python如何提升爬虫效率

单线程+多任务异步协程

• 协程

在函数(特殊函数)定义的时候,使用async修饰,函数调用后,内部语句不会立即执行,而是会返回一个协程对象

• 任务对象

任务对象=高级的协程对象(进一步封装)=特殊的函数
任务对象必须要注册到时间循环对象中
给任务对象绑定回调:爬虫的数据解析中

• 事件循环

当做是一个装载任务对象的容器
当启动事件循环对象的时候,存储在内的任务对象会异步执行

• 特殊函数内部不能写不支持异步请求的模块,如time,requests...否则虽然不报错但实现不了异步

time.sleep -- asyncio.sleep
requests -- aiohttp

import asyncio
import time

start_time = time.time()
async def get_request(url):
  await asyncio.sleep(2)
  print(url,'下载完成!')

urls = [
  'www.1.com',
  'www.2.com',
]

task_lst = [] # 任务对象列表
for url in urls:
  c = get_request(url) # 协程对象
  task = asyncio.ensure_future(c) # 任务对象
  # task.add_done_callback(...)  # 绑定回调
  task_lst.append(task)

loop = asyncio.get_event_loop() # 事件循环对象
loop.run_until_complete(asyncio.wait(task_lst)) # 注册,手动挂起

线程池+requests模块

# 线程池
import time
from multiprocessing.dummy import Pool

start_time = time.time()
url_list = [
  'www.1.com',
  'www.2.com',
  'www.3.com',
]
def get_request(url):
  print('正在下载...',url)
  time.sleep(2)
  print('下载完成!',url)

pool = Pool(3)
pool.map(get_request,url_list)
print('总耗时:',time.time()-start_time)

两个方法提升爬虫效率

起一个flask服务端

from flask import Flask
import time

app = Flask(__name__)

@app.route('/bobo')
def index_bobo():
  time.sleep(2)
  return 'hello bobo!'

@app.route('/jay')
def index_jay():
  time.sleep(2)
  return 'hello jay!'

@app.route('/tom')
def index_tom():
  time.sleep(2)
  return 'hello tom!'

if __name__ == '__main__':
  app.run(threaded=True)

aiohttp模块+单线程多任务异步协程

import asyncio
import aiohttp
import requests
import time

start = time.time()
async def get_page(url):
  # page_text = requests.get(url=url).text
  # print(page_text)
  # return page_text
  async with aiohttp.ClientSession() as s: #生成一个session对象
    async with await s.get(url=url) as response:
      page_text = await response.text()
      print(page_text)
  return page_text

urls = [
  'http://127.0.0.1:5000/bobo',
  'http://127.0.0.1:5000/jay',
  'http://127.0.0.1:5000/tom',
]
tasks = []
for url in urls:
  c = get_page(url)
  task = asyncio.ensure_future(c)
  tasks.append(task)
loop = asyncio.get_event_loop()
loop.run_until_complete(asyncio.wait(tasks))

end = time.time()
print(end-start)

# 异步执行!
# hello tom!
# hello bobo!
# hello jay!
# 2.0311079025268555

'''
aiohttp模块实现单线程+多任务异步协程
并用xpath解析数据
'''
import aiohttp
import asyncio
from lxml import etree
import time

start = time.time()
# 特殊函数:请求的发送和数据的捕获
# 注意async with await关键字
async def get_request(url):
  async with aiohttp.ClientSession() as s:
    async with await s.get(url=url) as response:
      page_text = await response.text()
      return page_text    # 返回页面源码

# 回调函数,解析数据
def parse(task):
  page_text = task.result()
  tree = etree.HTML(page_text)
  msg = tree.xpath('/html/body/ul//text()')
  print(msg)

urls = [
  'http://127.0.0.1:5000/bobo',
  'http://127.0.0.1:5000/jay',
  'http://127.0.0.1:5000/tom',
]
tasks = []
for url in urls:
  c = get_request(url)
  task = asyncio.ensure_future(c)
  task.add_done_callback(parse) #绑定回调函数!
  tasks.append(task)
loop = asyncio.get_event_loop()
loop.run_until_complete(asyncio.wait(tasks))

end = time.time()
print(end-start)

requests模块+线程池

import time
import requests
from multiprocessing.dummy import Pool

start = time.time()
urls = [
  'http://127.0.0.1:5000/bobo',
  'http://127.0.0.1:5000/jay',
  'http://127.0.0.1:5000/tom',
]
def get_request(url):
  page_text = requests.get(url=url).text
  print(page_text)
  return page_text

pool = Pool(3)
pool.map(get_request, urls)
end = time.time()
print('总耗时:', end-start)

# 实现异步请求
# hello jay!
# hello bobo!
# hello tom!
# 总耗时: 2.0467123985290527

小结

• 爬虫的加速目前掌握了两种方法:
  

aiohttp模块+单线程多任务异步协程
requests模块+线程池

• 爬虫接触的模块有三个:
  

requests
urllib
aiohttp

• 接触了一下flask开启服务器

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