Fs fixture prevents creation of Flask test client in fixtures

[ROldford]

Describe the bug
When the fs fixture is used as input into a fixture yielding a Flask test client, using the test client fixture causes the test to error with the error message "importlib.metadata.PackageNotFoundError: No package metadata was found for werkzeug"

Test stack trace:

_______________________________ ERROR at setup of test_app ________________________________

cls = <class '_pytest.runner.CallInfo'>
func = <function call_and_report.<locals>.<lambda> at 0x10bdac820>, when = 'setup'
reraise = (<class '_pytest.outcomes.Exit'>, <class 'KeyboardInterrupt'>)

    @classmethod
    def from_call(
        cls,
        func: Callable[[], TResult],
        when: Literal["collect", "setup", "call", "teardown"],
        reraise: type[BaseException] | tuple[type[BaseException], ...] | None = None,
    ) -> CallInfo[TResult]:
        """Call func, wrapping the result in a CallInfo.
    
        :param func:
            The function to call. Called without arguments.
        :type func: Callable[[], _pytest.runner.TResult]
        :param when:
            The phase in which the function is called.
        :param reraise:
            Exception or exceptions that shall propagate if raised by the
            function, instead of being wrapped in the CallInfo.
        """
        excinfo = None
        start = timing.time()
        precise_start = timing.perf_counter()
        try:
>           result: TResult | None = func()

/Users/ryanoldford/Programming/Work/Berlinguette/TryingThingsOut/fakefs_importlib_metadata_bug/.venv/lib/python3.10/site-packages/_pytest/runner.py:341: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
/Users/ryanoldford/Programming/Work/Berlinguette/TryingThingsOut/fakefs_importlib_metadata_bug/.venv/lib/python3.10/site-packages/_pytest/runner.py:242: in <lambda>
    lambda: runtest_hook(item=item, **kwds), when=when, reraise=reraise
/Users/ryanoldford/Programming/Work/Berlinguette/TryingThingsOut/fakefs_importlib_metadata_bug/.venv/lib/python3.10/site-packages/pluggy/_hooks.py:513: in __call__
    return self._hookexec(self.name, self._hookimpls.copy(), kwargs, firstresult)
/Users/ryanoldford/Programming/Work/Berlinguette/TryingThingsOut/fakefs_importlib_metadata_bug/.venv/lib/python3.10/site-packages/pluggy/_manager.py:120: in _hookexec
    return self._inner_hookexec(hook_name, methods, kwargs, firstresult)
/Users/ryanoldford/Programming/Work/Berlinguette/TryingThingsOut/fakefs_importlib_metadata_bug/.venv/lib/python3.10/site-packages/_pytest/unraisableexception.py:90: in pytest_runtest_setup
    yield from unraisable_exception_runtest_hook()
/Users/ryanoldford/Programming/Work/Berlinguette/TryingThingsOut/fakefs_importlib_metadata_bug/.venv/lib/python3.10/site-packages/_pytest/unraisableexception.py:70: in unraisable_exception_runtest_hook
    yield
/Users/ryanoldford/Programming/Work/Berlinguette/TryingThingsOut/fakefs_importlib_metadata_bug/.venv/lib/python3.10/site-packages/_pytest/logging.py:840: in pytest_runtest_setup
    yield from self._runtest_for(item, "setup")
/Users/ryanoldford/Programming/Work/Berlinguette/TryingThingsOut/fakefs_importlib_metadata_bug/.venv/lib/python3.10/site-packages/_pytest/logging.py:829: in _runtest_for
    yield
/Users/ryanoldford/Programming/Work/Berlinguette/TryingThingsOut/fakefs_importlib_metadata_bug/.venv/lib/python3.10/site-packages/_pytest/capture.py:893: in pytest_runtest_setup
    return (yield)
/Users/ryanoldford/Programming/Work/Berlinguette/TryingThingsOut/fakefs_importlib_metadata_bug/.venv/lib/python3.10/site-packages/_pytest/threadexception.py:87: in pytest_runtest_setup
    yield from thread_exception_runtest_hook()
/Users/ryanoldford/Programming/Work/Berlinguette/TryingThingsOut/fakefs_importlib_metadata_bug/.venv/lib/python3.10/site-packages/_pytest/threadexception.py:68: in thread_exception_runtest_hook
    yield
/Users/ryanoldford/Programming/Work/Berlinguette/TryingThingsOut/fakefs_importlib_metadata_bug/.venv/lib/python3.10/site-packages/_pytest/runner.py:160: in pytest_runtest_setup
    item.session._setupstate.setup(item)
/Users/ryanoldford/Programming/Work/Berlinguette/TryingThingsOut/fakefs_importlib_metadata_bug/.venv/lib/python3.10/site-packages/_pytest/runner.py:514: in setup
    col.setup()
/Users/ryanoldford/Programming/Work/Berlinguette/TryingThingsOut/fakefs_importlib_metadata_bug/.venv/lib/python3.10/site-packages/_pytest/python.py:1630: in setup
    self._request._fillfixtures()
/Users/ryanoldford/Programming/Work/Berlinguette/TryingThingsOut/fakefs_importlib_metadata_bug/.venv/lib/python3.10/site-packages/_pytest/fixtures.py:697: in _fillfixtures
    item.funcargs[argname] = self.getfixturevalue(argname)
/Users/ryanoldford/Programming/Work/Berlinguette/TryingThingsOut/fakefs_importlib_metadata_bug/.venv/lib/python3.10/site-packages/_pytest/fixtures.py:532: in getfixturevalue
    fixturedef = self._get_active_fixturedef(argname)
/Users/ryanoldford/Programming/Work/Berlinguette/TryingThingsOut/fakefs_importlib_metadata_bug/.venv/lib/python3.10/site-packages/_pytest/fixtures.py:617: in _get_active_fixturedef
    fixturedef.execute(request=subrequest)
/Users/ryanoldford/Programming/Work/Berlinguette/TryingThingsOut/fakefs_importlib_metadata_bug/.venv/lib/python3.10/site-packages/_pytest/fixtures.py:1091: in execute
    result = ihook.pytest_fixture_setup(fixturedef=self, request=request)
/Users/ryanoldford/Programming/Work/Berlinguette/TryingThingsOut/fakefs_importlib_metadata_bug/.venv/lib/python3.10/site-packages/pluggy/_hooks.py:513: in __call__
    return self._hookexec(self.name, self._hookimpls.copy(), kwargs, firstresult)
/Users/ryanoldford/Programming/Work/Berlinguette/TryingThingsOut/fakefs_importlib_metadata_bug/.venv/lib/python3.10/site-packages/pluggy/_manager.py:120: in _hookexec
    return self._inner_hookexec(hook_name, methods, kwargs, firstresult)
/Users/ryanoldford/Programming/Work/Berlinguette/TryingThingsOut/fakefs_importlib_metadata_bug/.venv/lib/python3.10/site-packages/_pytest/setuponly.py:36: in pytest_fixture_setup
    return (yield)
/Users/ryanoldford/Programming/Work/Berlinguette/TryingThingsOut/fakefs_importlib_metadata_bug/.venv/lib/python3.10/site-packages/_pytest/fixtures.py:1140: in pytest_fixture_setup
    result = call_fixture_func(fixturefunc, request, kwargs)
/Users/ryanoldford/Programming/Work/Berlinguette/TryingThingsOut/fakefs_importlib_metadata_bug/.venv/lib/python3.10/site-packages/_pytest/fixtures.py:891: in call_fixture_func
    fixture_result = next(generator)
/Users/ryanoldford/Programming/Work/Berlinguette/TryingThingsOut/fakefs_importlib_metadata_bug/conftest.py:16: in test_client
    yield app_fixture.test_client()
/Users/ryanoldford/Programming/Work/Berlinguette/TryingThingsOut/fakefs_importlib_metadata_bug/.venv/lib/python3.10/site-packages/flask/app.py:723: in test_client
    return cls(  # type: ignore
/Users/ryanoldford/Programming/Work/Berlinguette/TryingThingsOut/fakefs_importlib_metadata_bug/.venv/lib/python3.10/site-packages/flask/testing.py:131: in __init__
    "HTTP_USER_AGENT": f"Werkzeug/{_get_werkzeug_version()}",
/Users/ryanoldford/Programming/Work/Berlinguette/TryingThingsOut/fakefs_importlib_metadata_bug/.venv/lib/python3.10/site-packages/flask/testing.py:103: in _get_werkzeug_version
    _werkzeug_version = importlib.metadata.version("werkzeug")
/Users/ryanoldford/.pyenv/versions/3.10.5/lib/python3.10/importlib/metadata/__init__.py:984: in version
    return distribution(distribution_name).version
/Users/ryanoldford/.pyenv/versions/3.10.5/lib/python3.10/importlib/metadata/__init__.py:957: in distribution
    return Distribution.from_name(distribution_name)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

cls = <class 'importlib.metadata.Distribution'>, name = 'werkzeug'

    @classmethod
    def from_name(cls, name):
        """Return the Distribution for the given package name.
    
        :param name: The name of the distribution package to search for.
        :return: The Distribution instance (or subclass thereof) for the named
            package, if found.
        :raises PackageNotFoundError: When the named package's distribution
            metadata cannot be found.
        """
        for resolver in cls._discover_resolvers():
            dists = resolver(DistributionFinder.Context(name=name))
            dist = next(iter(dists), None)
            if dist is not None:
                return dist
        else:
>           raise PackageNotFoundError(name)
E           importlib.metadata.PackageNotFoundError: No package metadata was found for werkzeug

/Users/ryanoldford/.pyenv/versions/3.10.5/lib/python3.10/importlib/metadata/__init__.py:548: PackageNotFoundError

How To Reproduce
Import these packages: Flask 3.1.0, pyfakefs 5.8.0, pytest 8.3.5

Create the following files:
app.py

from contextlib import contextmanager
from typing import Generator
from flask import Flask
from local_types import ApiResponse


@contextmanager
def create_app() -> Generator[Flask, None, None]:
    app = Flask(__name__)
    
    @app.route("/", methods=["GET"])
    def basic_get() -> ApiResponse:
        return {"message": "Works!"}, 200
    
    yield app

local_types.py

from typing import Any, Generator, TypeVar

ApiResponse = tuple[dict[str, Any], int]


T = TypeVar("T")
YieldFixture = Generator[T, Any, Any]

conftest.py

import pytest
from local_types import YieldFixture
from flask import Flask
from flask.testing import FlaskClient
from app import create_app


@pytest.fixture(scope="function")
def app_fixture(fs) -> YieldFixture[Flask]:
    # mocking goes here
    with create_app() as _app:
        yield _app

@pytest.fixture(scope="function")
def test_client(app_fixture: Flask) -> YieldFixture[FlaskClient]:
    yield app_fixture.test_client()

test_app.py

from flask.testing import FlaskClient

def test_app(test_client: FlaskClient, fs):
    response = test_client.get("/")
    assert response.status_code == 200
    response_json = response.json
    assert isinstance(response_json, dict)
    assert response_json.get("message") == "Works!"

Run pytest and you will see the error. If you remove fs from the fixture parameters, the tests will pass normally.

Your environment
macOS-15.3.2-x86_64-i386-64bit
Python 3.10.5 (main, Apr 1 2025, 16:28:06) [Clang 16.0.0 (clang-1600.0.26.6)]
pyfakefs 5.8.0
pytest 8.3.5

[ROldford]

As a workaround, I patched the flask.testing module in the app fixture function as follows:

@pytest.fixture(scope="function")
def app_fixture(mocker, fs) -> YieldFixture[Flask]:
    # mocking goes here
    mocker.patch("flask.testing._werkzeug_version", "3.1.3")
    with create_app() as _app:
        yield _app

It looks like _werkzeug_version is used only by the _get_werkzeug_version() function in this Flask version, so this shouldn't affect any other code, but I'm still a bit wary of doing this.

[mrbean-bremen]

Thanks, I will have a look!

[mrbean-bremen]

Ok, the problem is that importlib.metadata accesses the metadata in the filesystem to get the version of Werkzeug. This does not work with pyfakefs, because the metadata does not exist in the fake filesystem.

This kind of problems can usually be fixed by mapping the relevant files into the fake filesystem, though in this case it is a bit ugly:

import importlib
...
from pyfakefs.fake_filesystem_unittest import Pause

@pytest.fixture(scope="function")
def app_fixture(fs) -> YieldFixture[Flask]:
    with Pause(fs):
        # need to pause the patching here as this accesses the real fs
        metadata = [f for f in importlib.metadata.distribution("Werkzeug").files if "METADATA" in str(f)][0]
        metadata_path = metadata.locate()
    fs.add_real_file(metadata_path)
    with create_app() as _app:
        yield _app

Maybe a convenience function to map the metadata of a package into the fake fs would make sense. At least there shall be a recipe for resolving this in the troubleshooting section.