tempfile.TemporaryFile does not work on Linux with a fake filesystem

[vtsatskin]

Describe the bug
When invoking tempfile.TemporaryFile on a fake filesystem under Linux, a NotImplementedError is raised. This does not occur on macOS.

How To Reproduce

Both of the following tests fail:

from pyfakefs.fake_filesystem import FakeFilesystem
import tempfile


def test_tempfile_defaults(fs: FakeFilesystem) -> None:
    tempfile.TemporaryFile()


def test_tempfile_custom_dir(fs: FakeFilesystem) -> None:
    fs.create_dir("/dir")
    tempfile.TemporaryFile(dir="/dir")

# passes, checking if temporary directory exists
def test_tempfile_get_dir(fs: FakeFilesystem) -> None:
    assert tempfile.gettempdir() == "/tmp"

With the following truncated output:

/home/nvidia/passengerai/device/.venv/python-3.6.9/lib/python3.6/tempfile.py:735: in TemporaryFile
    fd = _os.open(dir, flags2, 0o600)
        fs.create_dir("/dir")
>       tempfile.TemporaryFile(dir="/dir")

/home/nvidia/passengerai/device/tests/unit/media_streaming/storage_management/test_pyfakefs.py:11:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
/home/nvidia/passengerai/device/.venv/python-3.6.9/lib/python3.6/tempfile.py:735: in TemporaryFile
    fd = _os.open(dir, flags2, 0o600)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = <pyfakefs.fake_filesystem.FakeOsModule object at 0x7f7e8a0eb8>, path = '/dir'
flags = 4243586, mode = 384, dir_fd = None

    def open(self, path, flags, mode=None, dir_fd=None):
        """Return the file descriptor for a FakeFile.
    
        Args:
            path: the path to the file
            flags: low-level bits to indicate io operation
            mode: bits to define default permissions
                Note: only basic modes are supported, OS-specific modes are
                ignored
            dir_fd: If not `None`, the file descriptor of a directory,
                with `file_path` being relative to this directory.
                New in Python 3.3.
    
        Returns:
            A file descriptor.
    
        Raises:
            IOError: if the path cannot be found
            ValueError: if invalid mode is given
            NotImplementedError: if `os.O_EXCL` is used without `os.O_CREAT`
        """
        path = self._path_with_dir_fd(path, self.open, dir_fd)
        if mode is None:
            if self.filesystem.is_windows_fs:
                mode = 0o666
            else:
                mode = 0o777 & ~self._umask()
    
        open_modes = _OpenModes(
            must_exist=not flags & os.O_CREAT,
            can_read=not flags & os.O_WRONLY,
            can_write=flags & (os.O_RDWR | os.O_WRONLY),
            truncate=flags & os.O_TRUNC,
            append=flags & os.O_APPEND,
            must_not_exist=flags & os.O_EXCL
        )
        if open_modes.must_not_exist and open_modes.must_exist:
            raise NotImplementedError(
>               'O_EXCL without O_CREAT mode is not supported')
E           NotImplementedError: O_EXCL without O_CREAT mode is not supported

/home/nvidia/passengerai/device/.venv/python-3.6.9/lib/python3.6/site-packages/pyfakefs/fake_filesystem.py:3739: NotImplementedError
=================================== 2 failed, 1 passed in 2.63s ===================================

Your enviroment

Linux-4.9.140-tegra-aarch64-with-Ubuntu-18.04-bionic
Python 3.6.9 (default, Nov  7 2019, 10:44:02)
pyfakefs 3.7.1

This also happens on pyfakefs 4.0 from the master branch.

[mrbean-bremen]

Thanks for the report! Looks like tempfile is using some open mode that we don't correctly support - I will have a look.

[mrbean-bremen]

@vtsatskin - please check if current master works for you, and reopen if you still have problems!

[vtsatskin]

@mrbean-bremen thanks so much for the timely response and fix as usual. I've tested it and found that the issue has been resolved.

Upon further testing with my test suite, I found another bug related to tempfile.TemporaryFile and filed it in #511.