alert.py

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

driver = webdriver.Chrome()

driver.get('http://www.w3schools.com/js/tryit.asp?filename=tryjs_prompt')
driver.switch_to_frame('iframeResult')

element = WebDriverWait(driver, 10).until(
            EC.presence_of_element_located(
                (By.XPATH, '//button[text()="Try it"]')))
element.click()
wait = WebDriverWait(driver, 10)
alert = wait.until(EC.alert_is_present())

try:
    print alert.text
    alert.dismiss()
finally:
    driver.quit()