Selenium Firefox headless returns different results

Question

When i scrape page that contains products with usage of headless option i get different results.
For the same question one time i get results that are not sorted, and an

Answer 1

Ideally, using and not using firefox_options.headless = True shouldn't have any major effect on the elements within the DOM Tree getting rendered but may have a significant difference as far as the Viewport is concerned.

As an example, when GeckoDriver/Firefox is initialized along with the --headless option the default Viewport is width = 1366px, height = 768px where as when GeckoDriver/Firefox is initialized without the --headless option the default Viewport is width = 1382px, height = 744px.

• Example Code:

  from selenium import webdriver
  from selenium.webdriver.support.ui import WebDriverWait
  from selenium.webdriver.common.by import By
  from selenium.webdriver.support import expected_conditions as EC
  
  options = webdriver.FirefoxOptions()
  options.headless = True
  driver = webdriver.Firefox(options=options, executable_path=r'C:\Utility\BrowserDrivers\geckodriver.exe')
  driver.get("https://www.google.com/")
  WebDriverWait(driver, 10).until(EC.element_to_be_clickable((By.NAME, "q")))
  print ("Headless Firefox Initialized")
  size = driver.get_window_size()
  print("Window size: width = {}px, height = {}px".format(size["width"], size["height"]))
  driver.quit()
  driver = webdriver.Firefox(executable_path=r'C:\Utility\BrowserDrivers\geckodriver.exe')
  driver.get("https://www.google.com/")
  WebDriverWait(driver, 10).until(EC.element_to_be_clickable((By.NAME, "q")))
  print ("Firefox Initialized")
  size = driver.get_window_size()
  print("Window size: width = {}px, height = {}px".format(size["width"], size["height"]))
  driver.quit()
  

• Console Output:

  Headless Firefox Initialized
  Window size: width = 1366px, height = 768px
  Firefox Initialized
  Window size: width = 1382px, height = 744px
  

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Conclusion

From the above observation it can be inferred that with --headless option GeckoDriver/Firefox opens the Browsing Context with reduced Viewport and hence the number of elements identified can be less.

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Solution

While using GeckoDriver/Firefox to initiate a Browsing Context always open in maximized mode or configure through set_window_size() as follows:

from selenium import webdriver
from selenium.webdriver.support.ui import WebDriverWait
from selenium.webdriver.common.by import By
from selenium.webdriver.support import expected_conditions as EC

options = webdriver.FirefoxOptions()
options.headless = True
#options.add_argument("start-maximized")
options.add_argument("window-size=1400,600")
driver = webdriver.Firefox(options=options, executable_path=r'C:\Utility\BrowserDrivers\geckodriver.exe')
driver.get("https://www.google.com/")
driver.set_window_size(1920, 1080)

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tl; dr

You find a couple of relevant discussion on window size in:

• python: How to set window size in Selenium Chrome Python
• java: Not able to maximize Chrome Window in headless mode