Docker containers for test execution in Selenium Python - Test Execution Trace

Test Overview

This test opens a web page inside a Docker container running Selenium WebDriver. It verifies the page title to confirm the browser inside the container works correctly.

Test Code - unittest

Selenium Python

from selenium import webdriver
from selenium.webdriver.common.by import By
from selenium.webdriver.chrome.service import Service
from selenium.webdriver.chrome.options import Options
import unittest

class TestDockerSelenium(unittest.TestCase):
    def setUp(self):
        chrome_options = Options()
        chrome_options.add_argument('--headless')
        chrome_options.add_argument('--no-sandbox')
        chrome_options.add_argument('--disable-dev-shm-usage')
        self.driver = webdriver.Remote(
            command_executor='http://localhost:4444/wd/hub',
            options=chrome_options
        )

    def test_open_example(self):
        self.driver.get('https://example.com')
        title = self.driver.title
        self.assertEqual(title, 'Example Domain')

    def tearDown(self):
        self.driver.quit()

if __name__ == '__main__':
    unittest.main()

Execution Trace - 4 Steps

Step	Action	System State	Assertion	Result
1	Test starts and sets up Selenium WebDriver to connect to Docker container at localhost:4444	Docker container with Selenium Grid is running and ready to accept connections	—	PASS
2	Browser inside Docker container opens URL 'https://example.com'	Browser navigates to example.com page inside container	—	PASS
3	Test retrieves the page title from the browser	Page title is 'Example Domain'	Check if page title equals 'Example Domain'	PASS
4	Test closes the browser and ends session	Browser session inside Docker container is closed	—	PASS

Failure Scenario

Failing Condition: Docker container with Selenium is not running or unreachable at localhost:4444

Execution Trace Quiz - 3 Questions

What does the test verify inside the Docker container?

A The page URL contains 'docker'

B The page title is 'Example Domain'

C The browser version is Chrome

D The container has internet access

Key Result

Using Docker containers for test execution isolates browser environments, making tests consistent and easy to run on any machine without local browser setup.

Practice

1. What is the main benefit of using Docker containers for Selenium test execution?

easy

A. They replace the need for writing test scripts

B. They provide a clean, consistent environment for tests to run

C. They automatically fix test failures

D. They make tests run faster on any machine

Solution

1. Step 1: Understand Docker container purpose

   Docker containers isolate the environment so tests run the same everywhere.

2. Step 2: Compare options

   Only They provide a clean, consistent environment for tests to run correctly describes this benefit; others are incorrect or unrelated.

3. Final Answer:

   They provide a clean, consistent environment for tests to run -> Option B

4. Quick Check:

   Docker containers = consistent test environment [OK]

Hint: Docker isolates environment to avoid setup issues [OK]

Common Mistakes:

• Thinking Docker speeds up tests automatically
• Believing Docker fixes test code errors
• Confusing Docker with test script writing

2. Which Python Selenium WebDriver URL should you use to connect to a Selenium server running inside a Docker container?

easy

A. "http://localhost:4444/wd/hub"

B. "http://127.0.0.1:8080"

C. "http://docker:5555"

D. "http://0.0.0.0:80"

Solution

1. Step 1: Identify default Selenium server URL in Docker

   The Selenium server in Docker usually listens on port 4444 at localhost.

2. Step 2: Match URL options

   Only "http://localhost:4444/wd/hub" matches the standard Selenium Docker URL format.

3. Final Answer:

   "http://localhost:4444/wd/hub" -> Option A

4. Quick Check:

   Default Selenium Docker URL = localhost:4444/wd/hub [OK]

Hint: Selenium Docker server default port is 4444 [OK]

Common Mistakes:

• Using wrong port numbers
• Using IPs other than localhost
• Missing the /wd/hub path

3. Given this Python code snippet connecting to a Selenium Docker container, what will be the output if the container is not running?

from selenium import webdriver
from selenium.common.exceptions import WebDriverException

try:
    driver = webdriver.Remote(
        command_executor='http://localhost:4444/wd/hub',
        options=webdriver.ChromeOptions()
    )
    print("Connected")
except WebDriverException:
    print("Connection failed")

medium

A. Connection failed

B. Connected

C. SyntaxError

D. TimeoutError

Solution

1. Step 1: Understand connection attempt

   The code tries to connect to Selenium server at localhost:4444.

2. Step 2: Consider container not running

   If the container is down, connection fails and WebDriverException is raised, triggering except block.

3. Final Answer:

   Connection failed -> Option A

4. Quick Check:

   Container down causes WebDriverException = "Connection failed" [OK]

Hint: If container off, connection throws WebDriverException [OK]

Common Mistakes:

• Expecting 'Connected' even if container is off
• Confusing exception types
• Thinking code has syntax errors

4. You wrote this Docker command to run Selenium server:

docker run -d -p 4444:4444 selenium/standalone-chrome

But your Selenium Python tests fail to connect. What is the most likely issue?

medium

A. You used the wrong port mapping in Docker command

B. You forgot to install Selenium Python package

C. Docker container is not running or crashed

D. Your test code uses an incorrect WebDriver import

Solution

1. Step 1: Check Docker container status

   If tests can't connect, first check if container is running or crashed.

2. Step 2: Evaluate other options

   The other options are less likely because the Docker command uses correct port mapping and Selenium package or imports cause different errors.

3. Final Answer:

   Docker container is not running or crashed -> Option C

4. Quick Check:

   Connection fails if container stopped = Docker container is not running or crashed [OK]

Hint: Check container running status first when connection fails [OK]

Common Mistakes:

• Assuming code import errors cause connection failure
• Ignoring container crash possibility
• Confusing port mapping syntax

5. You want to run multiple Selenium Python tests in parallel using Docker containers. Which approach best ensures isolated, reliable test runs?

hard

A. Run tests locally without Docker to avoid container overhead

B. Run all tests in a single container sharing the same browser session

C. Use Docker but share the same Selenium server URL for all tests

D. Run one Docker container per test with unique ports and clean state

Solution

1. Step 1: Understand parallel test isolation

   Running one container per test ensures each test has a clean browser and no interference.

2. Step 2: Evaluate other options

   Sharing containers or sessions risks test interference and flaky results; running locally loses Docker benefits.

3. Final Answer:

   Run one Docker container per test with unique ports and clean state -> Option D

4. Quick Check:

   Parallel tests need isolated containers = Run one Docker container per test with unique ports and clean state [OK]

Hint: Use separate containers per test for isolation [OK]

Common Mistakes:

• Sharing one container causing test conflicts
• Ignoring container cleanup between tests
• Running tests locally losing environment consistency