Ch 3: Pytest Fixtures

Intro

The @pytest.fixture() decorator is used to tell pytest that a function is a fixture. When you include the fixture name in the parameter list of a test function, pytest knows to run it before running the test. Fixtures can do work, and can also return data to the test function.

Test fixtures refer to the mechanism pytest provides to allow the separation of “getting ready for” and “cleaning up after” code from your test functions.

Fixtures

GIVEN/WHEN/THEN → Useful to document behaviour of test

# tasks_db is a fixture defined in conftest.py

def test_add_returns_valid_id(tasks_db):
    """tasks.add(<valid task>) should return an integer."""
    # GIVEN an initialized tasks db
    # WHEN a new task is added
    # THEN returned task_id is of type int
    new_task = Task('do something')
    task_id = tasks.add(new_task)
    assert isinstance(task_id, int)

@pytest.fixture()
def tasks_db(tmpdir):
    """Connect to db before tests, disconnect after."""
    # Setup : start db
    tasks.start_tasks_db(str(tmpdir), 'tiny')

    yield  # this is where the testing happens

    # Teardown : stop db
    tasks.stop_tasks_db()

See setup/teardown

pytest --setup-show test.py

"""
platform darwin -- Python 3.8.4, pytest-5.4.3, py-1.9.0, pluggy-0.13.1
rootdir: /Users/dhruvthakur/code/pers/testing/code/ch3/a/tasks_proj/tests, inifile: pytest.ini
collected 3 items / 2 deselected / 1 selected

test_add.py
SETUP    S tmp_path_factory
        SETUP    F tmp_path (fixtures used: tmp_path_factory)
        SETUP    F tmpdir (fixtures used: tmp_path)
        SETUP    F tasks_db (fixtures used: tmpdir)
        func/test_add.py::test_add_returns_valid_id (fixtures used: request, tasks_db, tmp_path, tmp_path_factory, tmpdir).
        TEARDOWN F tasks_db
        TEARDOWN F tmpdir
        TEARDOWN F tmp_path
TEARDOWN S tmp_path_factory
"""

Exceptions can be raise from fixtures. These will be reported as ERROR by pytest, instead of a FAIL.

Combining multiple fixtures

########## FIXTURES ############

@pytest.fixture()
def tasks_db(tmpdir):
    """Connect to db before tests, disconnect after."""
    # Setup : start db
    tasks.start_tasks_db(str(tmpdir), 'tiny')

    yield  # this is where the testing happens

    # Teardown : stop db
    tasks.stop_tasks_db()

# Reminder of Task constructor interface
# Task(summary=None, owner=None, done=False, id=None)
# summary is required
# owner and done are optional
# id is set by database

@pytest.fixture()
def tasks_just_a_few():
    """All summaries and owners are unique."""
    return (
        Task('Write some code', 'Brian', True),
        Task("Code review Brian's code", 'Katie', False),
        Task('Fix what Brian did', 'Michelle', False))

@pytest.fixture()
def db_with_3_tasks(tasks_db, tasks_just_a_few):
    """Connected db with 3 tasks, all unique."""
    for t in tasks_just_a_few:
        tasks.add(t)

########### TEST ###########

def test_add_increases_count(db_with_3_tasks):
    """Test tasks.add() affect on tasks.count()."""
    # GIVEN a db with 3 tasks
    #  WHEN another task is added
    tasks.add(Task('throw a party'))

    #  THEN the count increases by 1
    assert tasks.count() == 4

########## TEST RUN ########

python -m pytest test_add.py::test_add_increases_count  --disable-pytest-warnings --setup-show
===================================================================== test session starts =====================================================================
platform darwin -- Python 3.8.4, pytest-5.4.3, py-1.9.0, pluggy-0.13.1
rootdir: /Users/dhruvthakur/code/pers/testing/code/ch3/a/tasks_proj/tests, inifile: pytest.ini
collected 1 item

test_add.py
SETUP    S tmp_path_factory
        SETUP    F tmp_path (fixtures used: tmp_path_factory)
        SETUP    F tmpdir (fixtures used: tmp_path)
        SETUP    F tasks_db (fixtures used: tmpdir)
        SETUP    F tasks_just_a_few
        SETUP    F db_with_3_tasks (fixtures used: tasks_db, tasks_just_a_few)
        func/test_add.py::test_add_increases_count (fixtures used: db_with_3_tasks, request, tasks_db, tasks_just_a_few, tmp_path, tmp_path_factory, tmpdir).
        TEARDOWN F db_with_3_tasks
        TEARDOWN F tasks_just_a_few
        TEARDOWN F tasks_db
        TEARDOWN F tmpdir
        TEARDOWN F tmp_path
TEARDOWN S tmp_path_factory

Fixture scopes

    import pytest


    @pytest.fixture(scope='function')
    def func_scope():
        """A function scope fixture."""


    @pytest.fixture(scope='module')
    def mod_scope():
        """A module scope fixture."""


    @pytest.fixture(scope='session')
    def sess_scope():
        """A session scope fixture."""


    @pytest.fixture(scope='class')
    def class_scope():
        """A class scope fixture."""


    def test_1(sess_scope, mod_scope, func_scope):
        """Test using session, module, and function scope fixtures."""


    def test_2(sess_scope, mod_scope, func_scope):
        """Demo is more fun with multiple tests."""

    @pytest.mark.usefixtures('class_scope')
    class TestSomething():
        """Demo class scope fixtures."""

        def test_3(self):
            """Test using a class scope fixture."""

        def test_4(self):
            """Again, multiple tests are more fun."""

"""
pytest --setup-show test_scope.py
    ======================== test session starts ========================
    collected 4 items

    test_scope.py
    SETUP    S sess_scope
      SETUP    M mod_scope
          SETUP    F func_scope
            test_scope.py::test_1
              (fixtures used: func_scope, mod_scope, sess_scope).
          TEARDOWN F func_scope
          SETUP    F func_scope
            test_scope.py::test_2
              (fixtures used: func_scope, mod_scope, sess_scope).
          TEARDOWN F func_scope
        SETUP    C class_scope
            test_scope.py::TestSomething::()::test_3 (fixtures used: class_scope).
            test_scope.py::TestSomething::()::test_4 (fixtures used: class_scope).
        TEARDOWN C class_scope
      TEARDOWN M mod_scope
    TEARDOWN S sess_scope
"""

Parameterising Fixtures

tasks_to_try = (Task('sleep', done=True),
                Task('wake', 'brian'),
                Task('breathe', 'BRIAN', True),
                Task('exercise', 'BrIaN', False))

task_ids = ['Task({},{},{})'.format(t.summary, t.owner, t.done)
            for t in tasks_to_try]

@pytest.fixture(params=tasks_to_try, ids=task_ids)
def b_task(request):
    """Using a list of ids."""
    return request.param

def test_add_b(tasks_db, b_task):
    """Using b_task fixture, with ids."""
    task_id = tasks.add(b_task)
    t_from_db = tasks.get(task_id)
    assert equivalent(t_from_db, b_task)

"""
test_add_variety2.py
SETUP    S tmpdir_factory
SETUP    S tasks_db_session (fixtures used: tmpdir_factory)
        SETUP    F tasks_db (fixtures used: tasks_db_session)
        SETUP    F b_task[Task(sleep,None,True)]
        func/test_add_variety2.py::test_add_b[Task(sleep,None,True)] (fixtures used: b_task, request, tasks_db, tasks_db_session, tmpdir_factory).
        TEARDOWN F b_task[Task(sleep,None,True)]
        TEARDOWN F tasks_db
        SETUP    F tasks_db (fixtures used: tasks_db_session)
        SETUP    F b_task[Task(wake,brian,False)]
        func/test_add_variety2.py::test_add_b[Task(wake,brian,False)] (fixtures used: b_task, request, tasks_db, tasks_db_session, tmpdir_factory).
        TEARDOWN F b_task[Task(wake,brian,False)]
        TEARDOWN F tasks_db
        SETUP    F tasks_db (fixtures used: tasks_db_session)
        SETUP    F b_task[Task(breathe,BRIAN,True)]
        func/test_add_variety2.py::test_add_b[Task(breathe,BRIAN,True)] (fixtures used: b_task, request, tasks_db, tasks_db_session, tmpdir_factory).
        TEARDOWN F b_task[Task(breathe,BRIAN,True)]
        TEARDOWN F tasks_db
        SETUP    F tasks_db (fixtures used: tasks_db_session)
        SETUP    F b_task[Task(exercise,BrIaN,False)]
        func/test_add_variety2.py::test_add_b[Task(exercise,BrIaN,False)] (fixtures used: b_task, request, tasks_db, tasks_db_session, tmpdir_factory).
        TEARDOWN F b_task[Task(exercise,BrIaN,False)]
        TEARDOWN F tasks_db
TEARDOWN S tasks_db_session
TEARDOWN S tmpdir_factory
"""