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.

Practice

1. Why is test independence important in pytest?

easy

A. It groups tests to run in a specific order.

B. It allows tests to share variables for faster execution.

C. It ensures tests do not affect each other and run reliably alone.

D. It makes tests run only when previous tests pass.

Solution

1. Step 1: Understand test independence concept

   Test independence means each test runs alone without relying on others.

2. Step 2: Identify why independence matters

   This prevents tests from failing due to side effects or order, making results reliable.

3. Final Answer:

   It ensures tests do not affect each other and run reliably alone. -> Option C

4. Quick Check:

   Test independence = tests run alone [OK]

Hint: Tests should run alone without relying on others [OK]

Common Mistakes:

• Thinking tests must share data to be efficient
• Believing tests run only if previous tests pass
• Assuming test order controls correctness

2. Which pytest feature helps keep tests independent by running setup code before each test?

easy

A. Using setup_method or setup_function

B. Using yield_fixture to share data

C. Using @pytest.mark.parametrize

D. Using pytest.skip() to skip tests

Solution

1. Step 1: Identify setup features in pytest

   Pytest runs setup code before each test using setup_method or setup_function.

2. Step 2: Understand their role in test independence

   Setup prepares fresh state for each test, avoiding shared state and keeping tests independent.

3. Final Answer:

   Using setup_method or setup_function -> Option A

4. Quick Check:

   Setup before each test = setup_method/setup_function [OK]

Hint: Setup code before each test keeps tests independent [OK]

Common Mistakes:

• Confusing parameterize with setup
• Using yield_fixture to share state incorrectly
• Skipping tests does not setup state

3. Given the code below, what will be the output when running both tests?

counter = 0

def test_first():
    global counter
    counter += 1
    assert counter == 1

def test_second():
    global counter
    counter += 1
    assert counter == 1

medium

A. First test passes, second test fails

B. Both tests pass

C. First test fails, second test passes

D. Both tests fail

Solution

1. Step 1: Analyze test_first behavior

   Initially, counter=0. test_first increments to 1 and asserts counter == 1, so it passes.

2. Step 2: Analyze test_second behavior

   counter is now 1 from previous test. test_second increments to 2 and asserts counter == 1, which fails.

3. Final Answer:

   First test passes, second test fails -> Option A

4. Quick Check:

   Shared state causes second test failure [OK]

Hint: Shared global state breaks test independence [OK]

Common Mistakes:

• Assuming counter resets automatically
• Thinking both tests run with fresh state
• Ignoring global variable effects

4. Identify the problem in this pytest code that breaks test independence:

shared_list = []

def test_add_one():
    shared_list.append(1)
    assert len(shared_list) == 1

def test_add_two():
    shared_list.append(2)
    assert len(shared_list) == 1

medium

A. shared_list should be a global variable inside tests

B. Assertions are incorrect; they should check for length 2

C. Tests run in parallel causing race conditions

D. shared_list is not cleared between tests causing length to grow

Solution

1. Step 1: Understand shared_list usage

   shared_list is defined outside tests and not reset, so it keeps growing with each test.

2. Step 2: Identify why independence breaks

   Because shared_list is not cleared, test_add_two sees length 2, but asserts length 1, causing failure.

3. Final Answer:

   shared_list is not cleared between tests causing length to grow -> Option D

4. Quick Check:

   Shared mutable state without reset breaks independence [OK]

Hint: Reset shared data between tests to keep independence [OK]

Common Mistakes:

• Assuming pytest resets global variables automatically
• Changing assertions instead of fixing state
• Confusing parallel runs with shared state issues

5. You want to ensure two pytest tests that modify a database table run independently. Which approach best maintains test independence?

hard

A. Run tests in a fixed order so changes apply sequentially

B. Use a fixture to create and rollback a transaction for each test

C. Share a global database connection and commit changes after all tests

D. Skip tests that depend on database state

Solution

1. Step 1: Understand database state isolation

   To keep tests independent, each test should not affect others' database state.

2. Step 2: Choose best isolation method

   Using a fixture that creates a transaction and rolls back after each test resets database state, ensuring independence.

3. Final Answer:

   Use a fixture to create and rollback a transaction for each test -> Option B

4. Quick Check:

   Transaction rollback per test = independence [OK]

Hint: Rollback DB changes after each test to isolate state [OK]

Common Mistakes:

• Relying on test order for correctness
• Sharing global DB connection without isolation
• Skipping tests instead of fixing independence