Ordering tests for parallel safety in PyTest - Build an Automation Script

Verify tests run in correct order to avoid parallel execution conflicts

Preconditions (3)

Step 1: Create three test functions: test_setup, test_action, test_cleanup

Step 2: Mark test_setup to run first, test_cleanup to run last

Step 3: Run tests in parallel using pytest-xdist with 3 workers

Step 4: Observe the order of test execution and resource conflicts

✅ Expected Result: Tests run in the specified order without resource conflicts or failures caused by parallel execution

Automation Requirements - pytest

Assertions Needed:

Verify test_setup runs before test_action

Verify test_cleanup runs after test_action

Verify no test fails due to resource conflicts

Best Practices:

Use pytest-ordering or pytest-dependency to control test order

Avoid shared mutable state between tests

Use fixtures with proper scope to manage setup and cleanup

Use explicit waits or locks if needed to prevent race conditions

Automated Solution

PyTest

import pytest

@pytest.mark.order(1)
def test_setup():
    global resource
    resource = []
    resource.append('setup done')
    assert 'setup done' in resource

@pytest.mark.order(2)
def test_action():
    resource.append('action done')
    assert resource == ['setup done', 'action done']

@pytest.mark.order(3)
def test_cleanup():
    resource.clear()
    assert resource == []

# To run tests in parallel use:
# pytest -n 3 --dist=loadscope

This script uses the pytest-order plugin to control the order of test execution explicitly. The @pytest.mark.order decorator sets the order: test_setup runs first, test_action second, and test_cleanup last.

The tests share a global resource list to simulate shared state. Each test asserts the expected state to ensure no conflicts occur.

Running with pytest -n 3 enables parallel execution with 3 workers. The ordering ensures tests do not run out of sequence, preventing race conditions.

This approach demonstrates how to safely order tests when running in parallel to avoid conflicts.

Common Mistakes - 3 Pitfalls

Not controlling test order when tests share state

Using global variables without proper synchronization

Assuming tests run sequentially by default

Bonus Challenge

Now add data-driven testing with 3 different resource states to verify ordering safety

Show Hint

Practice

1. Why is it important to order tests when running pytest in parallel?

easy

A. To make tests run slower

B. To avoid conflicts when tests share resources

C. To increase the number of tests

D. To skip tests automatically

Solution

1. Step 1: Understand test resource sharing

   Tests that share resources like files or databases can interfere with each other if run at the same time.

2. Step 2: Recognize the role of ordering

   Ordering tests ensures they run in a sequence that prevents conflicts and errors.

3. Final Answer:

   To avoid conflicts when tests share resources -> Option B

4. Quick Check:

   Ordering prevents resource conflicts [OK]

Hint: Order tests to prevent shared resource conflicts [OK]

Common Mistakes:

• Thinking ordering slows tests down
• Believing ordering increases test count
• Assuming ordering skips tests

2. Which of the following is the correct way to order a test to run third using pytest?

easy

A. @pytest.mark.order(3)

B. @pytest.order(3)

C. @pytest.mark.run(3)

D. @pytest.order_mark(3)

Solution

1. Step 1: Recall pytest ordering syntax

   The correct decorator to order tests is @pytest.mark.order(n) where n is the order number.

2. Step 2: Match the syntax to options

   Only @pytest.mark.order(3) uses the correct decorator @pytest.mark.order(3).

3. Final Answer:

   @pytest.mark.order(3) -> Option A

4. Quick Check:

   Use @pytest.mark.order(n) for ordering [OK]

Hint: Use @pytest.mark.order(n) to set test order [OK]

Common Mistakes:

• Using @pytest.order instead of @pytest.mark.order
• Confusing order with run decorators
• Misspelling the decorator name

3. Given these two tests, what will be the order of execution when run with pytest in parallel with ordering?

import pytest

@pytest.mark.order(2)
def test_second():
    assert True

@pytest.mark.order(1)
def test_first():
    assert True

medium

A. Tests run in random order

B. test_second runs before test_first

C. test_first runs before test_second

D. Tests fail due to ordering conflict

Solution

1. Step 1: Identify order markers

   test_first has order 1, test_second has order 2.

2. Step 2: Understand execution order

   Lower order numbers run before higher ones, so test_first runs before test_second.

3. Final Answer:

   test_first runs before test_second -> Option C

4. Quick Check:

   Lower order number runs first [OK]

Hint: Lower order number runs first in pytest [OK]

Common Mistakes:

• Assuming higher order runs first
• Thinking tests run randomly despite order
• Believing ordering causes test failure

4. You have two tests that share a database. You want to run them in parallel safely. Which of these is a problem in the code below?

import pytest

@pytest.mark.order(1)
def test_write_db():
    # writes data
    assert True

@pytest.mark.order(2)
def test_read_db():
    # reads data
    assert True

medium

A. Tests must have the same order number

B. The order decorators are incorrect syntax

C. Tests are missing assert statements

D. Tests are ordered but may still run in parallel causing conflicts

Solution

1. Step 1: Check ordering usage

   Tests use correct order decorators, so syntax is fine.

2. Step 2: Understand parallel execution impact

   Even with order, if tests run truly in parallel (e.g., with pytest-xdist), they may overlap and cause conflicts.

3. Final Answer:

   Tests are ordered but may still run in parallel causing conflicts -> Option D

4. Quick Check:

   Ordering alone doesn't guarantee parallel safety [OK]

Hint: Ordering doesn't prevent parallel overlap without locks [OK]

Common Mistakes:

• Thinking order decorators fix parallel conflicts
• Believing same order number is required
• Ignoring assert statements importance

5. You have three tests that modify a shared file. To run them safely in parallel, you want to order them and ensure no overlap. Which approach below best achieves this?

hard

A. Use @pytest.mark.order to run tests sequentially and add file locks

B. Remove order decorators and run all tests in parallel without locks

C. Use @pytest.mark.order with the same order number for all tests

D. Run tests without ordering but add random sleep delays

Solution

1. Step 1: Understand test resource sharing

   Tests modifying the same file can cause conflicts if run simultaneously.

2. Step 2: Combine ordering with locking

   Ordering ensures sequence, and file locks prevent overlap during execution.

3. Step 3: Evaluate options

   Use @pytest.mark.order to run tests sequentially and add file locks uses both ordering and locks, which is the safest approach.

4. Final Answer:

   Use @pytest.mark.order to run tests sequentially and add file locks -> Option A

5. Quick Check:

   Order plus locks ensure parallel safety [OK]

Hint: Combine order and locks for safe parallel file tests [OK]

Common Mistakes:

• Relying on order alone without locks
• Using same order number causing race conditions
• Adding random delays instead of proper synchronization