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.
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))