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Selenium Pythontesting~15 mins

Docker-based Grid in Selenium Python - Build an Automation Script

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Run Selenium test on Docker-based Selenium Grid
Preconditions (3)
Step 1: Write a Selenium test script that connects to the Selenium Grid hub URL
Step 2: Configure the test to use Chrome browser capabilities
Step 3: Navigate to 'https://example.com' in the test
Step 4: Verify the page title is 'Example Domain'
Step 5: Close the browser session
✅ Expected Result: The test connects to the Docker Selenium Grid, runs on the Chrome node, navigates to the page, verifies the title, and finishes without errors.
Automation Requirements - Selenium with Python
Assertions Needed:
Assert page title equals 'Example Domain'
Best Practices:
Use Remote WebDriver to connect to Selenium Grid hub
Use explicit waits if needed
Use proper capabilities for browser selection
Close the driver session after test
Automated Solution
Selenium Python
from selenium import webdriver
from selenium.webdriver.common.by import By
from selenium.webdriver.chrome.options import Options
from selenium.webdriver.support.ui import WebDriverWait
from selenium.webdriver.support import expected_conditions as EC

# Selenium Grid hub URL (adjust if needed)
hub_url = 'http://localhost:4444/wd/hub'

# Set Chrome options
chrome_options = Options()

# Create Remote WebDriver instance
with webdriver.Remote(
    command_executor=hub_url,
    options=chrome_options
) as driver:
    # Navigate to the test page
    driver.get('https://example.com')

    # Wait until title is present and verify it
    WebDriverWait(driver, 10).until(EC.title_is('Example Domain'))
    assert driver.title == 'Example Domain', f"Expected title 'Example Domain' but got '{driver.title}'"

    # Driver will quit automatically due to 'with' context

The code uses Selenium's Remote WebDriver to connect to the Selenium Grid hub running in Docker at http://localhost:4444/wd/hub. We set Chrome options to specify the browser type.

The test navigates to https://example.com and waits explicitly until the page title is exactly 'Example Domain'. This ensures the page loaded correctly before asserting.

The assertion checks the page title matches the expected value. Using a with block ensures the browser session closes automatically after the test, which is a good practice to avoid leftover sessions.

Common Mistakes - 4 Pitfalls
Using local WebDriver instead of Remote WebDriver
Hardcoding browser capabilities incorrectly or missing them
Not waiting for page elements or title before asserting
Not closing the WebDriver session after test
Bonus Challenge

Now add data-driven testing to run the same test on three different URLs with their expected titles.

Show Hint

Practice

(1/5)
1. What is the main purpose of using a Docker-based Selenium Grid?
easy
A. To run tests only on the local machine without network
B. To write test scripts without any browser
C. To replace Selenium WebDriver with a new tool
D. To run browser tests in isolated containers for better scalability

Solution

  1. Step 1: Understand Docker-based Grid concept

    Docker-based Grid runs browsers inside containers, isolating tests and allowing many tests to run in parallel.

  2. Step 2: Compare options with this concept

    Only To run browser tests in isolated containers for better scalability correctly describes running tests in isolated containers for scalability.

  3. Final Answer:

    To run browser tests in isolated containers for better scalability -> Option D

  4. Quick Check:

    Docker Grid = isolated, scalable browser tests [OK]
Hint: Docker Grid isolates browsers to run many tests fast [OK]
Common Mistakes:
  • Thinking Docker replaces Selenium WebDriver
  • Believing tests run only locally without network
  • Confusing Docker Grid with test script writing
2. Which Python code snippet correctly creates a Remote WebDriver session to connect to a Docker-based Selenium Grid at URL http://localhost:4444/wd/hub for Chrome browser?
easy
A. driver = webdriver.Remote('http://localhost:4444/wd/hub', browser='chrome')
B. driver = webdriver.Chrome('http://localhost:4444/wd/hub')
C. driver = webdriver.Remote(command_executor='http://localhost:4444/wd/hub', options=webdriver.ChromeOptions())
D. driver = webdriver.ChromeOptions().Remote('http://localhost:4444/wd/hub')

Solution

  1. Step 1: Recall correct Remote WebDriver syntax

    Remote WebDriver requires command_executor URL and browser options object, e.g., ChromeOptions.

  2. Step 2: Match options with correct syntax

    driver = webdriver.Remote(command_executor='http://localhost:4444/wd/hub', options=webdriver.ChromeOptions()) uses webdriver.Remote with command_executor and ChromeOptions correctly; others misuse classes or parameters.

  3. Final Answer:

    driver = webdriver.Remote(command_executor='http://localhost:4444/wd/hub', options=webdriver.ChromeOptions()) -> Option C

  4. Quick Check:

    Remote WebDriver needs command_executor and options [OK]
Hint: Use webdriver.Remote with command_executor and options [OK]
Common Mistakes:
  • Using webdriver.Chrome instead of webdriver.Remote for grid
  • Passing URL directly to webdriver.Chrome constructor
  • Incorrect chaining of ChromeOptions and Remote
3. Given this Python code connecting to a Docker Selenium Grid, what will print(driver.title) output if the page at https://example.com has title "Example Domain"? 
from selenium import webdriver

driver = webdriver.Remote(
    command_executor='http://localhost:4444/wd/hub',
    options=webdriver.FirefoxOptions()
)
driver.get('https://example.com')
print(driver.title)
driver.quit()
medium
A. Example Domain
B. https://example.com
C. None
D. Error: NoSuchWindowException

Solution

  1. Step 1: Understand code flow with Remote WebDriver

    The code opens Firefox browser via Docker Grid, navigates to example.com, then prints the page title.

  2. Step 2: Recall page title for https://example.com

    The official example.com page title is "Example Domain", so print(driver.title) outputs this string.

  3. Final Answer:

    Example Domain -> Option A

  4. Quick Check:

    driver.title = page title = "Example Domain" [OK]
Hint: Page title prints exactly what the website sets [OK]
Common Mistakes:
  • Confusing URL with page title
  • Expecting None if using Remote WebDriver
  • Assuming an error without browser session
4. You run this Python test script with Docker Selenium Grid but get a connection error. What is the likely cause? 
from selenium import webdriver

driver = webdriver.Remote(
    command_executor='http://localhost:5555/wd/hub',
    options=webdriver.ChromeOptions()
)
driver.get('https://example.com')
medium
A. Docker Selenium Grid is not running on port 5555
B. ChromeOptions cannot be used with Remote WebDriver
C. The URL 'https://example.com' is invalid
D. webdriver.Remote requires a local Chrome browser

Solution

  1. Step 1: Analyze connection error cause

    Connection errors usually mean the Selenium Grid server is unreachable at the given URL and port.

  2. Step 2: Check port and server status

    Port 5555 is uncommon; default Docker Grid uses 4444. If Grid is not running on 5555, connection fails.

  3. Final Answer:

    Docker Selenium Grid is not running on port 5555 -> Option A

  4. Quick Check:

    Connection error = wrong port or server down [OK]
Hint: Check Docker Grid port matches command_executor URL [OK]
Common Mistakes:
  • Thinking ChromeOptions can't be used remotely
  • Blaming website URL for connection errors
  • Assuming Remote needs local browser installed
5. You want to run tests on Chrome and Firefox browsers simultaneously using Docker Selenium Grid. Which Docker Compose service setup is correct to start both browser nodes connected to the hub?
hard
A. version: '3' services: selenium-hub: image: selenium/hub:4.10.0-20230628 ports: - '4444:4444' chrome: image: selenium/node-chrome:4.10.0-20230628 environment: - SE_EVENT_BUS_HOST=selenium-hub firefox: image: selenium/node-firefox:4.10.0-20230628 environment: - SE_EVENT_BUS_HOST=selenium-hub
B. version: '3' services: selenium-hub: image: selenium/hub:4.10.0-20230628 ports: - '4444:4444' chrome: image: selenium/node-chrome:4.10.0-20230628 depends_on: - selenium-hub environment: - SE_EVENT_BUS_HOST=selenium-hub - SE_EVENT_BUS_PUBLISH_PORT=4442 - SE_EVENT_BUS_SUBSCRIBE_PORT=4443 firefox: image: selenium/node-firefox:4.10.0-20230628 depends_on: - selenium-hub environment: - SE_EVENT_BUS_HOST=selenium-hub - SE_EVENT_BUS_PUBLISH_PORT=4442 - SE_EVENT_BUS_SUBSCRIBE_PORT=4443
C. version: '3' services: selenium-hub: image: selenium/hub:4.10.0-20230628 ports: - '4444:4444' chrome: image: selenium/node-chrome:4.10.0-20230628 depends_on: - selenium-hub firefox: image: selenium/node-firefox:4.10.0-20230628 depends_on: - selenium-hub
D. version: '3' services: selenium-hub: image: selenium/hub:4.10.0-20230628 ports: - '4444:4444' chrome: image: selenium/node-chrome:4.10.0-20230628 depends_on: - selenium-hub environment: - SE_EVENT_BUS_HOST=selenium-hub - SE_EVENT_BUS_PUBLISH_PORT=4443 - SE_EVENT_BUS_SUBSCRIBE_PORT=4442 firefox: image: selenium/node-firefox:4.10.0-20230628 depends_on: - selenium-hub environment: - SE_EVENT_BUS_HOST=selenium-hub - SE_EVENT_BUS_PUBLISH_PORT=4442 - SE_EVENT_BUS_SUBSCRIBE_PORT=4443

Solution

  1. Step 1: Understand Docker Compose setup for Selenium Grid

    The hub service exposes port 4444. Each node (chrome, firefox) must depend on hub and set environment variables for event bus host and ports.

  2. Step 2: Check environment variables and dependencies

    version: '3' services: selenium-hub: image: selenium/hub:4.10.0-20230628 ports: - '4444:4444' chrome: image: selenium/node-chrome:4.10.0-20230628 depends_on: - selenium-hub environment: - SE_EVENT_BUS_HOST=selenium-hub - SE_EVENT_BUS_PUBLISH_PORT=4442 - SE_EVENT_BUS_SUBSCRIBE_PORT=4443 firefox: image: selenium/node-firefox:4.10.0-20230628 depends_on: - selenium-hub environment: - SE_EVENT_BUS_HOST=selenium-hub - SE_EVENT_BUS_PUBLISH_PORT=4442 - SE_EVENT_BUS_SUBSCRIBE_PORT=4443 correctly sets SE_EVENT_BUS_HOST, SE_EVENT_BUS_PUBLISH_PORT=4442, and SE_EVENT_BUS_SUBSCRIBE_PORT=4443 for both nodes, matching Selenium Grid docs.

  3. Final Answer:

    version: '3' services: selenium-hub: image: selenium/hub:4.10.0-20230628 ports: - '4444:4444' chrome: image: selenium/node-chrome:4.10.0-20230628 depends_on: - selenium-hub environment: - SE_EVENT_BUS_HOST=selenium-hub - SE_EVENT_BUS_PUBLISH_PORT=4442 - SE_EVENT_BUS_SUBSCRIBE_PORT=4443 firefox: image: selenium/node-firefox:4.10.0-20230628 depends_on: - selenium-hub environment: - SE_EVENT_BUS_HOST=selenium-hub - SE_EVENT_BUS_PUBLISH_PORT=4442 - SE_EVENT_BUS_SUBSCRIBE_PORT=4443 -> Option B

  4. Quick Check:

    Correct event bus ports and depends_on = version: '3' services: selenium-hub: image: selenium/hub:4.10.0-20230628 ports: - '4444:4444' chrome: image: selenium/node-chrome:4.10.0-20230628 depends_on: - selenium-hub environment: - SE_EVENT_BUS_HOST=selenium-hub - SE_EVENT_BUS_PUBLISH_PORT=4442 - SE_EVENT_BUS_SUBSCRIBE_PORT=4443 firefox: image: selenium/node-firefox:4.10.0-20230628 depends_on: - selenium-hub environment: - SE_EVENT_BUS_HOST=selenium-hub - SE_EVENT_BUS_PUBLISH_PORT=4442 - SE_EVENT_BUS_SUBSCRIBE_PORT=4443 [OK]
Hint: Set SE_EVENT_BUS_HOST and ports exactly as docs show [OK]
Common Mistakes:
  • Omitting depends_on causing nodes to start before hub
  • Wrong or missing event bus port environment variables
  • Swapping publish and subscribe port numbers
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