看WebDriverWait(self.driver, wait_time).until(EC.visibility_of_element_located((by, locator)))源码的时候，不太明白visibility_of_element_located((by, locator))内为什么是两层括号，因为源码是这样写的：
class visibility_of_element_located(object):
""" An expectation for checking that an element is present on the DOM of a
page and visible. Visibility means that the element is not only displayed
but also has a height and width that is greater than 0.
locator - used to find the element
returns the WebElement once it is located and visible
"""
def __init__(self, locator):
self.locator = locator
def __call__(self, driver):
try:
return _element_if_visible(_find_element(driver, self.locator))
except StaleElementReferenceException:
return False
__call__()的作用是，让实例对象也像函数一样作为可调用对象来使用，正常来讲visibility_of_element_located(locator)(driver)才对，直到看了下until的源码，才发现有这一层：value = method(self._driver)，其中这个method就是EC.visibility_of_element_located((by, locator))实例对象，method(self._driver)就是EC.visibility_of_element_located((by, locator))(self._driver)，也就是把实例对象作为参数一样传了个参数，self._driver。其实前者((by, locator))是__init__()做初始化，只不过参数是个元组(by, locator)，这个元组作为实例对象的属性self.locator，而后者(self._driver)就是__call__()的作用
def until(self, method, message=''):
"""Calls the method provided with the driver as an argument until the \
return value is not False."""
screen = None
stacktrace = None
end_time = time.time() + self._timeout
while True:
try:
value = method(self._driver)
if value:
return value
except self._ignored_exceptions as exc:
screen = getattr(exc, 'screen', None)
stacktrace = getattr(exc, 'stacktrace', None)
time.sleep(self._poll)
if time.time() > end_time:
break
raise TimeoutException(message, screen, stacktrace)
当使用EC.visibility_of_element_located((by, locator))(self._driver)时，总共进行了以下的步骤：
1. 实例化一个EC.visibility_of_element_located，传入参数(by, locator)，将其赋值给属性self.locator
2. 通过__call__()，将参数self._driver（实际上这个self.driver就是WebDriverWait中的self.driver传进来的）和self.locator传给内层方法_find_element()
2.1 这个find_element(self._driver, self.locator)对应着源码中的driver和by，再通过*by解包(by, locator)，使其这样调用driver.find_element(by, locator)
def _find_element(driver, by):
"""Looks up an element. Logs and re-raises ``WebDriverException``
if thrown."""
try:
return driver.find_element(*by)
except NoSuchElementException as e:
raise e
except WebDriverException as e:
raise e
2.2 然后driver.find_element(by, locator)找到元素对象并返回给_find_element()方法
2.3 _find_element()方法拿到元素对象，此时作为参数传递给_element_if_visisble()方法，element_if_visisble()方法通过判断元素是否可见，返回True或False
until作为一个判断条件，来决定是否继续寻找可见元素，主要有这几种情况：如果EC.visibility_of_element_located((by, locator))(self._driver)返回为True，则返回对应的True；如果已超时，则抛出超时异常
