TypeError: unhashable type

[trulave]

Describe the bug
Call setUp fails on fake_filesystem_unittest.py::_find_modules when one of the modules has unhashable type. I don't know wether is a bad (Undefined) module or not, but I think this is weird behavior.

self = <pyfakefs.fake_filesystem_unittest.Patcher object at 0x7fdc28337d50>

    def _find_modules(self) -> None:
        """Find and cache all modules that import file system modules.
        Later, `setUp()` will stub these with the fake file system
        modules.
        """
        module_names = list(self._fake_module_classes.keys()) + [PATH_MODULE]
        for name, module in list(sys.modules.items()):
            try:
                if (
                    self.use_cache
                    and module in self.CACHED_MODULES
                    or not inspect.ismodule(module)
                ):
                    continue
            except Exception:
                # workaround for some py (part of pytest) versions
                # where py.error has no __name__ attribute
                # see https://github.com/pytest-dev/py/issues/73
                # and any other exception triggered by inspect.ismodule
                if self.use_cache:
>                   self.__class__.CACHED_MODULES.add(module)
E                   TypeError: unhashable type: 'Undefined'

List of unhashable modules:

>>> [i for i in sys.modules.items() if not isinstance(i[1], typing.Hashable)]
[('undefined', Undefined)]

How To Reproduce

1. Install suspicious unhashable module such as Undefined
2. Run tests which uses pyfakefs

Environment

Linux-6.2.0-37-generic-x86_64-with-glibc2.35
Python 3.11.7 (main, Dec  8 2023, 18:56:58) [GCC 11.4.0]
pyfakefs 5.3.2
pytest 7.4.3

[mrbean-bremen]

Thanks, good catch! I hadn't thought about this possibility... I have to think what to do in this case... maybe just ignore the module.