unittest-fixtures: A small library for loading unittest fixtures

Description

unittest-fixtures spun off from my Gentoo Build Publisher project. I use unittest, the test framework in the Python standard library, where it's customary to define a TestCase's fixtures in the .setUp() method. Having done it this way for years, it occurred to me one day that this goes against OCP. What if instead of cracking open the .setUp() method to add a fixture to a TestCase one could instead add a decorator? That's what unittest-fixtures allows one to do.

from unittest_fixtures import given

@given(dog)
class MyTest(TestCase):
    def test_method(self, fixtures):
        dog = fixtures.dog

In the above example, dog is a fixture function. Fixture functions are passed to the given decorator. When the test method is run, the fixtures are "instantiated" and attached to the fixtures keyword argument of the test method.

Fixture functions

Fixture functions are functions that one defines that return a "fixture". For example the above dog fixture might look like this:

from unittest_fixtures import fixture

@fixture()
def dog(fixtures):
    return Dog(name="Fido")

Fixture functions are always passed a Fixtures argument. Because fixtures can depend on other fixtures. For example:

@fixture(dog)
def person(fixtures):
    p = Person(name="Jane")
    p.pet = fixtures.dog
    return p

Fixture functions can have keyword parameters, but those parameters must have defaults.

@fixture
def dog(fixtures, name="Fido"):
    return Dog(name=name)

Then one's TestCase can use the where decorator to passed the parameter:

from unittest_fixtures import given, where

@given(dog)
@where(dog__name="Buddy")
class MyTest(TestCase):
    def test_method(self, fixtures):
        dog = fixtures.dog
        self.assertEqual("Buddy", dog.name)

Duplicating fixtures

The unittest-fixtures library allows one to use a fixture more than once. This can be done by passing the fixture as a keyword argument giving different names to the same fixture. Different parameters can be passed to them:

@given(fido=dog, buddy=dog)
@where(fido__name="Fido", buddy__name="Buddy")
class MyTest(TestCase):
    def test_method(self, fixtures):
        self.assertEqual("Buddy", fixtures.buddy.name)
        self.assertEqual("Fido", fixtures.fido.name)

Fixture-depending fixtures will all use the same fixture, but only if they have the same name. So in the above example, if we also gave the TestCase the person fixture, that person would have a different dog because it depends on a fixture called "dog". However this will work:

@given(dog, person)
class MyTest(TestCase):
    def test_method(self, fixtures):
        dog = fixtures.dog
        person = fixtures.person
        self.assertIs(person.pet, dog)

@where (fixture parameters)

The where decorator can be used to pass parameters to a fixture function. Fixture functions are not required to take arguments. To pass a parameter to a function, for example pass name to the dog fixture it's the name of the function, followed by __ followed by the parameter name. For example: dog__name. Fixture functions can also have a parameter that is the same name as the fixture itself. For example:

@given(settings)
@where(settings={"DEBUG": True, "SECRET": "sauce"})
class MyTest(TestCase):
    ...

Fixtures as context managers

Sometimes a fixture will need a setup and teardown process. If unittest-fixtures is supposed to remove the need to open setUp(), then it must also remove the need to open tearDown(). And it does this by by defining itself as a generator function. For example:

import tempfile

@fixture()
def tmpdir(fixtures):
    with tempfile.TemporaryDirectory() as tempdir:
        yield tempdir

Using the unittest.mock library is another good example of using context manager fixtures.

fixture-depending fixtures

As stated above, fixtures can depend on other fitures. This is done by "declaring" the dependencies in the fixture decorator. Fixtures are then passed as an argument to the fixture function:

@fixture(settings, tmpdir)
def jenkins(fixtures, root=None):
    root = root or fixtures.tmpdir
    settings = replace(fixtures.settings, STORAGE_PATH=root)
    return Jenkins.from_settings(settings)

The above example shows that one can get pretty fancy... or creative with one's fixture definitions.

Fixtures can also have named dependencies. So in the above example, if one wanted a different tmpdir than the "global" one:

@fixture(settings, jenkins_root=tmpdir)
def jenkins(fixtures, root=None):
    root = root or fixtures.jenkins_root
    settings = replace(fixtures.settings, STORAGE_PATH=root)
    return Jenkins.from_settings(settings)

If a TestCase used both jenkins and tmpdir:

@given(tmpdir, jenkins)
class MyTest(TestCase):
   def test_something(self, fixtures):
       self.assertNotEqual(fixtures.jenkins.root, fixtures.tmpdir)

Again if the two fixtures have different names then they are two separate fixtures. In general one should not use named fixtures unless one wants multiple fixtures of the same type.

@parametrized

Not so much a fixtures tool, however unittest-fixtures also comes with a @parametrized decorator that acts as a wrapper for unittest's subtests. A rather contrived example comes from unittest-fixtures own tests:

from unittest_fixtures import parametrized

class ParametrizeTests(TestCase):
    values = {1, 2}

    @parametrized([[1, values], [2, values], [None, values]])
    def test(self, value, values):
        if value is not None:
            self.assertIn(value, values)
            values.discard(value)
            return
        self.assertEqual(set(), values)

The fixtures kwarg may be overridden

The fixtures keyword argument is automatically passed to TestCase methods when the test is run. The name of the keyword argument can be overridden as follows:

@given(dog)
class MyTest(TestCase):
    unittest_fixtures_kwarg = "fx"

    def test_method(self, fx):
        dog = fx.dog

Recipes

The following are real-world examples of using unittest-fixtures.

Random Number Generator

From larry:

import random as stdlib_random
from unittest import mock

from unittest-fixtures import fixture


@fixture()
def random(fixtures, target="larry.color.random", seed=1):
    _random = stdlib_random.Random(seed)
    with mock.patch(target, new=_random):
        yield _random

Temporary Directory

Also from larry:

import tempfile

from unittest-fixtures import fixture


@fixture()
def tmpdir(fixtures):
    with tempfile.TemporaryDirectory() as tempdir:
        yield tempdir

Mock Environment Variables

From gbp-webhook-playsound:

import os


@fixture()
def environ(fixtures, environ=None, clear=True):
    with mock.patch.dict(os.environ, clear=clear):
        os.environ.update(environ or {})
        yield os.environ

Local Timezone

From gbpcli:

import datetime as dt
from unittest import mock


LOCAL_TIMEZONE = dt.timezone(dt.timedelta(days=-1, seconds=61200), "PDT")


@fixture()
def local_timezone(fixtures, local_timezone=LOCAL_TIMEZONE):
    with mock.patch("gbpcli.render.LOCAL_TIMEZONE", new=local_timezone):
        yield local_timezone

Django HttpRequest

From gbp-feeds:

from django.http import HttpRequest


@fixture()
def request(fixtures, path="/feed.atom", server_name="testserver", server_port=80):
    request = HttpRequest()
    request.path = path
    request.META["SERVER_NAME"] = server_name
    request.META["SERVER_PORT"] = server_port

    return request

sys.argv

From gbp-webhook:

import mock
import sys


@fixture()
def argv(fixtures, argv=None):
    argv = ["gbp", "webhook", "serve"] if argv is None else list(argv)

    with mock.patch.object(sys, "argv", new=argv):
        yield argv

mock.patch

Instead of applying mock.patch and/or mock.patch.object decorators to a TestCase, one can instead use a fixture. This way mocked objects appear as part of the fixtures test parameter instead of having separate arguments for mocks. The following recipe demonstrates how this can be done.

From gbp-testkit (a component of gentoo-build-publisher):

NO_OBJECT = object()  # sentinel value

@fixture()
def patch(_, target="", object=NO_OBJECT, **kwargs):
    if not target:
        patcher = None
        fake = mock.Mock(**kwargs)
    elif object is _NO_OBJECT:
        patcher = mock.patch(target, **kwargs)
        fake = patcher.start()
    else:
        patcher = mock.patch.object(object, target, **kwargs)
        fake = patcher.start()

    yield fake

    if patcher:
        patcher.stop()

So with this the above sys.argv fixture can be replaced with:

@given(argv=patch)
@where(argv__object=sys, argv__target="argv", argv__new=["gbp", "webhook", "serve"])
class MyTest(TestCase)
    def test_method(self, fixtures):
         status_code = main()

         self.assertEqual(status_code, 0)

Alternatively, insteady of passing the sys object one could also:

@where(argv__target="sys.argv", argv__new=["gbp", "webhook", "serve"])