背景
UI自动化过程中，必然会遇到加载问题、环境不稳定、网络慢的情况，如果不做任何处理就会因为没有找到元素而报错。
另外一种情况就是页面使用了ajax异步加载机制（现在都是resetful，客户端和服务端都是分离的），不知道页面是什么时候到达的。这时我们就要用到wait，而在Selenium 中，我们一共有三种等待，分别是固定等待、隐式等待和显式等待。
time.sleep
固定等待，本质：让当前的线程睡眠，实质是线程的阻塞（blocking），用wait 方式实现。
缺点：网络条件好浪费时间，严重影响项目的性能。
好处：调试脚本可以用。
implicitly_wait
隐式等待，本质：在脚本的开始设置一个最长等待时间，如果在规定时间内网页加载完成，则执行下一步，否则可能抛出异常。隐式等待对整个driver周期都起作用，在最开始设置一次就可以了，不要当作固定等待使用。
缺点：JavaScript一般都是放在我们的body的最后进行加载，实际这时页面的元素都已经加载完毕，我们却还在等待全部页面加载结束。
WebDriverWait
显式等待，本质：动态的等待，判断某一个元素是不是已经出现了，比如title是不是叫百度或百度搜索，根据动态的一些条件来轮询，它会不停的轮询去给我们检测，条件是成功还是失败，比如0.5s就检测一次这个元素在这个条件下是成功还是失败。同时设置轮询的超时时间。
如果同时设置了显式等待和隐式等待，就看谁设置的等待时间长，谁的超时等待时间长，就用谁的执行。
显式等待
模块
from selenium.webdriver.support.wait import WebDriverWait
WebDriverWait 的源码
# Licensed to the Software Freedom Conservancy (SFC) under one
# or more contributor license agreements. See the NOTICE file
# distributed with this work for additional information
# regarding copyright ownership. The SFC licenses this file
# to you under the Apache License, Version 2.0 (the
# "License"); you may not use this file except in compliance
# with the License. You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing,
# software distributed under the License is distributed on an
# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
# KIND, either express or implied. See the License for the
# specific language governing permissions and limitations
# under the License.
import time
from selenium.common.exceptions import NoSuchElementException
from selenium.common.exceptions import TimeoutException
POLL_FREQUENCY = 0.5 # How long to sleep inbetween calls to the method
IGNORED_EXCEPTIONS = (NoSuchElementException,) # exceptions ignored during calls to the method
class WebDriverWait(object):
def __init__(self, driver, timeout, poll_frequency=POLL_FREQUENCY, ignored_exceptions=None):
"""Constructor, takes a WebDriver instance and timeout in seconds.
:Args:
- driver - Instance of WebDriver (Ie, Firefox, Chrome or Remote)
- timeout - Number of seconds before timing out
- poll_frequency - sleep interval between calls
By default, it is 0.5 second.
- ignored_exceptions - iterable structure of exception classes ignored during calls.
By default, it contains NoSuchElementException only.
Example:
from selenium.webdriver.support.ui import WebDriverWait \n
element = WebDriverWait(driver, 10).until(lambda x: x.find_element_by_id("someId")) \n
is_disappeared = WebDriverWait(driver, 30, 1, (ElementNotVisibleException)).\ \n
until_not(lambda x: x.find_element_by_id("someId").is_displayed())
"""
self._driver = driver
self._timeout = timeout
self._poll = poll_frequency
# avoid the divide by zero
if self._poll == 0:
self._poll = POLL_FREQUENCY
exceptions = list(IGNORED_EXCEPTIONS)
if ignored_exceptions is not None:
try:
exceptions.extend(iter(ignored_exceptions))
except TypeError: # ignored_exceptions is not iterable
exceptions.append(ignored_exceptions)
self._ignored_exceptions = tuple(exceptions)
def __repr__(self):
return '<{0.__module__}.{0.__name__} (session="{1}")>'.format(
type(self), self._driver.session_id)
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)
def until_not(self, method, message=''):
"""Calls the method provided with the driver as an argument until the \
return value is False."""
end_time = time.time() + self._timeout
while True:
try:
value = method(self._driver)
if not value:
return value
except self._ignored_exceptions:
return True
time.sleep(self._poll)
if time.time() > end_time:
break
raise TimeoutException(message)
参数：
三种等待方式的实例
from selenium import webdriver
from time import sleep
from selenium.webdriver.support.wait import WebDriverWait
from selenium.webdriver.support import expected_conditions as EC
# 三种等待方法
class TestCase(object):
def __init__(self):
self.driver = webdriver.Chrome()
self.driver.get('https://www.baidu.com')
sleep(2)
def test_sleep(self):
self.driver.find_element_by_id('kw').send_keys("selenium")
sleep(2) # 线程的阻塞 blocking wait 方式实现
self.driver.find_element_by_id('su').click()
sleep(3)
self.driver.quit()
def time_implicitly(self):
self.driver.implicitly_wait(10)
self.driver.find_element_by_id('kw').send_keys("webdriver")
self.driver.find_element_by_id('su').click()
# sleep(2)
self.driver.quit()
def time_wait(self):
wait = WebDriverWait(self.driver,2,0.5)
wait.until(EC.title_is('百度一下，你就知道'))
self.driver.find_element_by_id('kw').send_keys("webdriver")
self.driver.find_element_by_id('su').click()
sleep(2)
self.driver.quit()
if __name__=="__main__":
case = TestCase()
# case.test_sleep()
# case.time_implicitly()
case.time_wait()
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