Selenium 设置元素等待的三种方式

    1. sleep 强制等待
    2. implicitly_wait() 隐性等待
    3. WebDriverWait()显示等待

三种方式的优缺点

1. sleep 强制等待

from selenium import webdriver
from time import sleep
driver = webdriver.Chrome()
sleep(2)    #设置等待2秒钟
driver.get('http://www.baidu.com')

优点: 
           代码简介,简单明了

缺点: 
           如果设置sleep等待时间过短,元素还没加载出来,程序报错,sleep设置等待时间过长,元素早就加载出来了,程序还在等待,浪费是时间,影响代码整体的运行效率

个人看法: 
           简单粗暴,根据网站的响应速度和自己的网速来设置合理的休眠时间

2. implicitly_wait() 隐性等待

from selenium import webdriver
from time import sleep
driver = webdriver.Chrome()
driver.implicitly_wait(20) #设置等待20秒钟
driver.get('http://www.baidu.com')

优点: 
          1.代码简介
          2.在代码前部分加implicitly_wait(10) ,整个的程序运行过程中都会有效(作用于全局,直接在初始化driver的后面加,后面的代码都会受影响),都会等待元素加载完成
          3.在设置的时间内没有加载到整个页面,则会报NosuchElementError。如果元素在第10s被加载出来,自动执行下面的脚本,不会一直等待10s

缺点:
          1. 非要加载到整个页面才执行代码,这样影响代码的执行效率,一般情况下,我们想要的结果是只需加载到了我要定位的元素就执行代码,不需要等待整个页面的完全加载出来再执行代码。

个人看法: 
          1.不适合用在数据在ajax的网站中,比如翻页什么的,某个元素一直存在,但是数据一直在变,这样的话只要加载出来第一页,后面翻页的数据全部会和第一页的数据相同,因为代码判断了这个元素已经被加载出来了,不会等ajax去加载

3. WebDriverWait()显示等待

from selenium import webdriver
from selenium.webdriver.support import expected_conditions as EC
from selenium.webdriver.support.ui import WebDriverWait    #WebDriverWait注意大小写
from selenium.webdriver.common.by import By
driver = webdriver.Chrome()
driver.get('http://www.baidu.com')
try:
  element = 
  WebDriverWait(driver,10).until(EC.presence_of_element_located((By.ID,'kw')))
  element.send_keys('123')
  driver.find_element_by_id('su').click()
except Exception as message:
  print('元素定位报错%s'%message)
finally:
  pass

优点:
           代码执行效率快。无需等待整个页面加载完成,只需加载到你要定位的元素就可以执行代码。是最智能的设置元素等待的方式。

缺点:
           1.要导入from selenium.webdriver.support import expected_conditions as EC

 from selenium.webdriver.support.ui import WebDriverWait
 from selenium.webdriver.common.by import By

            必须要导入以上3个包,导包路径相当的复杂,啰嗦而且麻烦
           2.写等待时间的代码也是复杂。步骤稍微有点多。

element=WebDriverWait(driver,10).until(EC.presence_of_element_located((By.ID,‘kw')))
element.send_keys(‘123')

个人看法: 相比于两种,这种方式可以算的上好的了,但是就是麻烦,写的代码太多,使用的话可以和第一种方式sleep混合使用,不过我还是喜欢用sleep,本身使用selenium就是没办法破开网站,或者使用selenium比直接破解的方式更好才使用这种,我个人是能不用就不用,抓取速度太慢了。

附上我抓取一个网站的代码,这网站作者的成果抓不到,只好用这种方式来抓了:

from selenium import webdriver
import time
from lxml.html import etree
import copy
import json
from selenium.webdriver.common.by import By
from selenium.webdriver.support.ui import WebDriverWait
from selenium.webdriver.support import expected_conditions as EC
 
def getAuthors():
  j1 = set()
  f = open('Author.json', 'r', encoding='utf-8')
  data = f.read()
  data_list = data.split('\n')
  for dt in data_list:
    j1.add(dt)
  f.close()
  print('j1= ', len(j1))
  j2 = set()
  f1 = open('yzq.json', 'r', encoding='utf-8')
  data1 = f1.read()
  data_list1 = data1.split('\n')
  for dt in data_list1:
    j2.add(dt)
  print('j2= ', len(j2))
  countSet = j1 - j2
  print('countset= ', len(countSet))
  AuthorsData = []
  for dt in countSet:
    dt_json = json.loads(dt)
    if int(dt_json["成果"]) > 0:
      AuthorsData.append(dt_json)
  # dt = {'img': 'https://www.scholarmate.com/avatars/99/92/62/37572.jpg', 'name': '吴伟',
  #    'url': 'https://www.scholarmate.com/P/aeiUZr', 'org': '复旦大学, 教授', '项目': 20, '成果': 234, 'H指数': '24'}
  print('AuthorData= ', len(AuthorsData))
  return AuthorsData
 
def parseHtml(html, i):
  temp_list = []
  html_data = etree.HTML(html)
  project_html = html_data.xpath('//div[@class="pub-idx__main"]')
  for p in project_html:
    # pro_name = p.xpath('./div[@class="pub-idx__main_title"]/a/@title')[0]
    pro_name = p.xpath('.//a/@title')[0].strip().replace(r'\xa0', '')
    # pro_url = p.xpath('./div[@class="pub-idx__main_title"]/a/@href')[0]
    pro_url = p.xpath('.//a/@href')[0]
    pro_author = p.xpath('./div[2]/@title')[0].strip().replace('\xa0', '')
    # pro_author = p.xpath('.//div[@class="pub-idx__main_author"]/@title')
    pro_inst = p.xpath('./div[3]/@title')[0]
    temp_dict = {
      'num': i,
      'pro_name': pro_name,
      'pro_url': pro_url,
      'pro_author': pro_author,
      'pro_inst': pro_inst
    }
    temp_list.append(copy.deepcopy(temp_dict))
  return temp_list 
 
def parseData(author_data):
  try:
    url = author_data['url']
    ach_num = int(author_data['成果'])
    pages = ach_num // 10
    pages_ys = ach_num % 10
    if pages_ys > 0:
      pages += 1
    driver = webdriver.Chrome()
    # driver.implicitly_wait(10)
    driver.get(url)
    psn_data = []
    for i in range(1, pages+1):
      if i == 1:
        # 防止抓取到半路的时候页面没有响应,这部分数据就直接扔掉
        try:
          # time.sleep(2)
          driver.find_element_by_xpath('//*[@id="pubTab"]').click()
          # time.sleep(3)
          # 有以下这些选择
          # WebDriverWait(driver, 5).until(EC.presence_of_element_located((By.ID, 'pub-idx__main')))
          # WebDriverWait(driver, 5).until(EC.presence_of_element_located((By.CLASS_NAME, 'pub-idx__main')))
          # WebDriverWait(driver, 5).until(EC.presence_of_element_located((By.CSS_SELECTOR, './/pub-idx__main')))
          # 这个也不适合这个网站,还是会抓到重复的
          WebDriverWait(driver, 5).until(EC.presence_of_element_located((By.XPATH, '//div[@class="pub-idx__main"]')))
          html = driver.page_source
          temp_dict = parseHtml(html, i)
          psn_data.append(copy.deepcopy(temp_dict))
        except:
          import traceback
          print(traceback.print_exc())
          pass
      else:
        # driver.find_element_by_xpath('//*[@id="pubTab"]').click()
        # 将页面拉到底部
        try:
          js = "var q=document.documentElement.scrollTop=100000"
          driver.execute_script(js)
          # time.sleep(1)
          driver.find_element_by_xpath('//div[@class="pagination__pages_next"]').click()
          # time.sleep(2)
          WebDriverWait(driver, 5).until(EC.presence_of_element_located((By.XPATH, '//div[@class="pub-idx__main"]')))
          html = driver.page_source
          temp_dict = parseHtml(html, i)
          psn_data.append(copy.deepcopy(temp_dict))
        except:
          pass
    driver.close()
    psn_data = {
      'init_data': author_data,
      'psn_data': psn_data
    }
    print(psn_data)
    psn_data_string = json.dumps(psn_data, ensure_ascii=False)
    with open('data.json', 'a+', encoding='utf-8') as f:
      f.write('{}\n'.format(psn_data_string))
 
    author_data_string = json.dumps(author_data, ensure_ascii=False)
    with open('yzq.json', 'a+', encoding='utf-8') as f:
      f.write('{}\n'.format(author_data_string))
 
  except:
    pass
    # import traceback
    # print(traceback.print_exc())
    # au_strign = json.dumps(author_data, ensure_ascii=False)
    # author_data_string = json.dumps(au_strign, ensure_ascii=False)
    # with open('error.json', 'a+', encoding='utf-8') as f:
    #   f.write('{}\n'.format(author_data_string))
 
def main():
  # authors的值:给出三条
  # {"img": "https://www.scholarmate.com/avatars/e4/fe/1e/1000002077830.png", "name": "胡婷",
  # "url": "https://www.scholarmate.com/P/QFFbae", "org": "四川大学, 主治医师", "项目": "0", "成果": "11", "H指数": "0"}
  # {"img": "https://www.scholarmate.com/avatars/01/ea/59/1000002180047.png", "name": "白晓涓",
  # "url": "https://www.scholarmate.com/P/73me22", "org": "", "项目": "6", "成果": "8", "H指数": "0"}
  # {"img": "https://www.scholarmate.com/avatars/fe/0d/89/1000000732306.png", "name": "原鹏飞",
  # "url": "https://www.scholarmate.com/P/77nIFr", "org": "国家统计局统计科学研究所, 副研究员", "项目": "0", "成果": "90", "H指数": "0"}
 
  AuthorsData = getAuthors()
  for authors in AuthorsData:
    print('author= ', authors)
    parseData(authors)
 
if __name__ == '__main__':
  main()

友情链接:

https://www.cnblogs.com/zhaof/p/6953241.html

https://blog.csdn.net/xiezhiming1234/article/details/83865314

https://www.cnblogs.com/April-Chou-HelloWorld/p/8855760.html

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