Cause test failure from pytest autouse fixture

Question

pytest allows the creation of fixtures that are automatically applied to every test in a test suite (via the autouse keyword argument). This is useful for implementing setup and teardown actions that affect every test case. More details can be found in the pytest documentation.

In theory, the same infrastructure would also be very useful for verifying post-conditions that are expected to exist after each test runs. For example, maybe a log file is created every time a test runs, and I want to make sure it exists when the test ends.

Don't get hung up on the details, but I hope you get the basic idea. The point is that it would be tedious and repetitive to add this code to each test function, especially when autouse fixtures already provide infrastructure for applying this action to every test. Furthermore, fixtures can be packaged into plugins, so my check could be used by other packages.

The problem is that it doesn't seem to be possible to cause a test failure from a fixture. Consider the following example:

@pytest.fixture(autouse=True)
def check_log_file():
    # Yielding here runs the test itself
    yield

    # Now check whether the log file exists (as expected)
    if not log_file_exists():
        pytest.fail("Log file could not be found")

In the case where the log file does not exist, I don't get a test failure. Instead, I get a pytest error. If there are 10 tests in my test suite, and all of them pass, but 5 of them are missing a log file, I will get 10 passes and 5 errors. My goal is to get 5 passes and 5 failures.

So the first question is: is this possible? Am I just missing something? This answer suggests to me that it is probably not possible. If that's the case, the second question is: is there another way? If the answer to that question is also "no": why not? Is it a fundamental limitation of pytest infrastructure? If not, then are there any plans to support this kind of functionality?

Answer 1

In pytest, a yield-ing fixture has the first half of its definition executed during setup and the latter half executed during teardown. Further, setup and teardown aren't considered part of any individual test and thus don't contribute to its failure. This is why you see your exception reported as an additional error rather than a test failure.

---

On a philosophical note, as (cleverly) convenient as your attempted approach might be, I would argue that it violates the spirit of test setup and teardown and thus even if you could do it, you shouldn't. The setup and teardown stages exist to support the execution of the test—not to supplement its assertions of system behavior. If the behavior is important enough to assert, the assertions are important enough to reside in the body of one or more dedicated tests.

If you're simply trying to minimize the duplication of code, I'd recommend encapsulating the assertions in a helper method, e.g., assert_log_file_cleaned_up(), which can be called from the body of the appropriate tests. This will allow the test bodies to retain their descriptive power as specifications of system behavior.

Answer 2

AFAIK it isn't possible to tell pytest to treat errors in particular fixture as test failures.

I also have a case where I would like to use fixture to minimize test code duplication but in your case pytest-dependency may be a way to go.

Moreover, test dependencies aren't bad for non-unit tests and be careful with autouse because it makes tests harder to read and debug. Explicit fixtures in test function header give you at least some directions to find executed code.

Answer 3

I prefer using context managers for this purpose:

from contextlib import contextmanager

@contextmanager
def directory_that_must_be_clean_after_use():
    directory = set()
    yield directory
    assert not directory

def test_foo():
    with directory_that_must_be_clean_after_use() as directory:
        directory.add("file")

If you absoulutely can't afford to add this one line for every test, it's easy enough to write this as a plugin.

Put this in your conftest.py:

import pytest

directory = set()

# register the marker so that pytest doesn't warn you about unknown markers
def pytest_configure(config):
    config.addinivalue_line("markers", 
            "directory_must_be_clean_after_test: the name says it all")

# this is going to be run on every test 
@pytest.hookimpl(hookwrapper=True)
def pytest_runtest_call(item):
    directory.clear()
    yield
    if item.get_closest_marker("directory_must_be_clean_after_test"):
        assert not directory

And add the according marker to your tests:

# test.py
import pytest
from conftest import directory

def test_foo():
    directory.add("foo file")

@pytest.mark.directory_must_be_clean_after_test
def test_bar():
    directory.add("bar file")

Running this will give you:

fail.py::test_foo PASSED
fail.py::test_bar FAILED
...
>           assert not directory
E           AssertionError: assert not {'bar file'}

conftest.py:13: AssertionError

You don't have to use markers, of course, but these allow controlling the scope of the plugin. You can have the markers per-class or per-module as well.