Python爬虫基础之requests

一、随时随地爬取一个网页下来

　　怎么爬取网页？对网站开发了解的都知道，浏览器访问Url向服务器发送请求，服务器响应浏览器请求并返回一堆HTML信息，其中包括html标签，css样式，js脚本等。我们之前用的是Python标准基础库Urllib实现的，

现在我们使用Python的Requests HTTP库写个脚本开始爬取网页。Requests的口号很响亮“让HTTP服务人类“，够霸气。

二、Python Requests库的基本使用

1.GET和POST请求方式

GET请求

1 import requests
2 
3 payload = {"t": "b", "w": "Python urllib"}
4 response = requests.get('http://zzk.cnblogs.com/s', params=payload)
5 # print(response.url)  # 打印 http://zzk.cnblogs.com/s?w=Python+urllib&t=b&AspxAutoDetectCookieSupport=1
6 print(response.text)

Python requests的GET请求，不需要在作为请求参数前，对dict参数进行urlencode()和手动拼接到请求url后面，get()方法会直接对params参数这样做。

POST请求

1 import requests
2 
3 payload = {"t": "b", "w": "Python urllib"}
4 response = requests.post('http://zzk.cnblogs.com/s', data=payload)
5 print(response.text)  # u'......'

Python requests的POST请求，不需要在作为请求参数前，对dict参数进行urlencode()和encode()将字符串转换成字节码。raw属性返回的是字节码，text属性直接返回unicode格式的字符串，而不需要再进行decode()将返回的bytes字节码转化为unicode。

相对于Python urllib而言，Python requests更加简单易用。

2.设置请求头headers

1 import requests
2 
3 payload = {"t": "b", "w": "Python urllib"}
4 headers = {'user_agent':'Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.110 Safari/537.36'}
5 response = requests.get('http://zzk.cnblogs.com/s', params=payload, headers=headers)
6 print(response.request.headers)

get方法的请求头，可以通过传递字典格式的参数给headers来实现。response.headers返回服务器响应的请求头信息，response.request.headers返回客户端的请求头信息。

3.设置会话cookie

1 import requests
2 
3 cookies = {'cookies_are': 'working'}
4 response = requests.get('http://zzk.cnblogs.com/', cookies=cookies)
5 print(response.text)

requests.get()方法cookies参数除了支持dict()字典格式，还支持传递一个复杂的RequestsCookieJar对象，可以指定域名和路径属性。

1 import requests
2 import requests.cookies
3 
4 cookieJar = requests.cookies.RequestsCookieJar()
5 cookieJar.set('cookies_are', 'working', domain='cnblogs', path='/cookies')
6 response = requests.get('http://zzk.cnblogs.com/', cookies=cookieJar)
7 print(response.text)

4.设置超时时间timeout

1 import requests
2 
3 response = requests.get('http://zzk.cnblogs.com/', timeout=0.001)
4 print(response.text)

三、Python Requests库的高级使用

1.Session Object

1 from requests import Request,Session
2 
3 s = Session()
4 
5 s.get('http://httpbin.org/cookies/set/sessioncookie/123456789')
6 r = s.get('http://httpbin.org/cookies')
7 
8 print(r.text)
9 # '{"cookies": {"sessioncookie": "123456789"}}'

通过Session，我们可以在多个请求之间传递cookies信息，不过仅限于同一域名下，否则不会附带上cookie。如果碰到需要登录态的页面，我们可以在登陆的时候保存登录态，再访问其他页面时附带上就好。

2.Prepared Requested

 1 from requests import Request,Session
 2 
 3 url = 'http://zzk.cnblogs.com/s'
 4 payload = {"t": "b", "w": "Python urllib"}
 5 headers = {
 6         'user-agent': 'Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.110 Safari/537.36',
 7         'Content-Type':'application/x-www-form-urlencoded'
 8 }
 9 s = Session()
10 request = Request('GET', url, headers=headers, data=payload)
11 prepped = request.prepare()
12 
13 # do something with prepped.headers
14 del prepped.headers['Content-Type']
15 response = s.send(prepped, timeout=3)
16 print(response.request.headers)

Request对象的prepare()方法返回的对象允许在发送请求前做些额外的工作，例如更新请求体body或者请求头headers.

3.Set Proxy

 1 import requests
 2 
 3 
 4 # set headers
 5 user_agent = 'Mozilla/5.0 (iPhone; CPU iPhone OS 8_0 like Mac OS X) AppleWebKit/600.1.3 (KHTML, like Gecko) Version/8.0 Mobile/12A4345d Safari/600.1.4'
 6 headers = {'User-Agent': user_agent}
 7 url = 'http://passport.xxx.com/auth/valid.json'
 8 proxies = {'http': 'http://10.1.1.1:80'}  # set http proxy
 9 params = {'uin': 'xxxxxx', 'passwd': 'xxxxxx', 'imgcode': 'ijyk', '_1': '12',
10           '_2': '10279', '_3': '23935743', 'url': ''}
11 
12 response = requests.post(url, headers=headers, data=params, proxies=proxies)
13 response.raise_for_status()
14 if response.status_code == requests.codes.ok:
15     print(response.text)

requests.get()和post()方法均支持http proxy代理，只要传递proxies = {'http': 'http://10.1.1.1:80'}字典对象，可以实现把请求传递给代理服务器，代理服务器从10.1.1.1:80取回响应数据返回来。

四、Python Requests库的实际应用

1.GET请求封装

 1 def do_get_request(self, url, headers=None, timeout=3, is_return_text=True, num_retries=2):
 2         if url is None:
 3             return None
 4         print('Downloading:', url)
 5         if headers is None:  # 默认请求头
 6             headers = {
 7                 'user-agent': 'Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.110 Safari/537.36'}
 8         response = None
 9         try:
10             response = requests.get(url,headers=headers,timeout=timeout)
11 
12             response.raise_for_status()  # a 4XX client error or 5XX server error response,raise requests.exceptions.HTTPError
13             if response.status_code == requests.codes.ok:
14                 if is_return_text:
15                     html = response.text
16                 else:
17                     html = response.json()
18             else:
19                 html = None
20         except requests.Timeout as err:
21             print('Downloading Timeout:', err.args)
22             html = None
23         except requests.HTTPError as err:
24             print('Downloading HTTP Error,msg:{0}'.format(err.args))
25             html = None
26             if num_retries > 0:
27                 if 500 <= response.status_code < 600:
28                     return self.do_get_request(url, headers=headers, num_retries=num_retries - 1)  # 服务器错误，导致请求失败，默认重试2次
29         except requests.ConnectionError as err:
30             print('Downloading Connection Error:', err.args)
31             html = None
32 
33         return html

2.POST请求封装

 1  def do_post_request(self, url, data=None, headers=None, timeout=3, is_return_text=True, num_retries=2):
 2         if url is None:
 3             return None
 4         print('Downloading:', url)
 5         # 如果请求数据未空，直接返回
 6         if data is None:
 7             return
 8         if headers is None:
 9             headers = {
10                 'user-agent': 'Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.110 Safari/537.36'}
11         response = None
12         try:
13             response = requests.post(url,data=data, headers=headers, timeout=timeout)   # 设置headers timeout无效
14 
15             response.raise_for_status()  # a 4XX client error or 5XX server error response,raise requests.exceptions.HTTPError
16             if response.status_code == requests.codes.ok:
17                 if is_return_text:
18                     html = response.text
19                 else:
20                     html = response.json()
21             else:
22                 print('else')
23                 html = None
24         except requests.Timeout as err:
25             print('Downloading Timeout:', err.args)
26             html = None
27         except requests.HTTPError as err:
28             print('Downloading HTTP Error,msg:{0}'.format(err.args))
29             html = None,
30             if num_retries > 0:
31                 if 500 <= response.status_code < 600:
32                     return self.do_post_request(url, data=data, headers=headers,
33                                                 num_retries=num_retries - 1)  # 服务器错误，导致请求失败，默认重试2次
34         except requests.ConnectionError as err:
35             print('Downloading Connection Error:', err.args)
36             html = None
37 
38         return html

3.登录态cookie

 1 def save_cookies(self, requeste_cookiejar, filename):
 2     with open(filename, 'wb')as f:
 3         pickle.dump(requeste_cookiejar, f)
 4 
 5 def load_cookies(self, filename):
 6     with open(filename, 'rb') as f:
 7         return pickle.load(f)
 8 
 9 # save request cookies
10 r = requests.get(url)
11 save_cookies(r.cookies,filename)
12 
13 # load cookies and do a request
14 requests.get(url,cookies=load_cookies(filename))