UI自动化测试中，Selenium的三种等待方式详解

Posted 2022-12-04 一始如衷

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

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# 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
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# 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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