Reliably detect page load or time out, Selenium 2

Question

I am writing a generic web-scraper using Selenium 2 (version 2.33 Python bindings, Firefox driver). It is supposed to take an arbitrary URL, load the page, and report all of the outbound links. Because the URL is arbitrary, I cannot make any assumptions whatsoever about the contents of the page, so the usual advice (wait for a specific element to be present) is inapplicable.

I have code which is supposed to poll document.readyState until it reaches "complete" or a 30s timeout has elapsed, and then proceed:

def readystate_complete(d):
    # AFAICT Selenium offers no better way to wait for the document to be loaded,
    # if one is in ignorance of its contents.
    return d.execute_script("return document.readyState") == "complete"

def load_page(driver, url):
    try:
        driver.get(url)
        WebDriverWait(driver, 30).until(readystate_complete)
    except WebDriverException:
        pass

    links = []
    try:
        for elt in driver.find_elements_by_xpath("//a[@href]"):
            try: links.append(elt.get_attribute("href"))
            except WebDriverException: pass
    except WebDriverException: pass
    return links

This sort-of works, but on about one page out of five, the .until call hangs forever. When this happens, usually the browser has not in fact finished loading the page (the "throbber" is still spinning) but tens of minutes can go by and the timeout does not trigger. But sometimes the page does appear to have loaded completely and the script still does not go on.

What gives? How do I make the timeout work reliably? Is there a better way to request a wait-for-page-to-load (if one cannot make any assumptions about the contents)?

Note: The obsessive catching-and-ignoring of WebDriverException has proven necessary to ensure that it extracts as many links from the page as possible, whether or not JavaScript inside the page is doing funny stuff with the DOM (e.g. I used to get "stale element" errors in the loop that extracts the HREF attributes).

NOTE: There are a lot of variations on this question both on this site and elsewhere, but they've all either got a subtle but critical difference that makes the answers (if any) useless to me, or I've tried the suggestions and they don't work. Please answer exactly the question I have asked.

Answer 1

1. The "recommended" (however still ugly) solution could be to use explicit wait:

   from selenium.webdriver.common.by import By
   from selenium.webdriver.support.ui import WebDriverWait 
   from selenium.webdriver.support import expected_conditions
   
   old_value = browser.find_element_by_id('thing-on-old-page').text
   browser.find_element_by_link_text('my link').click()
   WebDriverWait(browser, 3).until(
       expected_conditions.text_to_be_present_in_element(
           (By.ID, 'thing-on-new-page'),
           'expected new text'
       )
   )
   

2. The naive attempt would be something like this:

   def wait_for(condition_function):
       start_time = time.time()
       while time.time() < start_time + 3:
           if condition_function():
               return True
           else:
               time.sleep(0.1)
       raise Exception(
           'Timeout waiting for {}'.format(condition_function.__name__)
       )
   
   
   def click_through_to_new_page(link_text):
       browser.find_element_by_link_text('my link').click()
   
       def page_has_loaded():
           page_state = browser.execute_script(
               'return document.readyState;'
           ) 
           return page_state == 'complete'
   
       wait_for(page_has_loaded)
   

3. Another, better one would be (credits to @ThomasMarks):

   def click_through_to_new_page(link_text):
       link = browser.find_element_by_link_text('my link')
       link.click()
   
       def link_has_gone_stale():
           try:
               # poll the link with an arbitrary call
               link.find_elements_by_id('doesnt-matter') 
               return False
           except StaleElementReferenceException:
               return True
   
       wait_for(link_has_gone_stale)
   

4. And the final example includes comparing page ids as below (which could be bulletproof):

   class wait_for_page_load(object):
   
       def __init__(self, browser):
           self.browser = browser
   
       def __enter__(self):
           self.old_page = self.browser.find_element_by_tag_name('html')
   
       def page_has_loaded(self):
           new_page = self.browser.find_element_by_tag_name('html')
           return new_page.id != self.old_page.id
   
       def __exit__(self, *_):
           wait_for(self.page_has_loaded)
   

   And now we can do:

   with wait_for_page_load(browser):
       browser.find_element_by_link_text('my link').click()
   

Above code samples are from Harry's blog.