Test independence in PyTest - Test Execution Trace

Test Overview

This test suite demonstrates test independence by ensuring each test runs separately without relying on shared state. It verifies that modifying data in one test does not affect others.

Test Code - pytest

PyTest

import pytest

@pytest.fixture
 def sample_list():
     return [1, 2, 3]

def test_append_item(sample_list):
    sample_list.append(4)
    assert sample_list == [1, 2, 3, 4]

def test_original_list_unchanged(sample_list):
    assert sample_list == [1, 2, 3]

Execution Trace - 3 Steps

Step	Action	System State	Assertion	Result
1	Test runner starts and collects tests	No tests executed yet	-	PASS
2	Runs test_append_item with fresh sample_list fixture	sample_list = [1, 2, 3]	Check sample_list after append is [1, 2, 3, 4]	PASS
3	Runs test_original_list_unchanged with fresh sample_list fixture	sample_list = [1, 2, 3]	Check sample_list is still [1, 2, 3]	PASS

Failure Scenario

Failing Condition: If sample_list fixture is shared and modified by test_append_item, test_original_list_unchanged will fail

Execution Trace Quiz - 3 Questions

Why does test_original_list_unchanged pass after test_append_item modifies the list?

A Because pytest fixtures provide a fresh list for each test

B Because the list is reset manually in the second test

C Because the tests run in parallel and do not affect each other

D Because the list is immutable

Key Result

Always design tests so they do not depend on each other's data or state. Use fixtures or setup methods that provide fresh data for each test to ensure reliability and easier debugging.