Python模块WSGI使用详解

WSGI（Web Server Gateway Interface）：Web服务网关接口，是Python中定义的服务器程序和应用程序之间的接口。

Web程序开发中，一般分为服务器程序和应用程序。服务器程序负责对socket服务的数据进行封装和整理，而应用程序则负责对Web请求进行逻辑处理。

Web应用本质上也是一个socket服务器，用户的浏览器就是一个socket客户端。

我们先用socket编程实现一个简单的Web服务器：

import socket 

def handle_request(client):
  buf = client.recv(1024)
  print(buf)
  msg = "HTTP/1.1 200 OK\r\n\r\n" #HTTP头信息
  client.send(('%s' % msg).encode())
  msg = "Hello, World!"
  client.send(('%s' % msg).encode()) 

def main():
  ip_port = ("localhost", 8000)
  sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
  sock.bind(ip_port)
  sock.listen(5) 

  while True:
    conn, addr = sock.accept()
    handle_request(conn)
    conn.close() 

if __name__ == "__main__":
  main()

上述代码中，main()函数就是服务器函数，handle_request()就是应用程序。
下面我们再用python的wsgiref模块来实现跟上述代码一样的Web服务器：

from wsgiref.simple_server import make_server 

def handle_request(env, res):
  res("200 OK",[("Content-Type","text/html")])
  body = "<h1>Hello World!</h1>"
  return [body.encode("utf-8")] 

if __name__ == "__main__":
  httpd = make_server("",8000,handle_request)
  print("Serving http on port 80000")
  httpd.serve_forever()

上面两份代码实现的效果是一样的，调用wsgiref模块则明显节省了代码量，是整个程序更加简洁。
wsgiref模块封装了socket服务端的代码，只留下一个调用的接口，省去了程序员的麻烦，程序员可以将精力放在Web请求的逻辑处理中。

以上述的代码为例，详细看一下wsgiref模块的源码中一些关键的地方：

if __name__ == "__main__":
  httpd = make_server("",8000,handle_request)
  print("Serving http on port 80000")
  httpd.serve_forever()

1、整个程序的入口为make_server()函数：

def make_server(host, port, app, server_class=WSGIServer, handler_class=WSGIRequestHandler):
  """Create a new WSGI server listening on `host` and `port` for `app`"""
  server = server_class((host, port), handler_class) #默认创建一个WSGIServer类
  server.set_app(app) #将应用程序，即逻辑处理函数传给类
  return server

2、make_server()函数默认生成一个WSGIServer类：

class WSGIServer(HTTPServer):
class HTTPServer(socketserver.TCPServer):
class TCPServer(BaseServer):

WSGIServer，HTTPServer两个类没有初始化函数，调用父类的初始化函数，TCPServer类的__init__()函数拓展了BaseServer

类的__init__()函数：

#BaseServer类的__init__()函数：
def __init__(self, server_address, RequestHandlerClass):
  """Constructor. May be extended, do not override."""
  self.server_address = server_address
  self.RequestHandlerClass = RequestHandlerClass
  self.__is_shut_down = threading.Event()
  self.__shutdown_request = False

#TCPServer类的__init__()函数：
def __init__(self, server_address, RequestHandlerClass, bind_and_activate=True):
  """Constructor. May be extended, do not override."""
  BaseServer.__init__(self, server_address, RequestHandlerClass)
  self.socket = socket.socket(self.address_family,self.socket_type)
    if bind_and_activate:
      try:
        self.server_bind()
        self.server_activate()
      except:
        self.server_close()
        raise

TCPServer类的初始化函数还调用了server_bind(self)，server_bind(self)两个函数：

def server_bind(self):
  """Called by constructor to bind the socket.May be overridden."""
  if self.allow_reuse_address:
    self.socket.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
  self.socket.bind(self.server_address)
  self.server_address = self.socket.getsockname()
def self.server_activate(self):
  """Called by constructor to activate the server.May be overridden."""
  self.socket.listen(self.request_queue_size)

可以看到server.bind()函数调用了socket.bind()函数，而server_activate()调用了socket.listen()函数：

3、server.set_app(app)，此处传入Web请求的处理逻辑：

def set_app(self,application):
  self.application = application

4、httpd.serve_forever()函数调用BaseServer类的_handle_request_noblock()函数处理多路请求：

def _handle_request_noblock(self):
  try:
    request, client_address = self.get_request() #get_request()调用了socket.accept()函数
  except OSError:
    return
  if self.verify_request(request, client_address):
    try:
      self.process_request(request, client_address)
    except:
      self.handle_error(request, client_address)
      self.shutdown_request(request)
  else:
    self.shutdown_request(request) 

def process_request(self, request, client_address):
  self.finish_request(request, client_address)
  self.shutdown_request(request)#shutdown_request()调用socket.close()关闭socket 

def finish_request(self, request, client_address):
  """Finish one request by instantiating RequestHandlerClass."""
  self.RequestHandlerClass(request, client_address, self) 

5、process_request()函数调用了finish_request()函数，简介调用了make_server函数的默认参数WSGIRequestHandler类：

class WSGIRequestHandler(BaseHTTPRequestHandler):
class BaseHTTPRequestHandler(socketserver.StreamRequestHandler):
class StreamRequestHandler(BaseRequestHandler):

#调用BaseRequestHandler类的初始化函数：
def __init__(self, request, client_address, server):
  self.request = request
  self.client_address = client_address
  self.server = server
  self.setup()
  try:
    self.handle()
  finally:
    self.finish()

6、初始化函数调用之后调用WSGIRequestHandler类的handle()函数获取server的逻辑处理函数：

def handle(self):
  """Handle a single HTTP request"""
  try:
    handler = ServerHandler(self.rfile, stdout, self.get_stderr(), self.get_environ())
    handler.request_handler = self   # backpointer for logging
    handler.run(self.server.get_app()) #此处调用server的逻辑处理函数
  finally:
    stdout.detach()

7、BaseHandler类的handler.run()函数执行逻辑处理：

def run(self, application):
   try:
    self.setup_environ()
    self.result = application(self.environ, self.start_response)
    self.finish_response()
  except:
    try:
      self.handle_error()
    except:
      self.close()
      raise  # ...and let the actual server figure it out. 

self.environ：一个包含所有HTTP请求信息的dict对象
self.start_response：一个发送HTTP响应的函数。

在application函数中，调用：

res("200 OK",[("Content-Type","text/html")])

这样就发送了HTTP响应的头信息

8、BaseHandler类的setup_environ()函数获取HTTP请求的头信息：

def setup_environ(self):
  """Set up the environment for one request"""
  env = self.environ = self.os_environ.copy() 

os_environ= read_environ() 

read_environ()函数： 

def read_environ():
  """Read environment, fixing HTTP variables"""
  enc = sys.getfilesystemencoding()
  esc = 'surrogateescape'
  try:
    ''.encode('utf-8', esc)
  except LookupError:
    esc = 'replace'
  environ = {} 

  # Take the basic environment from native-unicode os.environ. Attempt to
  # fix up the variables that come from the HTTP request to compensate for
  # the bytes->unicode decoding step that will already have taken place.
  for k, v in os.environ.items():
    if _needs_transcode(k): 

      # On win32, the os.environ is natively Unicode. Different servers
      # decode the request bytes using different encodings.
      if sys.platform == 'win32':
        software = os.environ.get('SERVER_SOFTWARE', '').lower() 

        # On IIS, the HTTP request will be decoded as UTF-8 as long
        # as the input is a valid UTF-8 sequence. Otherwise it is
        # decoded using the system code page (mbcs), with no way to
        # detect this has happened. Because UTF-8 is the more likely
        # encoding, and mbcs is inherently unreliable (an mbcs string
        # that happens to be valid UTF-8 will not be decoded as mbcs)
        # always recreate the original bytes as UTF-8.
        if software.startswith('microsoft-iis/'):
          v = v.encode('utf-8').decode('iso-8859-1') 

        # Apache mod_cgi writes bytes-as-unicode (as if ISO-8859-1) direct
        # to the Unicode environ. No modification needed.
        elif software.startswith('apache/'):
          pass 

        # Python 3's http.server.CGIHTTPRequestHandler decodes
        # using the urllib.unquote default of UTF-8, amongst other
        # issues.
        elif (
          software.startswith('simplehttp/')
          and 'python/3' in software
        ):
          v = v.encode('utf-8').decode('iso-8859-1') 

        # For other servers, guess that they have written bytes to
        # the environ using stdio byte-oriented interfaces, ending up
        # with the system code page.
        else:
          v = v.encode(enc, 'replace').decode('iso-8859-1') 

      # Recover bytes from unicode environ, using surrogate escapes
      # where available (Python 3.1+).
      else:
        v = v.encode(enc, esc).decode('iso-8859-1') 

    environ[k] = v
  return environ

9、BaseHandler类的start_response()函数：

def start_response(self, status, headers,exc_info=None):
  """'start_response()' callable as specified by PEP 3333"""
  if exc_info:
    try:
      if self.headers_sent:
        # Re-raise original exception if headers sent
        raise exc_info[0](exc_info[1]).with_traceback(exc_info[2])
    finally:
      exc_info = None    # avoid dangling circular ref
  elif self.headers is not None:
    raise AssertionError("Headers already set!") 

  self.status = status
  self.headers = self.headers_class(headers)
  status = self._convert_string_type(status, "Status")
  assert len(status)>=4,"Status must be at least 4 characters"
  assert status[:3].isdigit(), "Status message must begin w/3-digit code"
  assert status[3]==" ", "Status message must have a space after code" 

  if __debug__:
    for name, val in headers:
      name = self._convert_string_type(name, "Header name")
      val = self._convert_string_type(val, "Header value")
  return self.write

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