Lesson 8 · Pytest deep dive

pytest Tutorial #8: Fixture Scopes & Finalisation

On testing real Python code with pytest. session and class scope, finalization order, and a real scope-mismatch gotcha. Let's get into it. scope='session'…

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CoursePytest deep dive
Lesson8 of 18
FormatJupyter notebook · 7 code cells

pytest

What you'll learn

The Helper From Last Time
session Scope - Once Per Entire Test Run
class Scope - Once Per Test Class
Finalization Order - Stacked yield Fixtures
request.addfinalizer - an Alternative to yield
A Real Gotcha - Scope Mismatch
A Real Pattern - an Expensive Session-Scoped Setup
Wrap-Up: What You Learned

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pytest Deep-Dive, Video 8: Fixture Scopes and Finalization#

Video eight of an eighteen-part series on testing real Python code with pytest.
session and class scope, finalization order, and a real scope-mismatch gotcha.
Let's get into it.

Part 1: The Helper From Last Time#

import pytest, tempfile, os
def run_pytest(code_str, args=None):
    with tempfile.NamedTemporaryFile(mode='w', suffix='_test.py', delete=False) as f:
        f.write(code_str)
        path = f.name
    result = pytest.main([path, *(args or [])])
    os.unlink(path)
    return result

Part 2: session Scope - Once Per Entire Test Run#

code = '''
import pytest

init_count = {"n": 0}

@pytest.fixture(scope='session')
def shared_resource():
    init_count["n"] += 1
    return {'value': 42}

def test_first_use(shared_resource):
    assert shared_resource['value'] == 42
    assert init_count['n'] == 1

def test_second_use(shared_resource):
    assert shared_resource['value'] == 42
    assert init_count['n'] == 1

def test_third_use(shared_resource):
    assert init_count['n'] == 1
'''
exit_code = run_pytest(code, ['-v', '--no-header'])
print(exit_code)

============================= test session starts =============================
collecting ... collected 3 items

..\..\..\Users\makmw\AppData\Local\Temp\tmpouwn0bcy_test.py::test_first_use PASSED [ 33%]
..\..\..\Users\makmw\AppData\Local\Temp\tmpouwn0bcy_test.py::test_second_use PASSED [ 66%]
..\..\..\Users\makmw\AppData\Local\Temp\tmpouwn0bcy_test.py::test_third_use PASSED [100%]

============================== 3 passed in 0.22s ==============================
0

Part 3: class Scope - Once Per Test Class#

code = '''
import pytest

call_log = []

@pytest.fixture(scope='class')
def class_resource():
    call_log.append('init')
    return {'ready': True}

class TestGroupOne:
    def test_a(self, class_resource):
        assert class_resource['ready']

    def test_b(self, class_resource):
        assert class_resource['ready']

class TestGroupTwo:
    def test_c(self, class_resource):
        assert class_resource['ready']

def test_log_length():
    assert len(call_log) == 2
'''
exit_code = run_pytest(code, ['-v', '--no-header'])
print(exit_code)

============================= test session starts =============================
collecting ... collected 4 items

..\..\..\Users\makmw\AppData\Local\Temp\tmp7oe37to6_test.py::TestGroupOne::test_a PASSED [ 25%]
..\..\..\Users\makmw\AppData\Local\Temp\tmp7oe37to6_test.py::TestGroupOne::test_b PASSED [ 50%]
..\..\..\Users\makmw\AppData\Local\Temp\tmp7oe37to6_test.py::TestGroupTwo::test_c PASSED [ 75%]
..\..\..\Users\makmw\AppData\Local\Temp\tmp7oe37to6_test.py::test_log_length PASSED [100%]

============================== 4 passed in 0.11s ==============================
0

Part 4: Finalization Order - Stacked yield Fixtures#

code = '''
import pytest

order = []

@pytest.fixture
def outer():
    order.append('setup outer')
    yield
    order.append('teardown outer')

@pytest.fixture
def inner(outer):
    order.append('setup inner')
    yield
    order.append('teardown inner')

def test_order(inner):
    order.append('test body')

def test_check_order():
    assert order == ['setup outer', 'setup inner', 'test body', 'teardown inner', 'teardown outer']
'''
exit_code = run_pytest(code, ['-v', '--no-header'])
print(exit_code)

============================= test session starts =============================
collecting ... collected 2 items

..\..\..\Users\makmw\AppData\Local\Temp\tmpr07i4awj_test.py::test_order PASSED [ 50%]
..\..\..\Users\makmw\AppData\Local\Temp\tmpr07i4awj_test.py::test_check_order PASSED [100%]

============================== 2 passed in 0.10s ==============================
0

Part 5: request.addfinalizer - an Alternative to yield#

code = '''
import pytest

cleanup_order = []

@pytest.fixture
def resource_with_finalizers(request):
    request.addfinalizer(lambda: cleanup_order.append('first registered'))
    request.addfinalizer(lambda: cleanup_order.append('second registered'))
    return {'ready': True}

def test_uses_resource(resource_with_finalizers):
    assert resource_with_finalizers['ready']

def test_cleanup_order_after():
    assert cleanup_order == ['second registered', 'first registered']
'''
exit_code = run_pytest(code, ['-v', '--no-header'])
print(exit_code)

============================= test session starts =============================
collecting ... collected 2 items

..\..\..\Users\makmw\AppData\Local\Temp\tmp21oss2p5_test.py::test_uses_resource PASSED [ 50%]
..\..\..\Users\makmw\AppData\Local\Temp\tmp21oss2p5_test.py::test_cleanup_order_after PASSED [100%]

============================== 2 passed in 0.11s ==============================
0

Part 6: A Real Gotcha - Scope Mismatch#

code = '''
import pytest

@pytest.fixture
def per_test_id():
    return object()

@pytest.fixture(scope='session')
def broken_session_fixture(per_test_id):
    return per_test_id

def test_uses_broken_fixture(broken_session_fixture):
    assert broken_session_fixture is not None
'''
exit_code = run_pytest(code, ['-v', '--no-header'])
print(exit_code)

============================= test session starts =============================
collecting ... collected 1 item

..\..\..\Users\makmw\AppData\Local\Temp\tmpi7gsd0ei_test.py::test_uses_broken_fixture ERROR [100%]

=================================== ERRORS ====================================
_________________ ERROR at setup of test_uses_broken_fixture __________________
ScopeMismatch: You tried to access the function scoped fixture per_test_id with a session scoped request object. Requesting fixture stack:
tmpi7gsd0ei_test.py:8:  def broken_session_fixture(per_test_id)
Requested fixture:
tmpi7gsd0ei_test.py:4:  def per_test_id()
=========================== short test summary info ===========================
ERROR ..\..\..\Users\makmw\AppData\Local\Temp\tmpi7gsd0ei_test.py::test_uses_broken_fixture - Failed: ScopeMismatch: You tried to access the function scoped fixture per_...
============================== 1 error in 0.11s ===============================
1

Part 7: A Real Pattern - an Expensive Session-Scoped Setup#

code = '''
import pytest

build_count = {"n": 0}

def expensive_lookup_table():
    build_count["n"] += 1
    return {n: n * n for n in range(1, 1001)}

@pytest.fixture(scope='session')
def lookup_table():
    return expensive_lookup_table()

def test_lookup_5(lookup_table):
    assert lookup_table[5] == 25

def test_lookup_100(lookup_table):
    assert lookup_table[100] == 10000

def test_lookup_1000(lookup_table):
    assert lookup_table[1000] == 1000000

def test_built_only_once(lookup_table):
    assert build_count['n'] == 1
'''
exit_code = run_pytest(code, ['-v', '--no-header'])
print(exit_code)

============================= test session starts =============================
collecting ... collected 4 items

..\..\..\Users\makmw\AppData\Local\Temp\tmp4r444uw__test.py::test_lookup_5 PASSED [ 25%]
..\..\..\Users\makmw\AppData\Local\Temp\tmp4r444uw__test.py::test_lookup_100 PASSED [ 50%]
..\..\..\Users\makmw\AppData\Local\Temp\tmp4r444uw__test.py::test_lookup_1000 PASSED [ 75%]
..\..\..\Users\makmw\AppData\Local\Temp\tmp4r444uw__test.py::test_built_only_once PASSED [100%]

============================== 4 passed in 0.12s ==============================
0

Wrap-Up: What You Learned#

scope='session' runs a fixture exactly once for the whole pytest invocation, shared by every requesting test.
scope='class' runs once per test class, shared by every method inside that class.
Stacked yield fixtures tear down in reverse of setup order, exactly like nested with blocks.
request.addfinalizer registers cleanup callbacks directly, run in LIFO order, as an alternative to yield.
A session-scoped fixture cannot depend on a function-scoped one; pytest raises ScopeMismatch at collection.
Session scope is ideal for genuinely expensive, safely-shareable, read-only setup like a big lookup table.
That wraps up fixture scopes and finalization. Next up: Test Organization.

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