How Can I Test If A Pytest Fixture Raises An Exception?

Use case: In a pytest test suite I have a @fixture which raises exceptions if command line options for its configuration are missing. I've written a test for this fixture using xfa

Solution 1:

initial setup

Suppose you have a simplified project with conftest.py containing the following code:

import pytest


def pytest_addoption(parser):
    parser.addoption('--foo', action='store', dest='foo', default='bar',
                     help='--foo should be always bar!')

@pytest.fixture
def foo(request):
    fooval = request.config.getoption('foo')
    if fooval != 'bar':
        raise ValueError('expected foo to be "bar"; "{}" provided'.format(fooval))

It adds a new command line arg --foo and a fixture foo returning the passed arg, or bar if not specified. If anything else besides bar passed via --foo, the fixture raises a ValueError.

You use the fixture as usual, for example

def test_something(foo):
    assert foo == 'bar'

Now let's test that fixture.

preparations

In this example, we need to do some simple refactoring first. Move the fixture and related code to some file called something else than conftest.py, for example, my_plugin.py:

# my_plugin.py

import pytest


def pytest_addoption(parser):
    parser.addoption('--foo', action='store', dest='foo', default='bar',
                     help='--foo should be always bar!')

@pytest.fixture
def foo(request):
    fooval = request.config.getoption('foo')
    if fooval != 'bar':
        raise ValueError('expected foo to be "bar"; "{}" provided'.format(fooval))

In conftest.py, ensure the new plugin is loaded:

# conftest.py

pytest_plugins = ['my_plugin']

Run the existing test suite to ensure we didn't break anything, all tests should still pass.

activate pytester

pytest provides an extra plugin for writing plugin tests, called pytester. It is not activated by default, so you should do that manually. In conftest.py, extend the plugins list with pytester:

# conftest.py

pytest_plugins = ['my_plugin', 'pytester']

writing the tests

Once pytester is active, you get a new fixture available called testdir. It can generate and run pytest test suites from code. Here's what our first test will look like:

# test_foo_fixture.py

def test_all_ok(testdir):

    testdata = '''
               def test_sample(foo):
                   assert True
               '''

    testconftest = '''
                   pytest_plugins = ['my_plugin']
                   '''

    testdir.makeconftest(testconftest)
    testdir.makepyfile(testdata)
    result = testdir.runpytest()
    result.assert_outcomes(passed=1)

It should be pretty obvious what happens here: we provide the tests code as string and testdir will generate a pytest project from it in some temporary directory. To ensure our foo fixture is available in the generated test project, we pass it in the generated conftest same way as we do in the real one. testdir.runpytest() starts the test run, producing a result that we can inspect.

Let's add another test that checks whether foo will raise a ValueError:

def test_foo_valueerror_raised(testdir):
    testdata = '''
               def test_sample(foo):
                   assert True
               '''

    testconftest = '''
                   pytest_plugins = ['my_plugin']
                   '''

    testdir.makeconftest(testconftest)
    testdir.makepyfile(testdata)
    result = testdir.runpytest('--foo', 'baz')                                                                                                                                
    result.assert_outcomes(error=1)
    result.stdout.fnmatch_lines([
        '*ValueError: expected foo to be "bar"; "baz" provided'
    ])

Here we execute the generated tests with --foo baz and verify afterwards if one test ended with an error and the error output contains the expected error message.