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 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 server2、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 = application4、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
acf6
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