Parallel execution in CI in Selenium Python

Introduction

Running tests at the same time saves time and finds problems faster.

You want to check your website on many browsers quickly.

You have many tests and want results faster.

You want to use your computer or server better by running tests together.

You want to catch errors early by testing many parts at once.

You use a Continuous Integration (CI) system to test code automatically.

Syntax

Selenium Python

from selenium import webdriver
from concurrent.futures import ThreadPoolExecutor

def run_test(browser_name):
    if browser_name == 'chrome':
        driver = webdriver.Chrome()
    elif browser_name == 'firefox':
        driver = webdriver.Firefox()
    else:
        return 'Browser not supported'
    driver.get('https://example.com')
    title = driver.title
    driver.quit()
    return title

browsers = ['chrome', 'firefox']

with ThreadPoolExecutor() as executor:
    results = list(executor.map(run_test, browsers))

print(results)

This example uses Python's ThreadPoolExecutor to run tests in parallel.

Each test opens a browser, runs, and closes it independently.

Examples

This runs two tests in parallel using submit method.

Selenium Python

from concurrent.futures import ThreadPoolExecutor

# Run two tests at the same time
with ThreadPoolExecutor(max_workers=2) as executor:
    executor.submit(run_test, 'chrome')
    executor.submit(run_test, 'firefox')

This runs tests on three browsers at once and collects results.

Selenium Python

browsers = ['chrome', 'firefox', 'edge']

with ThreadPoolExecutor(max_workers=3) as executor:
    results = list(executor.map(run_test, browsers))

print(results)

Sample Program

This script runs tests on Chrome and Firefox at the same time. Each test opens the browser, gets the page title, then closes the browser. The results print after both finish.

Selenium Python

from selenium import webdriver
from concurrent.futures import ThreadPoolExecutor

def run_test(browser_name):
    if browser_name == 'chrome':
        driver = webdriver.Chrome()
    elif browser_name == 'firefox':
        driver = webdriver.Firefox()
    else:
        return f'{browser_name} not supported'
    driver.get('https://example.com')
    title = driver.title
    driver.quit()
    return f'{browser_name} title: {title}'

browsers = ['chrome', 'firefox']

with ThreadPoolExecutor(max_workers=2) as executor:
    results = list(executor.map(run_test, browsers))

for result in results:
    print(result)

Important Notes

Make sure your CI environment supports running browsers in parallel.

Use separate browser instances to avoid conflicts.

Limit the number of parallel tests to avoid overloading your system.

Summary

Parallel execution runs tests at the same time to save time.

Use ThreadPoolExecutor in Python to run Selenium tests in parallel.

Good for testing multiple browsers or many tests quickly in CI.

Practice

1. What is the main benefit of running Selenium tests in parallel during Continuous Integration (CI)?

easy

A. It reduces the total test execution time by running tests simultaneously.

B. It makes tests run slower but more accurately.

C. It ensures tests run only on one browser at a time.

D. It disables all tests except one to save resources.

Solution

1. Step 1: Understand parallel execution in CI

   Parallel execution means running multiple tests at the same time instead of one after another.

2. Step 2: Identify the benefit of parallel execution

   Running tests simultaneously saves time by reducing total test duration.

3. Final Answer:

   It reduces the total test execution time by running tests simultaneously. -> Option A

4. Quick Check:

   Parallel execution = faster testing [OK]

Hint: Parallel means running tests at the same time [OK]

Common Mistakes:

• Thinking parallel slows tests down
• Believing tests run only on one browser
• Confusing parallel with disabling tests

2. Which Python module is commonly used to run Selenium tests in parallel within a CI pipeline?

easy

A. ThreadPoolExecutor

B. unittest

C. multiprocessing

D. time

Solution

1. Step 1: Identify Python modules for parallel execution

   Python offers several modules, but ThreadPoolExecutor from concurrent.futures is popular for parallel threads.

2. Step 2: Match module to Selenium parallel testing

   ThreadPoolExecutor allows running multiple Selenium tests in parallel threads efficiently.

3. Final Answer:

   ThreadPoolExecutor -> Option A

4. Quick Check:

   ThreadPoolExecutor = parallel threads [OK]

Hint: ThreadPoolExecutor is for parallel threads in Python [OK]

Common Mistakes:

• Choosing multiprocessing which uses processes, not threads
• Selecting unittest which is a test framework, not parallel executor
• Picking time which is for delays, not parallelism

3. Given the code snippet below, what will be the output?

from concurrent.futures import ThreadPoolExecutor
import time

def test_func(name):
    time.sleep(1)
    return f"Test {name} done"

with ThreadPoolExecutor(max_workers=3) as executor:
    results = list(executor.map(test_func, ['A', 'B', 'C', 'D']))
print(results)

medium

A. Error due to max_workers less than tasks

B. ['Test A done', 'Test B done', 'Test C done']

C. ['Test A done', 'Test B done', 'Test C done', 'Test D done']

D. ['Test D done']

Solution

1. Step 1: Understand ThreadPoolExecutor with max_workers=3

   It runs up to 3 threads at once, but can handle more tasks by queuing them.

2. Step 2: Analyze executor.map behavior

   executor.map runs test_func on all 4 inputs and returns results in order after all finish.

3. Final Answer:

   ['Test A done', 'Test B done', 'Test C done', 'Test D done'] -> Option C

4. Quick Check:

   All tasks run, results list includes all [OK]

Hint: ThreadPoolExecutor queues extra tasks beyond max_workers [OK]

Common Mistakes:

• Assuming only 3 results because max_workers=3
• Expecting error due to more tasks than workers
• Thinking results order changes

4. You wrote this code to run Selenium tests in parallel but get an error: RuntimeError: cannot schedule new futures after shutdown. What is the likely cause?

from concurrent.futures import ThreadPoolExecutor
executor = ThreadPoolExecutor(max_workers=2)
executor.shutdown()
executor.submit(test_function)

medium

A. ThreadPoolExecutor requires a context manager to work.

B. max_workers=2 is too low for submitting tasks.

C. test_function is not defined before submit call.

D. Calling submit after shutdown closes the executor, so no new tasks allowed.

Solution

1. Step 1: Understand executor.shutdown() effect

   shutdown() stops the executor from accepting new tasks and cleans up threads.

2. Step 2: Analyze submit call after shutdown

   Calling submit after shutdown causes RuntimeError because executor is closed.

3. Final Answer:

   Calling submit after shutdown closes the executor, so no new tasks allowed. -> Option D

4. Quick Check:

   submit after shutdown = error [OK]

Hint: Don't submit tasks after executor.shutdown() [OK]

Common Mistakes:

• Thinking max_workers causes this error
• Assuming test_function undefined causes this error
• Believing context manager is required

5. You want to run Selenium tests in parallel on Chrome and Firefox browsers in your CI pipeline using Python. Which approach correctly sets up parallel execution for both browsers?

hard

A. Run tests sequentially by switching browser drivers inside a single test function.

B. Use ThreadPoolExecutor to run a test function twice, each time initializing a different browser driver inside the function.

C. Use a single WebDriver instance shared by all tests to save memory.

D. Run tests in parallel but initialize browsers outside the test functions to avoid overhead.

Solution

1. Step 1: Understand parallel testing with multiple browsers

   Each test must create its own browser driver instance to avoid conflicts.

2. Step 2: Use ThreadPoolExecutor to run tests in parallel

   Running test functions in parallel, each initializing its own browser, allows simultaneous testing on Chrome and Firefox.

3. Final Answer:

   Use ThreadPoolExecutor to run a test function twice, each time initializing a different browser driver inside the function. -> Option B

4. Quick Check:

   Separate drivers per thread = correct parallel setup [OK]

Hint: Initialize browser inside each parallel test function [OK]

Common Mistakes:

• Sharing one WebDriver instance across threads
• Switching browsers sequentially defeats parallelism
• Initializing browsers outside test functions causes conflicts