to set MutationObserver, How to inject javascript before page-loading using Selenium

Question

I'm trying to set MutationObserver for observing page mutation while loading.

In order to do that, MutationObserver should be configured before page loading.

With selenium-chromedriver, couldn't find the way to inject JS for such purpose.

I know chrome extension can do that but extensions won't work on headless mode.

That's the problem.

Answer 1

It's possible via the DevTool API by calling Page.addScriptToEvaluateOnNewDocument

from selenium import webdriver
from selenium.webdriver.remote.webdriver import WebDriver
import json

def send(driver, cmd, params={}):
  resource = "/session/%s/chromium/send_command_and_get_result" % driver.session_id
  url = driver.command_executor._url + resource
  body = json.dumps({'cmd': cmd, 'params': params})
  response = driver.command_executor._request('POST', url, body)
  if response['status']:
    raise Exception(response.get('value'))
  return response.get('value')

def add_script(driver, script):
  send(driver, "Page.addScriptToEvaluateOnNewDocument", {"source": script})

WebDriver.add_script = add_script


# launch Chrome
driver = webdriver.Chrome()

# add a script which will be executed when the page starts loading
driver.add_script("""
  if (window.self === window.top) { // if main document
    console.log('add script');
  }
  """)

# load a page
driver.get("https://stackoverflow.com/questions")