Headless browser execution in Selenium Python - Test Execution Trace

Test Overview

This test opens a browser in headless mode, navigates to a webpage, finds a heading element, and verifies its text content.

Test Code - unittest

Selenium Python

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

class HeadlessBrowserTest(unittest.TestCase):
    def setUp(self):
        options = Options()
        options.headless = True
        self.driver = webdriver.Chrome(options=options)

    def test_heading_text(self):
        self.driver.get('https://example.com')
        heading = self.driver.find_element(By.TAG_NAME, 'h1')
        self.assertEqual(heading.text, 'Example Domain')

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

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

Execution Trace - 5 Steps

Step	Action	System State	Assertion	Result
1	Test starts and sets up Chrome browser in headless mode	Chrome browser is launched without UI (headless)	—	PASS
2	Browser navigates to 'https://example.com'	Page loads with heading 'Example Domain'	—	PASS
3	Finds the <h1> element on the page	Element with tag 'h1' containing text 'Example Domain' is located	—	PASS
4	Checks if the heading text equals 'Example Domain'	Heading text is 'Example Domain'	Assert heading.text == 'Example Domain'	PASS
5	Test tears down and closes the browser	Headless browser session ends	—	PASS

Failure Scenario

Failing Condition: The <h1> element is not found or text does not match 'Example Domain'

Execution Trace Quiz - 3 Questions

What does headless mode mean in this test?

A The browser runs with a graphical interface

B The browser runs without opening a visible window

C The browser runs slower than normal

D The browser runs only on mobile devices

Key Result

Using headless mode allows tests to run faster and without opening a visible browser window, which is useful for automated testing environments.

Practice

1. What is the main advantage of running Selenium tests in headless mode?

easy

A. It requires more system resources than normal mode

B. It allows manual interaction with the browser during tests

C. Tests run faster without opening a visible browser window

D. It disables all JavaScript on the web page

Solution

1. Step 1: Understand headless mode purpose

   Headless mode runs browser tests without showing the browser window, saving time and resources.

2. Step 2: Compare options

   Only Tests run faster without opening a visible browser window correctly states that tests run faster without opening a visible window. Other options are incorrect or unrelated.

3. Final Answer:

   Tests run faster without opening a visible browser window -> Option C

4. Quick Check:

   Headless mode = faster tests without UI [OK]

Hint: Headless means no browser window visible during tests [OK]

Common Mistakes:

• Thinking headless disables JavaScript
• Assuming headless mode allows manual browser control
• Believing headless uses more resources

2. Which of the following is the correct way to enable headless mode in Selenium with Python using Chrome?

easy

A. options = webdriver.ChromeOptions()
options.add_argument('--headless')
driver = webdriver.Chrome(options=options)

B. driver = webdriver.Chrome(headless=True)

C. options = webdriver.ChromeOptions()
options.headless = True
driver = webdriver.Chrome(options=options)

D. driver = webdriver.Chrome()
driver.set_headless(True)

Solution

1. Step 1: Review Selenium headless syntax

   To enable headless mode, you add the '--headless' argument to ChromeOptions and pass it to the driver.

2. Step 2: Analyze options

   options = webdriver.ChromeOptions() options.add_argument('--headless') driver = webdriver.Chrome(options=options) correctly uses options.add_argument('--headless') and passes options to Chrome. options = webdriver.ChromeOptions() options.headless = True driver = webdriver.Chrome(options=options) incorrectly sets options.headless (not standard). Options B and D use invalid syntax.

3. Final Answer:

   options = webdriver.ChromeOptions() options.add_argument('--headless') driver = webdriver.Chrome(options=options) -> Option A

4. Quick Check:

   Use add_argument('--headless') to enable headless [OK]

Hint: Use add_argument('--headless') on ChromeOptions [OK]

Common Mistakes:

• Setting options.headless = True (not standard)
• Passing headless=True directly to webdriver.Chrome()
• Calling non-existent set_headless method

3. What will be the output of the following code snippet?

from selenium import webdriver
options = webdriver.ChromeOptions()
options.add_argument('--headless')
driver = webdriver.Chrome(options=options)
driver.get('https://example.com')
print(driver.title)
driver.quit()

medium

A. Opens a visible browser window and prints the title

B. Raises an error because headless mode cannot get page title

C. Prints an empty string because page is not loaded in headless mode

D. Prints the title of the page 'Example Domain' without opening a browser window

Solution

1. Step 1: Understand headless execution effect

   Headless mode runs browser without UI but fully loads pages and executes scripts.

2. Step 2: Analyze code behavior

   The code loads 'https://example.com' and prints the page title. Headless mode does not prevent getting the title, so it prints 'Example Domain'.

3. Final Answer:

   Prints the title of the page 'Example Domain' without opening a browser window -> Option D

4. Quick Check:

   Headless loads page and gets title correctly [OK]

Hint: Headless runs full browser without UI, so title is accessible [OK]

Common Mistakes:

• Assuming headless mode blocks page loading
• Expecting errors when accessing page title in headless
• Thinking headless opens visible browser window

4. You wrote this code to run Chrome in headless mode but the browser window still appears. What is the likely mistake?

from selenium import webdriver
options = webdriver.ChromeOptions()
driver = webdriver.Chrome(options=options)
options.add_argument('--headless')
driver.get('https://example.com')

medium

A. Headless mode is not supported on Chrome

B. The headless argument was added after creating the driver, so it was ignored

C. You must call driver.set_headless(True) after creating the driver

D. You forgot to call driver.quit() to close the window

Solution

1. Step 1: Check order of setting options

   The headless argument must be added to options before creating the driver instance.

2. Step 2: Identify mistake in code

   Here, options.add_argument('--headless') is called after driver creation, so the driver does not run headless.

3. Final Answer:

   The headless argument was added after creating the driver, so it was ignored -> Option B

4. Quick Check:

   Add headless argument before driver creation [OK]

Hint: Add headless option before creating driver instance [OK]

Common Mistakes:

• Adding headless argument after driver creation
• Calling non-existent set_headless method
• Assuming headless unsupported on Chrome

5. You want to run Selenium tests in headless mode on a Linux server without a display. Which combination of steps is correct to ensure tests run smoothly?

hard

A. Use ChromeOptions with '--headless' argument and set '--no-sandbox' and '--disable-dev-shm-usage' arguments

B. Use FirefoxOptions with '--headless' and run tests without any additional arguments

C. Run Chrome without headless mode but use Xvfb to simulate display

D. Use ChromeOptions with '--headless' only; no other arguments needed

Solution

1. Step 1: Understand Linux server constraints

   Linux servers often lack GUI and have limited shared memory, so Chrome needs extra flags to run headless smoothly.

2. Step 2: Identify required ChromeOptions

   Adding '--no-sandbox' and '--disable-dev-shm-usage' helps avoid permission and memory issues on Linux servers.

3. Step 3: Evaluate options

   Use ChromeOptions with '--headless' argument and set '--no-sandbox' and '--disable-dev-shm-usage' arguments correctly includes these flags with '--headless'. Use FirefoxOptions with '--headless' and run tests without any additional arguments misses Linux-specific flags. Run Chrome without headless mode but use Xvfb to simulate display uses Xvfb but not headless mode. Use ChromeOptions with '--headless' only; no other arguments needed misses important flags.

4. Final Answer:

   Use ChromeOptions with '--headless' argument and set '--no-sandbox' and '--disable-dev-shm-usage' arguments -> Option A

5. Quick Check:

   Linux headless Chrome needs extra flags for stability [OK]

Hint: Add '--no-sandbox' and '--disable-dev-shm-usage' on Linux [OK]

Common Mistakes:

• Using headless without Linux-specific flags
• Assuming Firefox needs no extra setup
• Relying on Xvfb instead of headless mode