WSGI简介

当我们实现一个Web应用（application）的时候，通常不会考虑如何接受HTTP请求、解析HTTP请求、发送HTTP响应等等，我们只关心处理逻辑，而不用去关心HTTP规范的细节。

之所以有这层透明，是因为Web Server和Web Application之间有一套规范的接口，这套接口帮我们隐藏了很多HTTP相关的细节。这套接口规范就是WSGI（Web Server Gateway Interface）。

Web Server和Web Application都实现WSGI规范，然后各司其职：

• Web Server：接收来自客户端的HTTP，然后将请求交给Web Application
• Web Application：根据请求来调用相应的处理逻辑，生成response；通过Web Server把response发送给客户端

下面就一步步看下WSGI规范的更多内容。

Application Interface

上面了解到，Web Server和Web Application端都要遵守WSGI规范。对于实现WSGI的Web Application端，必须是一个callable的对象（类，函数，方法等等，实现__call__魔术方法的对象），这个callable对象需要满足下面两个条件：

• 包含两个参数
  • 一个dict对象，Web Server会将HTTP请求相关的信息添加到这个字典中，供Web application使用
  • 一个callback函数，Web application通过这个函数将HTTP status code和headers发送给Web Server
• 以字符串的形式返回response，并且包含在可迭代的list中

下面就是一个实现Application Interface的一个application函数：

# This is an application object. It could have any name, except when using mod_wsgi where it must be "application"
# The application object accepts two arguments
# This is an application object. It could have any name, except when using mod_wsgi where it must be "application"
# The application object accepts two arguments
def application( 
        # environ points to a dictionary containing CGI like environment variables
        # which is filled by the server for each received request from the client
        environ,
        # start_response is a callback function supplied by the server
        # which will be used to send the HTTP status and headers to the server
        start_response):

    # build the response body possibly using the environ dictionary
    response_body = 'The request method was %s' % environ['REQUEST_METHOD']
    
    # HTTP response code and message
    status = '200 OK'
    
    # These are HTTP headers expected by the client.
    # They must be wrapped as a list of tupled pairs:
    # [(Header name, Header value)].
    response_headers = [('Content-Type', 'text/plain'), ('Content-Length', str(len(response_body)))]
    
    # Send them to the server using the supplied function
    start_response(status, response_headers)
    
    # Return the response body.
    # Notice it is wrapped in a list although it could be any iterable.
    return [response_body]

看看Environment dict

在Python中就有一个WSGI server，我们可以直接使用。

在下面的这个例子中，WSGI server监听了"localhost:8080"，并绑定了一个支持WSGI规范的application对象；application对象就会处理来自8080端口，并将"Environment dict"的内容生产response传给WSGI server。

# WSGI server in Python
from wsgiref.simple_server import make_server

def application(environ, start_response):

    # Sorting and stringifying the environment key, value pairs
    response_body = ['%s: %s' % (key, value)
                    for key, value in sorted(environ.items())]
    response_body = '
'.join(response_body)
    
    status = '200 OK'
    response_headers = [('Content-Type', 'text/plain'),
                ('Content-Length', str(len(response_body)))]
    start_response(status, response_headers)
    
    return [response_body]

# Instantiate the WSGI server.
# It will receive the request, pass it to the application
# and send the application's response to the client
httpd = make_server(
    'localhost', # The host name.
    8080, # A port number where to wait for the request.
    application # Our application object name, in this case a function.
    )

# Wait for a single request, serve it and quit.
httpd.handle_request()

# Keep the server always alive with serve_forever()
# httpd.serve_forever()

注意，在application对象返回的时候，我们使用的是"return [response_body]"，当我们改成"return response_body"之后，一样可以工作，但是效率会很低，因为返回的时候会去迭代response字符串中的每一个字符。所以，当处理response字符串的时候，最好是将它包在一个可迭代对象中，例如list。

通过浏览器访问后，就可以得到"Environment dict"的内容，这些都是WSGI server提供的信息，包括了HTTP请求的相关信息。

处理GET请求

当我们执行一个如下的GET请求：

http://127.0.0.1:8080/?name=wilber&hobbies=software

QUERY_STRING（URL中"?"之后的部分）和REQUEST_METHOD这些信息会包含在"Environment dict"，从application中可以很方便的得到这些信息。

在application中，可以使用cgi模块中的parse_qs函数得到一个由QUERY_STRING生成的字典，方便我们取出请求的变量信息。

同时，为了避免客户端的输入可能存在的脚本注入，可以使用cgi模块中的escape函数对输入进行一次过滤。

下面直接看例子：

from wsgiref.simple_server import make_server
from cgi import parse_qs, escape

html = """
<html>
<body>
   <form method="get" action="/">
      <p>
         Name: <input type="text" name="name">
         </p>
      <p>
         Hobbies:
         <input name="hobbies" type="checkbox" value="running"> running
         <input name="hobbies" type="checkbox" value="swimming"> swimming
         <input name="hobbies" type="checkbox" value="reading"> reading
         </p>
      <p>
         <input type="submit" value="Submit">
         </p>
      </form>
   <p>
      Name: %s<br>
      Hobbies: %s
      </p>
   </body>
</html>"""

def application(environ, start_response):
    print "QUERY_STRING: %s" %environ['QUERY_STRING']
    print "REQUEST_METHOD: %s" %environ['REQUEST_METHOD']

    # Returns a dictionary containing lists as values.
    d = parse_qs(environ['QUERY_STRING'])
    
    # In this idiom you must issue a list containing a default value.
    name = d.get('name', [''])[0] # Returns the first name value.
    hobbies = d.get('hobbies', []) # Returns a list of hobbies.
    
    # Always escape user input to avoid script injection
    name = escape(name)
    hobbies = [escape(hobby) for hobby in hobbies]
    
    response_body = html % (name or 'Empty',
                ', '.join(hobbies or ['No Hobbies']))
    
    status = '200 OK'
    
    # Now content type is text/html
    response_headers = [('Content-Type', 'text/html'),
                ('Content-Length', str(len(response_body)))]
    start_response(status, response_headers)
    
    return [response_body]

httpd = make_server('localhost', 8080, application)
# Now it is serve_forever() in instead of handle_request().
# In Windows you can kill it in the Task Manager (python.exe).
# In Linux a Ctrl-C will do it.
httpd.serve_forever()

从结果中可以看到，请求URL中的QUERY_STRING被WSGI server填入了"Environment dict"中。

处理POST请求

当执行一个POST请求的时候，query string是不会出现在URL里面的，而是会包含在request body中。

对于WSGI server，request body存放在"Environment dict"中（environ['wsgi.input']），environ['wsgi.input']对应的是一个file object，可以通过读取文件的方式读取request body。同时，environ.get('CONTENT_LENGTH', 0)中存放着request body的size，我们可以根据这个值来读取适当长度的request body。

看下面的例子：

from wsgiref.simple_server import make_server
from cgi import parse_qs, escape

html = """
<html>
<body>
   <form method="post" action="parsing_post.wsgi">
      <p>
         Name: <input type="text" name="name">
         </p>
      <p>
         Hobbies:
         <input name="hobbies" type="checkbox" value="running"> running
         <input name="hobbies" type="checkbox" value="swimming"> swimming
         <input name="hobbies" type="checkbox" value="reading"> reading
         </p>
      <p>
         <input type="submit" value="Submit">
         </p>
      </form>
   <p>
      Name: %s<br>
      Hobbies: %s
      </p>
   </body>
</html>
"""

def application(environ, start_response):
    # the environment variable CONTENT_LENGTH may be empty or missing
    try:
        request_body_size = int(environ.get('CONTENT_LENGTH', 0))
    except (ValueError):
        request_body_size = 0
        
    # When the method is POST the query string will be sent
    # in the HTTP request body which is passed by the WSGI server
    # in the file like wsgi.input environment variable.
    request_body = environ['wsgi.input'].read(request_body_size)
    d = parse_qs(request_body)
    
    print "wsgi.input %s" %environ['wsgi.input']
    print "request_body_size %s" %environ.get('CONTENT_LENGTH', 0)
    print "request_body %s" %request_body
    
    
    name = d.get('name', [''])[0] # Returns the first name value.
    hobbies = d.get('hobbies', []) # Returns a list of hobbies.
    
    # Always escape user input to avoid script injection
    name = escape(name)
    hobbies = [escape(hobby) for hobby in hobbies]
    
    response_body = html % (name or 'Empty',
                ', '.join(hobbies or ['No Hobbies']))
    
    status = '200 OK'
    
    response_headers = [('Content-Type', 'text/html'),
                ('Content-Length', str(len(response_body)))]
    start_response(status, response_headers)
    
    return [response_body]

httpd = make_server('localhost', 8080, application)
httpd.serve_forever()

通过结果，我们可以看到environ字典中对应的"wsgi.input"和"CONTENT_LENGTH"，以及读取出来的"request body"。

总结

本文介绍了WSGI的一些基本内容，以及如何解析GET和POST请求中的参数。

通过WSGI这个规范，Web application的开发人员可以不用关心HTTP协议中的细节问题。