Ordering tests for parallel safety in PyTest - Test Execution Trace

Test Overview

This test suite checks that tests run safely in parallel by using an autouse fixture to reset shared state before each test. It verifies that tests are independent, do not depend on execution order, and avoid interference.

Test Code - pytest

PyTest

import pytest

shared_resource = []

@pytest.fixture(scope="function", autouse=True)
def reset_resource():
    shared_resource.clear()
    assert len(shared_resource) == 0
    yield

@pytest.mark.parametrize('item', [1, 2, 3])
def test_append_item(item):
    # Each test starts with empty shared_resource due to fixture
    shared_resource.append(item)
    assert len(shared_resource) == 1
    assert item in shared_resource


def test_shared_resource_length():
    # This test verifies the resource is reset (empty)
    assert len(shared_resource) == 0

Execution Trace - 8 Steps

Step	Action	System State	Assertion	Result
1	Fixture reset_resource runs before test_append_item[1], clears shared_resource	shared_resource is empty []	Assert length == 0 (fixture)	PASS
2	test_append_item[1] appends 1	shared_resource contains [1]	Assert len == 1 and 1 in shared_resource	PASS
3	Fixture reset_resource runs before test_append_item[2], clears shared_resource	shared_resource is empty []	Assert length == 0 (fixture)	PASS
4	test_append_item[2] appends 2	shared_resource contains [2]	Assert len == 1 and 2 in shared_resource	PASS
5	Fixture reset_resource runs before test_append_item[3], clears shared_resource	shared_resource is empty []	Assert length == 0 (fixture)	PASS
6	test_append_item[3] appends 3	shared_resource contains [3]	Assert len == 1 and 3 in shared_resource	PASS
7	Fixture reset_resource runs before test_shared_resource_length, clears shared_resource	shared_resource is empty []	Assert length == 0 (fixture)	PASS
8	test_shared_resource_length checks length	shared_resource is empty []	Assert length == 0	PASS

Failure Scenario

Failing Condition: Without the reset_resource fixture, tests interfere via shared state; e.g., second test_append_item sees previous appends

Execution Trace Quiz - 3 Questions

What ensures shared_resource is reset before each test?

A Global variable initialization

B pytest.fixture(scope='function', autouse=True)

C Test ordering markers

D Parametrization

Key Result

Autouse fixtures with function scope reset shared state before each test, ensuring independence, no order dependency, and safety for parallel execution (e.g., pytest -n auto). Prefer stateless tests when possible.