selenium-判断元素是否可见（五）

很多 case 在运行时都会出现页面还没加载完成，但是脚本已经跑完，并且报未找到元素

这是就需要增加判断，在预定的时间内如果页面显示了某元素后再让脚本继续执行，则为判断元素是否可见或者说页面是否显示了某元素

以百度首页，搜素框为例：

from selenium import webdriver
from selenium.webdriver.support import expected_conditions as EC

driver = webdriver.Chrome()
driver.get('https://www.baidu.com/')
baidu_input = driver.find_element_by_id('kw')
EC.visibility_of_element_located(baidu_input)

driver.close()

EC.visibility_of_element_located(baidu_input) 只是判断元素是否可见，若果这样写明显存在不合理的地方。如果代码运行很快，页面还未加载完就会出现该元素可见找不到。
所以通常需要结合 WebDriverWait 一起使用

from selenium import webdriver
from selenium.webdriver.support import expected_conditions as EC
from selenium.webdriver.support.wait import WebDriverWait
from selenium.webdriver.common.by import By

driver = webdriver.Chrome()
driver.get('https://www.baidu.com/')
baidu_input = (By.ID, 'kw')
WebDriverWait(driver,10).until(EC.visibility_of_element_located(baidu_input))

driver.close()

查看 WebDriverWait 类，他需要传入driver，超时时间timeout，而 unit 只需要传入定位元素，如下代码 WebDriverWait 类所示

所以在使用 WebDriverWait 时需要对元素定位使用 By 定位，剔除通过 driver 再定位的方法，如上代码所示

WebDriverWait 类

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# to you under the Apache License, Version 2.0 (the
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# "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 

            element = WebDriverWait(driver, 10).until(lambda x: x.find_element_by_id("someId")) 

            is_disappeared = WebDriverWait(driver, 30, 1, (ElementNotVisibleException)). 

                        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)

元素是否可见的方法

　　visibility_of_element_located : 判断某个元素是否可见
　　invisibility_of_element_located : 判断某个元素是否不存在或不可见
　　visibility_of : 判断元素是否可见，通过 driver 查找元素，如：EC.visibility_of(driver.find_element_by_id('kw'))
　　visibility_of_all_elements_located() ：判断定位的所有元素都存在于DOM树中并且可见，可存在则以list形式返回
　　visibility_of_any_elements_located() ： 判断定位的所有元素中，至少有一个存在于DOM树中并且可见，list形式返回存在的元素