前言
在定位元素的時候，經常會遇到各種異常，為什么會發生這些異常，遇到異常又該如何處理呢？
本篇通過學習selenium的exceptions模塊，了解異常發生的原因。
selenium+python高級教程》已出書：selenium webdriver基於Python源碼案例
（購買此書送對應PDF版本）
一、發生異常
1.打開博客首頁，定位“新隨筆”元素，此元素id="blog_nav_newpost"
2.為了故意讓它定位失敗，我在元素屬性后面加上xx
3.運行失敗后如下圖所示，程序在查找元素的這一行發生了中斷，不會繼續執行click事件了
二、捕獲異常
1.為了讓程序繼續執行，我們可以用try...except...捕獲異常。捕獲異常后可以打印出異常原因，這樣以便於分析異常原因
2.從如下異常內容可以看出，發生異常原因是：NoSuchElementException
selenium.common.exceptions.NoSuchElementException: Message: Unable to locate element: {"method":"id","selector":"blog_nav_newpostxx"}
3.從selenium.common.exceptions 導入 NoSuchElementException類
三、參考代碼：
# coding:utf-8
from selenium import webdriver
from selenium.common.exceptions import NoSuchElementException
driver = webdriver.Firefox()
driver.get("http://www.cnblogs.com/yoyoketang/")
# 定位首頁"新隨筆"
try:
element = driver.find_element("id", "blog_nav_newpostxx")
except NoSuchElementException as msg:
print u"查找元素異常%s"%msg
# 點擊該元素 # 交流QQ群：232607095
else:
element.click()
四、selenium常見異常
1.NoSuchElementException：沒有找到元素
2.NoSuchFrameException：沒有找到iframe
3.NoSuchWindowException:沒找到窗口句柄handle
4.NoSuchAttributeException:屬性錯誤
5.NoAlertPresentException：沒找到alert彈出框
6.lementNotVisibleException：元素不可見
7.ElementNotSelectableException：元素沒有被選中
8.TimeoutException：查找元素超時
五、其它異常與源碼
1.在Lib目錄下：\Lib\site-packages\selenium\common有興趣的可以看看
# 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.
"""
Exceptions that may happen in all the webdriver code.
"""
class WebDriverException(Exception):
"""
Base webdriver exception.
"""
def __init__(self, msg=None, screen=None, stacktrace=None):
self.msg = msg
self.screen = screen
self.stacktrace = stacktrace
def __str__(self):
exception_msg = "Message: %s\n" % self.msg
if self.screen is not None:
exception_msg += "Screenshot: available via screen\n"
if self.stacktrace is not None:
stacktrace = "\n".join(self.stacktrace)
exception_msg += "Stacktrace:\n%s" % stacktrace
return exception_msg
class ErrorInResponseException(WebDriverException):
"""
Thrown when an error has occurred on the server side.
This may happen when communicating with the firefox extension
or the remote driver server.
"""
def __init__(self, response, msg):
WebDriverException.__init__(self, msg)
self.response = response
class InvalidSwitchToTargetException(WebDriverException):
"""
Thrown when frame or window target to be switched doesn't exist.
"""
pass
class NoSuchFrameException(InvalidSwitchToTargetException):
"""
Thrown when frame target to be switched doesn't exist.
"""
pass
class NoSuchWindowException(InvalidSwitchToTargetException):
"""
Thrown when window target to be switched doesn't exist.
To find the current set of active window handles, you can get a list
of the active window handles in the following way::
print driver.window_handles
"""
pass
class NoSuchElementException(WebDriverException):
"""
Thrown when element could not be found.
If you encounter this exception, you may want to check the following:
* Check your selector used in your find_by...
* Element may not yet be on the screen at the time of the find operation,
(webpage is still loading) see selenium.webdriver.support.wait.WebDriverWait()
for how to write a wait wrapper to wait for an element to appear.
"""
pass
class NoSuchAttributeException(WebDriverException):
"""
Thrown when the attribute of element could not be found.
You may want to check if the attribute exists in the particular browser you are
testing against. Some browsers may have different property names for the same
property. (IE8's .innerText vs. Firefox .textContent)
"""
pass
class StaleElementReferenceException(WebDriverException):
"""
Thrown when a reference to an element is now "stale".
Stale means the element no longer appears on the DOM of the page.
Possible causes of StaleElementReferenceException include, but not limited to:
* You are no longer on the same page, or the page may have refreshed since the element
was located.
* The element may have been removed and re-added to the screen, since it was located.
Such as an element being relocated.
This can happen typically with a javascript framework when values are updated and the
node is rebuilt.
* Element may have been inside an iframe or another context which was refreshed.
"""
pass
class InvalidElementStateException(WebDriverException):
"""
"""
pass
class UnexpectedAlertPresentException(WebDriverException):
"""
Thrown when an unexpected alert is appeared.
Usually raised when when an expected modal is blocking webdriver form executing any
more commands.
"""
def __init__(self, msg=None, screen=None, stacktrace=None, alert_text=None):
super(UnexpectedAlertPresentException, self).__init__(msg, screen, stacktrace)
self.alert_text = alert_text
def __str__(self):
return "Alert Text: %s\n%s" % (self.alert_text, super(UnexpectedAlertPresentException, self).__str__())
class NoAlertPresentException(WebDriverException):
"""
Thrown when switching to no presented alert.
This can be caused by calling an operation on the Alert() class when an alert is
not yet on the screen.
"""
pass
class ElementNotVisibleException(InvalidElementStateException):
"""
Thrown when an element is present on the DOM, but
it is not visible, and so is not able to be interacted with.
Most commonly encountered when trying to click or read text
of an element that is hidden from view.
"""
pass
class ElementNotSelectableException(InvalidElementStateException):
"""
Thrown when trying to select an unselectable element.
For example, selecting a 'script' element.
"""
pass
class InvalidCookieDomainException(WebDriverException):
"""
Thrown when attempting to add a cookie under a different domain
than the current URL.
"""
pass
class UnableToSetCookieException(WebDriverException):
"""
Thrown when a driver fails to set a cookie.
"""
pass
class RemoteDriverServerException(WebDriverException):
"""
"""
pass
class TimeoutException(WebDriverException):
"""
Thrown when a command does not complete in enough time.
"""
pass
class MoveTargetOutOfBoundsException(WebDriverException):
"""
Thrown when the target provided to the `ActionsChains` move()
method is invalid, i.e. out of document.
"""
pass
class UnexpectedTagNameException(WebDriverException):
"""
Thrown when a support class did not get an expected web element.
"""
pass
class InvalidSelectorException(NoSuchElementException):
"""
Thrown when the selector which is used to find an element does not return
a WebElement. Currently this only happens when the selector is an xpath
expression and it is either syntactically invalid (i.e. it is not a
xpath expression) or the expression does not select WebElements
(e.g. "count(//input)").
"""
pass
class ImeNotAvailableException(WebDriverException):
"""
Thrown when IME support is not available. This exception is thrown for every IME-related
method call if IME support is not available on the machine.
"""
pass
class ImeActivationFailedException(WebDriverException):
"""
Thrown when activating an IME engine has failed.
"""
pass
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