python3爬虫-1

注:erroe:5**  服务器端错误   4**地址错误或错误发生在客户端

1.基本的网页下载

from urllib import request

from urllib.error import URLError

def download(url):

　　reponse=  request.Request(url)

　　html = request.urlopen(response).read().decode('utf-8')

　　return html

 2.当不能访问的链接过多时，需要对爬虫进行设置，当错误链接超过限定次数时，停止

def download(url, num_retried=2):
    print('download:',url)
    try:
        response = request.Request(url)
        html = request.urlopen(response).read().decode('utf-8')
    except error.URLError as e:
        print('Download Error:',e.reason)
        html =None
        if num_retried >0:
            if hasattr(e,'code') and 500 <=e.code <600:
                return download(url,num_retried-1)
    return html

 3.有用户代理的重试下载def download(url, num_retried=2,user_agent = 'wswp'):    print('download:',url)

    headers = {'User-agent':user_agent}
    response = request.Request(url, headers=headers)
    try:
        html = request.urlopen(response).read()
    except error.URLError as e:
        print('downloadError',e.reason)
        html = None
        if num_retried > 0:
            if hasattr(e,'code') and 500<= e.code <600:
                download(url,user_agent,num_retried-1)
    return html
4.网络地图爬虫

def crawl_sitemap(url):
    sitemap = download(url)
    print(type(sitemap))
    pattern = re.compile('<loc>(.*?)</loc>')
    links = pattern.findall(str(sitemap))
    for link in links:
        html = download(link)
5.ID遍历爬虫

import itertools
def crawl_ID(url):
    for page in itertools.count(1):
        urls =url+'-%d' % page
        html = download(urls)
        if html is None:
            break
        else:
            pass
6.升级版网络爬虫，知道链接错误5次才会停止下载

def crawl_ID_1(url,max_error):
    num_error = 0
    for page in itertools.count(1):
        urls = url+'-%d' % page
        html = download(urls)
        if html is None:
            num_error +=1
            if num_error == max_error:
                break
        else:
            num_error = 0
7.链接爬虫

import re
def link_crawler(seed_url, link_regex):
    crawl_queue = [seed_url]
    while crawl_queue:
        url = crawl_queue.pop()
        html = download(url)
        for link in get_links(html):
            if re.match(link_regex, link):
                crawl_queue.append(link)

def get_links(html):
    webpage_regex = re.compile('<a[^>] +href=["\'](.*?)["\']', re.IGNORECASE)
    return webpage_regex.findall(str(html))
8.升级版链接爬虫，合成绝对链接

def link_crawler_1(seed_url, link_regex):
    crawl_queue = [seed_url]
    while crawl_queue:
        url = crawl_queue.pop()
        html = download(url)
        for link in get_links(html):
            if re.match(link_regex, link):
                link = urlparse.urljoin(seed_url, link)
                crawl_queue.append(link)

def get_links(html):
    webpage_regex = re.compile('<a[^>] +href=["\'](.*?)["\']', re.IGNORECASE)
    return webpage_regex.findall(str(html))
9.解析robot.txt文件
在shell中进行

import urllib.robotparser
rp =urllib.robotparser.RobotFileParser()
rp.set_url('http://example.webscraping.com/robot.txt')
rp.read()
url = 'http://example.webscraping.com'
user_agent = 'BadCrawler'
rp.can_fetch(user_agent,url)

10.支持代理

from urllib import request
from urllib.error import URLError

def download(url,user_agent='wswp',proxy=None,num_retries=2):
    print('download',url)
    header = {'User-agent',user_agent}
    response = request.Request(url,headers=header)
    opener =urllib.request.build_opener(response)
    if proxy:
        proxy_params = {urlparse(url).scheme: proxy}
        opener.add_handler(urllib.request.ProxyHandler(proxy_params))
    try:
        html = opener.open(response).read()
    except urllib.error.URLError as e:
        print('Download error:', e.reason)
        html =None
        if num_retries >0:
            if hasattr(e, 'code') and 500 <= e.code < 600:
                html = download(url, user_agent,proxy,num_retries-1)
    return html
11.下载限速

import datetime
import time
class Throttle:
    def __init__(self,delay):
        self.delay = delay
        self.domains = {}

    def wait(self,url):
        domain = urlparse(url).netloc
        last_accessed = self.domains.get(domain)

        if self.delay >0 and last_accessed is not None:
            sleep_secs = self.delay - (datetime.datetime.now() - last_accessed).seconds
            if sleep_secs > 0:
                time.sleep(sleep_secs)

        self.domains[domain] = datetime.datetime.now()
12.避免重复下载

def link_crawler_1(seed_url, link_regex):
    crawl_queue = [seed_url]
　　seen= set[seed_url]
    while crawl_queue:
        url = crawl_queue.pop()
        html = download(url)
        for link in get_links(html):
            if re.match(link_regex, link):
                link = urlparse.urljoin(seed_url, link)
　　　　　　　　　　if link not in seen:
　　　　　　　　　　　　seen.add(link)
                crawl_queue.append(link)

13.避免爬虫陷阱

def link_crawler(*args,max_depth):
    max_depth = 2
    seen={}
    ...
    depth =seen[url]
    if depth != max_depth:
        for link in links:
            seen[link] = depth+1
            crawl_queue.append(link)


总：融合上面各方法，就可以做出一个比较棒的爬虫