syntax for new ExpectedCondition class in Selenium webdriver python

If you want to wait for arbitrary conditions, you don't have to use ExpectedCondition at all. You can just pass a function to the until method:

from selenium.webdriver.support.ui import WebDriverWait

def condition(driver):
    ret = False
    # ...
    # Actual code to check condition goes here and should
    # set `ret` to a truthy value if the condition is true
    # ...
    return ret

WebDriverWait(driver, 10).until(condition)

The code above will call condition repeatedly until either of the following is true:

• condition returns a value that evaluates true,

• 10 seconds have elapsed (in which case an exception is raised).

I haven't had occassion to try it, but I think you could use EC.alert_is_present:

import selenium.webdriver.support.expected_conditions as EC
import selenium.webdriver.support.ui as UI

wait = UI.WebDriverWait(driver, 10)
alert = wait.until(EC.alert_is_present())

Inside webdriver/support/expected_conditions.py, alert_is_present is defined like this:

class alert_is_present(object):
    """ Expect an alert to be present."""
    def __init__(self):
        pass

    def __call__(self, driver):
        try:
            alert = driver.switch_to_alert()
            alert.text
            return alert
        except NoAlertPresentException:
            return False

You could write it more simply as

def alert_is_present(driver):
    try:
        alert = driver.switch_to_alert()
        alert.text
        return alert
    except NoAlertPresentException:
        return False

This might give you an idea of how such a condition could be written.