3 minutes to understand the three waiting methods of Web Automation

Preface

Nowadays, many web pages are dynamically loaded , If the content of the page changes , It takes time to render .

Before we do web When automating tests , Because the code is automatically executed , When the code is executing , It is possible that the element loaded in the previous operation has not been loaded , You're going to report a mistake NoSuchElementException, If you report this mistake , It is likely that the positioning expression is incorrect , Or the page elements have not been loaded .

To solve this problem We can improve the stability of the script by setting some waiting mechanisms . do web Automation uses the following three waiting methods ：

• 1, Mandatory waiting ：time.sleep()
• 2, An implicit wait : driver.implicitly_wait(10)
• 3, Explicit waiting ：（ Wait for the element to be listed in a certain state : There is 、 so 、 Click and so on ）

WebDriverWait(driver, 30, 0.2).until(
    EC.visibility_of_element_located(
        (By.XPATH, '//input[@id="u"]')
    )
)

Next, we will introduce the use of three waiting methods in detail .

1、 Mandatory waiting

Mandatory waiting , That is to say python Medium time.sleep(), When the program is executed to time.sleep() The program will go to sleep ( That is what we call forced waiting ), When the sleep time has passed, continue to execute .

Disadvantages of using forced waiting , The program is in the process of execution , No matter whether the element to be operated appears , As long as the implementation of the time.sleep Will wait , To a certain extent, it increases the execution time of the program .

Case study ：

import time
from selenium import webdriver
driver = webdriver.Chrome()
driver.get("http://www.baidu.com")
#  Mandatory waiting 5 second 
time.sleep(5)
driver.find_element_by_xpath('//div[@id="u1"]/a[@name="tj_login"]').click()

2 、 An implicit wait

An implicit wait ： to driver Object to set a global waiting time , stay driver The object is valid throughout its life cycle . Whether it's through driver Look for the element , Or do some other operations on the element , If the element does not exist , Implicit waiting will make driver Wait for a certain amount of time before finding an element . Go and have a look every once in a while , Did it show up , No, just keep waiting , When the element appears, the wait ends , So let's keep going . If the waiting time exceeds the set maximum time , Then the error of waiting timeout will be thrown ！

Case study ：

import time
from selenium import webdriver
driver = webdriver.Chrome()
#  Set the maximum implicit wait time to 10 second 
driver.implicitly_wait(10)

driver.get("http://www.baidu.com")

( One )、 The basic use of explicit waiting

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


# ============= Step unwrapping ================
#  First step ： Create a waiting object 
wait = WebDriverWait(driver, 30, 0.2)
#  The second step ： Define elements to find objects 
# located = (" Positioning mode ",' Positioning expression ')
#  Such as through xpath
located = (By.XPATH, '//input[@id="u"]')
#  The third step ： Waiting conditions for positioning 
conditions = EC.visibility_of_element_located(located)
#  Step four ： Find by waiting for the timer object 
wait.until(conditions)


# ============ One line of code ==================
WebDriverWait(driver, 30, 0.2).until(
    EC.visibility_of_element_located(
        (By.XPATH, '//input[@id="u"]')
    )
)

( Two )、 Show common conditions for waiting

summary

• Mandatory waiting ： Fool Waiting , Not intelligent

• An implicit wait ： The global general , Just set it up once , Very smart ( Wait and go , Don't waste time ), Deficiencies can only be used to find elements , Wait for the element to be found .

• Explicit waiting ： Wait for the element to be in a specific condition , Very smart ( Wait and go , Don't waste time )

• problem ： When to use implicit waiting at work , When to use explicit wait ？
  Implicit is general , Create good driver Then set it , If the implicit wait code goes wrong , According to the wrong conditions , Add the corresponding explicit wait .

• problem ： Why wait ？
  Improve the stability of the program ( Not because of the network and other reasons , Page loading elements is slow , Element not found , Code execution error )

Learning resource sharing

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