Python模块WSGI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 socketdef 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_serverdef 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socketdef 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