = mockOut()

        def wait(self):
            return 0

    subprocess.Popen = lambda *args, **kwargs : mockPopen()

def tearDownModule():

    subprocess.Popen = origSubprocessPopen

[mrbean-bremen]

Well, if you just want to ignore that call, that is a valid solution, which is used often enough in Python.
If you really need to access the file system in that call, you can instead pause mocking during the call (as has been introduced in pyfakefs as proposed by @majuscule).