Python

Problem

While developing software using test-driven development (or at least, tightly integrating development of tests with production code), mocks/stubs are often used with unit tests for isolation and rapid iteration. Removing the overhead of the setup and tear-down of running an external database or other expensive resource makes it bearable to run tests frequently enough to write confidently.

Once enough of the software works, however, it should be integration-tested with live (ideally local and rebuilt for every test) resources to ensure that assumptions made in implementing mocks are valid.

The pytest documentation shows an example of simply skipping slow tests based on a command-line parameter (which could easily be extended to switch between two sets of tests) but having two sets of tests and fixtures is not very DRY (since the tests themselves are likely the same or very nearly the same).

Solution

A better solution, however, would be to switch between using live and mocked fixture resources. With pytest, this is not only possible but fairly easy (although some digging is required to to figure out) – the chain of fixtures can be computed at runtime simply by manipulating the list fixturenames, which is an attribute of the request fixture, thereby switching between using live resources and mocked resources. Making the switch based on a command-line parameter is also easy.

For example, let’s say we have a mock_database fixture and a live_database fixture:

import pytest

@pytest.fixture
def mock_database():
    ...

@pytest.fixture
def live_database():
    ...

We will also need to add a command-line parameter (which needs to be in the top-most conftest.py:

def pytest_addoption(parser):
    parser.addoption("--online", action="store_true", default=False,
        help="run tests on-line with live resources")

What we need now is a fixture to be the “switch”, which is what tests or other fixtures will use (probably the a more generic name like database):

@pytest.fixture
def database(request):
    if request.config.getoption('online'):
        request.fixturenames.append('live_database')
    else:
        request.fixturenames.append('mock_database')

And then our tests will use the generic database fixture:

def test_lookup(database):
    ...