Worker distribution strategies in PyTest - Test Execution Trace

Test Overview

This test checks that pytest distributes test cases evenly across multiple workers when using the pytest-xdist plugin. It verifies that each worker runs a subset of tests and all tests complete successfully.

Test Code - PyTest

PyTest

import pytest

@pytest.mark.parametrize('num', range(6))
def test_sample(num):
    assert num >= 0

# Run command: pytest -n 3
# This runs tests distributed across 3 workers

Execution Trace - 5 Steps

Step	Action	System State	Assertion	Result
1	Test run starts with pytest-xdist plugin enabled using 3 workers	Terminal shows pytest starting with 3 parallel workers	—	PASS
2	Pytest collects 6 test cases from test_sample function	Test collection complete, 6 tests found	Verify total tests collected equals 6	PASS
3	Tests are distributed evenly across 3 workers (2 tests per worker)	Each worker assigned 2 tests to run	Check that each worker runs exactly 2 tests	PASS
4	Each worker executes its assigned tests in parallel	Workers running tests concurrently	Each test assertion (num >= 0) passes	PASS
5	All workers complete tests and report results	Terminal shows all 6 tests passed	Verify all tests passed with no failures	PASS

Failure Scenario

Failing Condition: If pytest-xdist is not installed or workers fail to distribute tests evenly

Execution Trace Quiz - 3 Questions

How many tests does each worker run when using 3 workers for 6 tests?

A 2 tests per worker

B 3 tests per worker

C 6 tests per worker

D 1 test per worker

Key Result

Using pytest-xdist to distribute tests across multiple workers helps run tests faster and ensures workload is balanced, but requires proper plugin installation and configuration.

Practice

1. What does the --dist=loadscope option do in pytest-xdist worker distribution?

easy

A. It distributes tests randomly to all workers.

B. It runs all tests sequentially on a single worker.

C. It groups tests by their scope and distributes them to workers.

D. It groups tests by file size before distribution.

Solution

1. Step 1: Understand the meaning of loadscope

   The loadscope mode groups tests by their scope, such as class or module, so related tests run together.

2. Step 2: Compare with other distribution modes

   Unlike random or file-based grouping, loadscope keeps related tests together for better caching and setup reuse.

3. Final Answer:

   It groups tests by their scope and distributes them to workers. -> Option C

4. Quick Check:

   loadscope = group by scope [OK]

Hint: Loadscope groups tests by scope like class or module [OK]

Common Mistakes:

• Confusing loadscope with random distribution
• Thinking loadscope groups by file size
• Assuming loadscope runs tests sequentially

2. Which of the following is the correct pytest command to run tests with 4 workers using file-based distribution?

easy

A. pytest -n 4 --dist=loadfile

B. pytest --dist=loadfile -n four

C. pytest -n=4 --dist=loadscope

D. pytest -n 4 --dist=loadgroup

Solution

1. Step 1: Identify correct syntax for number of workers

   The correct syntax is -n 4 to specify 4 workers; spelling out 'four' is invalid.

2. Step 2: Match distribution mode to file-based

   The file-based distribution mode is loadfile, so --dist=loadfile is correct.

3. Final Answer:

   pytest -n 4 --dist=loadfile -> Option A

4. Quick Check:

   -n 4 and --dist=loadfile correct syntax [OK]

Hint: Use -n number and --dist=loadfile for file grouping [OK]

Common Mistakes:

• Using spelled-out numbers like 'four'
• Mixing distribution modes incorrectly
• Using equals sign with -n option

3. Given this pytest command: pytest -n 3 --dist=loadfile, and three test files test_a.py, test_b.py, test_c.py, how will tests be distributed?

medium

A. Tests run sequentially on a single worker.

B. All workers run tests from all files randomly.

C. Tests are grouped by class across files.

D. Each worker runs tests from one file exclusively.

Solution

1. Step 1: Understand loadfile distribution

   Loadfile mode assigns tests grouped by file to different workers, so each worker gets whole files.

2. Step 2: Match number of workers to files

   With 3 workers and 3 files, each worker will get one file's tests exclusively.

3. Final Answer:

   Each worker runs tests from one file exclusively. -> Option D

4. Quick Check:

   loadfile = group by file [OK]

Hint: Loadfile means one file per worker [OK]

Common Mistakes:

• Thinking tests are split randomly
• Confusing loadfile with loadscope
• Assuming tests run sequentially

4. You run pytest -n 2 --dist=loadscope but notice tests from the same class run on different workers. What is the likely cause?

medium

A. Tests are not properly grouped because the class scope is not detected.

B. The -n option must be set to 1 for loadscope.

C. The --dist option is ignored when using multiple workers.

D. Tests are always distributed randomly regardless of options.

Solution

1. Step 1: Understand loadscope grouping behavior

   Loadscope groups tests by scope like class or module, so tests in the same class should run together.

2. Step 2: Identify why grouping fails

   If tests from the same class run on different workers, pytest likely failed to detect the class scope properly, causing wrong grouping.

3. Final Answer:

   Tests are not properly grouped because the class scope is not detected. -> Option A

4. Quick Check:

   Undetected scope breaks loadscope grouping [OK]

Hint: Undetected scope causes loadscope to fail grouping [OK]

Common Mistakes:

• Thinking -n must be 1 for loadscope
• Believing --dist is ignored with multiple workers
• Assuming distribution is always random

5. You want to run tests in custom groups using pytest-xdist. Which command and option combination allows you to define and use custom test groups for worker distribution?

hard

A. pytest -n 3 --dist=loadgroup --tx group1 --tx group2 --tx group3

B. pytest -n 3 --dist=loadgroup

C. pytest -n 3 --dist=loadfile --group=custom

D. pytest -n 3 --dist=loadscope --group=custom

Solution

1. Step 1: Identify the distribution mode for custom groups

   The loadgroup mode is designed for custom grouping of tests for distribution.

2. Step 2: Understand correct command usage

   Using --dist=loadgroup with -n 3 enables pytest-xdist to distribute tests based on user-defined groups configured elsewhere (e.g., in pytest hooks).

3. Final Answer:

   pytest -n 3 --dist=loadgroup -> Option B

4. Quick Check:

   loadgroup enables custom test groups [OK]

Hint: Use --dist=loadgroup to enable custom test groups [OK]

Common Mistakes:

• Adding invalid --group option
• Using --tx incorrectly for grouping
• Confusing loadgroup with loadfile or loadscope