Hooks

The hook-based plugin architecture of pytest empowers you to heavily customize the test runners behavior.

In this particular case we’ll make use of the pytest_collection_modifyitems hook. It is called after collection has been performed and hence provides access to the actual test items - objects which represent a single test.

Fixtures

Hooks have access to pytest’s builtin fixtures such as config. We case use it to explicitly run a hook for deselecting test items:

config.hook.pytest_deselected

Items

Watch out! This hook does not return the test items. Instead you want to populate the given list as needed: items[:] = selected_items

Implicit Fixture Usage

Pytests modular design allows you to re-use fixtures. The fixturenames attribute of a test item not only holds the fixtures which are specified in its signature but also the ones that are used implicitly.

Example Implementation

# conftest.py

def pytest_collection_modifyitems(items, config):
    selected_items = []
    deselected_items = []

    for item in items:
        if 'new_fixture' in getattr(item, 'fixturenames', ()):
            selected_items.append(item)
        else:
            deselected_items.append(item)
    config.hook.pytest_deselected(items=deselected_items)
    items[:] = selected_items

# test_foobar.py

import pytest


@pytest.fixture
def new_fixture():
    return 'foobar'


@pytest.fixture
def another_fixture(new_fixture):
    return 'foobar'


def test_abc(new_fixture):
    assert True


def test_def(another_fixture):
    assert True


def test_that_does_not_use_fixture():
    assert False

Test Report

$ py.test -v
============================ test session starts =============================
platform darwin -- Python 3.5.0, pytest-2.8.7, py-1.4.31, pluggy-0.3.1
collected 3 items

test_foobar.py::test_abc PASSED
test_foobar.py::test_def PASSED

=================== 2 passed, 1 deselected in 0.01 seconds ===================

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