Stop 07 · Web Automation TestingTopics · 4/10

Waits & Synchronization

In short

Most flaky UI tests aren't caused by bugs. They're caused by a script clicking before the page is ready. Waiting the right way fixes the biggest headache in automation.

Ensuring a script waits for a page or element to be ready before interacting with it — the single most common source of flaky, unreliable automated tests when handled incorrectly.

The purple blob waits on a bicycle at a lowered railway crossing barrier as a train passes.

Think of it like this

The railway crossing gate

You don't count to 300 and then ride across. You wait until the gate actually lifts. Sometimes that's one minute, sometimes ten. A good wait watches for a condition, not a clock.

Watch it happen

An explicit wait polling the page

Script

WebDriverWait

Page

click Start

1/6The click starts a slow load. The result appears a few seconds later.

The big ideas

What you need to know

sleep is a guess

time.sleep(5) is too long on fast days and too short on slow ones. Avoid it.

Explicit waits

WebDriverWait plus a condition like visibility or clickability waits only as long as needed.

Don't mix waits

Selenium's docs warn against mixing implicit and explicit waits. It can cause long, unpredictable delays.

Sensible timeouts

Pick a limit like 10 seconds. If it times out, that's a real signal worth investigating.

See it in code

Walk through the code

python

1from selenium import webdriver2from selenium.webdriver.common.by import By3from selenium.webdriver.support.ui import WebDriverWait4from selenium.webdriver.support import expected_conditions as EC5 6driver = webdriver.Chrome()7driver.get("https://the-internet.herokuapp.com/dynamic_loading/1")8driver.find_element(By.CSS_SELECTOR, "#start button").click()9 10wait = WebDriverWait(driver, 10)11finish = wait.until(EC.visibility_of_element_located((By.ID, "finish")))12 13assert finish.text == "Hello World!"14driver.quit()

1/3Clicking Start triggers a loading bar that takes a few seconds.

The text is in the page but hidden while loading, so we wait for it to become visible.

Save yourself the pain

Common mistakes

Oops

Adding time.sleep(10) everywhere until the test stops failing.

Do this instead

Wait for a specific condition, like visibility or clickability, with WebDriverWait.

Oops

Waiting for an element to exist when it's present but hidden.

Do this instead

Pick the right condition: presence, visibility and clickability are different things.

Your mission

Kill a flaky test

~25 min

Quick check

Why is an explicit wait better than time.sleep(5)?

Go deeper

1Replace a hardcoded sleep() in a script with an explicit wait condition.
2Learn the difference between implicit waits, explicit waits, and fixed sleeps.
3Understand why fixed sleeps are almost always the wrong choice — too slow or too fragile.