Development with Python

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Do you remember that we set an implicitly_wait on the browser, way back in
Do you remember I mentioned it was unreliable?
implicitly_wait works reasonably well for any calls to any of the Selenium find_el
ement_ calls, but it doesn’t apply to browser.current_url . Seleniumdoesn’t “wait” after
you tell it to click an element, so what’s happened is that the browser hasn’t finished
loading the new page yet, so current_url is still the old page. We need to use some
more wait code, like we did for the various Persona pages.
At this point it’s time for a “wait for” helper function. To see how this is going to work,
it helps to see how I expect to use it (outside-in!):
functional_tests/test_my_lists.py (ch20l012).
# She sees that her list is in there, named according to its
# first list item
self .browser .find_element_by_link_text('Reticulate splines' ).click()
self .wait_for(
lambda: self.assertEqual(self.browser.current_url, first_list_url)
)
We’re going to take our assertEqual call and turn it into a lambda function, then pass
it into our wait_for helper.
functional_tests/base.py (ch20l013).
import time
from selenium.common.exceptions import WebDriverException
[ ...]
def wait_for(self, function_with_assertion , timeout=DEFAULT_WAIT):
start_time = time.time()
while time.time() - start_time < timeout:
try:
return function_with_assertion ()
except (AssertionError, WebDriverException ):
time .sleep(0.1)
# one more try, which will raise any errors if they are outstanding
return function_with_assertion ()
wait_for then tries to execute that function, but instead of letting the test fail if the
assertion fails, it catches the AssertionError that assertEqual would ordinarily raise,
waits for a brief moment, and then loops around retrying it. The while loop lasts until
a given timeout. It also catches any WebDriverException that might happen if, say, an
element hasn’t appeared on the page yet. It tries one last time after the timeout has
expired, this time without the try/except, so that if there really is still an AssertionEr
ror , the test will fail appropriately.
A Common Selenium Problem: Race Conditions | 379