Test independence in PyTest - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Why is test independence important?

Imagine you have multiple tests for a calculator app. Why should each test be independent from others?

A Because tests should share data to reduce code duplication.

B So that one test's failure doesn't cause others to fail, making debugging easier.

C To make tests run slower and use more resources.

D To ensure tests always run in a fixed order.

💡 Hint

Think about how one broken test might affect others if they depend on each other.

Test independence means tests do not rely on each other's results or data. This helps isolate failures and makes debugging easier.

❓ Predict Output

intermediate

What is the output of these pytest tests?

Consider these two pytest tests that share a global variable. What will be the test results?

PyTest

counter = 0

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

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

A Both tests fail

B Both tests pass

C test_increment fails, test_increment_again passes

D test_increment passes, test_increment_again fails

💡 Hint

Think about the value of counter when each test runs.

The first test sets counter to 1 and passes. The second test runs after and increments counter to 2, so the assertion counter == 1 fails.

🔧 Debug

advanced

Identify the cause of flaky tests due to shared state

These pytest tests sometimes fail randomly. What is the main cause?

PyTest

shared_list = []

def test_add_item():
    shared_list.append(1)
    assert 1 in shared_list

def test_clear_list():
    shared_list.clear()
    assert shared_list == []

A Tests share mutable global state causing interference between tests.

B Tests use different assert statements causing confusion.

C Tests do not import pytest properly.

D Tests run in parallel causing race conditions.

💡 Hint

Look at the variable shared_list and how it is used.

Both tests modify the same global list. If test_add_item runs before test_clear_list, the list is cleared. But if order changes, tests may fail unpredictably.

❓ framework

advanced

How to ensure test independence in pytest?

Which pytest feature helps to create independent tests by providing fresh data for each test?

A Using global variables to share data between tests.

B Running tests in a fixed order using <code>pytest-order</code> plugin.

C Using fixtures with <code>scope='function'</code> to provide fresh setup for each test.

D Disabling assertions to speed up tests.

💡 Hint

Think about how pytest can prepare data or state before each test runs.

Fixtures with function scope run before each test, giving a clean environment and avoiding shared state.

❓ assertion

expert

Which assertion best checks test independence in pytest?

You want to verify that a test does not affect global state. Which assertion is best?

PyTest

global_state = {'count': 0}

def test_modify_state():
    global_state['count'] += 1
    # Which assertion below best ensures test independence?

A assert global_state['count'] == 0

B assert global_state['count'] > 0

C assert isinstance(global_state, dict)

D assert global_state != {}

💡 Hint

Think about what value count should have if tests are independent.

If tests are independent, the global count should reset to 0 before each test. Checking it equals 0 ensures no leftover changes.

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