py.test - how to use a context manager in a funcarg/fixture

Question

Closely related: In python, is there a good idiom for using context managers in setup/teardown

---

I have a context manager that is used in tests to fix the time/timezone. I want to have it in a pytest funcarg (or fixture, we are using pytest 2.2.3 but I can translate backwards). I could just do this:

def pytest_funcarg__fixedTimezone(request):
    # fix timezone to match Qld, no DST to worry about and matches all
    # Eastern states in winter.
    fixedTime = offsetTime.DisplacedRealTime(tz=' Australia/Brisbane')

    def setup():
        fixedTime.__enter__()
        return fixedTime

    def teardown(fixedTime):
        # this seems rather odd?
        fixedTime.__exit__(None, None, None)

... but it's a bit icky. In the related Q jsbueno points out: The problem is that your code has no provision to call the object's __exit__ method properly if an exception occurs.

His answer uses a metaclass approach. But this is not that useful for pytest where often tests are just functions, not classes. So what would be the pytest-y way to solve this? Something involving runtest hooks?

Answer 1

Since 2.4, py.test has yield style fixture support. We could use a with context inside it directly.

@pytest.yield_fixture
def passwd():
    with open("/etc/passwd") as f:
        yield f.readlines()

Since 3.0, py.test deprecated the @pytest.yield_fixture usage. We could use @pytest.fixture as a context manager directly.

@pytest.fixture
def passwd():
    with open("/etc/passwd") as f:
        yield f.readlines()

Answer 2

I'm afraid there is currently no elegant way of using context managers in fixtures. However the finalizers will run if the test fails:

import contextlib, pytest

@contextlib.contextmanager
def manager():
    print 'manager enter'
    yield 42
    print 'manager exit'

@pytest.fixture
def fix(request):
    m = manager()
    request.addfinalizer(lambda: m.__exit__(None, None, None))
    return m.__enter__()

def test_foo(fix):
    print fix
    raise Exception('oops')

If you run this with pytest -s you will see that the __exit__() call happens.