class WSGIServer(HTTPServer):

 """BaseHTTPServer that implements the Python WSGI protocol"""

 application = None

 def server_bind(self):
  """Override server_bind to store the server name."""
  HTTPServer.server_bind(self)
  self.setup_environ()

 def setup_environ(self): # 初始化環境變量
  # Set up base environment
  env = self.base_environ = {}
  env['SERVER_NAME'] = self.server_name
  env['GATEWAY_INTERFACE'] = 'CGI/1.1'
  env['SERVER_PORT'] = str(self.server_port)
  env['REMOTE_HOST']=''
  env['CONTENT_LENGTH']=''
  env['SCRIPT_NAME'] = ''

 def get_app(self):
  return self.application

 def set_app(self,application): # 注入application的class，注意是class
  self.application = application

class WSGIRequestHandler(BaseHTTPRequestHandler):

 server_version = "WSGIServer/" + __version__

 def get_environ(self):
  pass

 def handle(self):
  """Handle a single HTTP request"""

  self.raw_requestline = self.rfile.readline(65537)
  if len(self.raw_requestline) > 65536:
   ...
   self.send_error(414)
   return

  if not self.parse_request(): # An error code has been sent, just exit
   return

  handler = ServerHandler(
   self.rfile, self.wfile, self.get_stderr(), self.get_environ(),
   multithread=False,
  ) # 創建新的業務handler
  handler.request_handler = self  
  handler.run(self.server.get_app()) # 創建application對象

def get_environ(self):
 env = self.server.base_environ.copy() # wsgi-server的環境變量
 env['SERVER_PROTOCOL'] = self.request_version
 env['SERVER_SOFTWARE'] = self.server_version
 env['REQUEST_METHOD'] = self.command
 
 ...
 
 host = self.address_string()
 if host != self.client_address[0]:
  env['REMOTE_HOST'] = host
 env['REMOTE_ADDR'] = self.client_address[0]

 if self.headers.get('content-type') is None:
  env['CONTENT_TYPE'] = self.headers.get_content_type()
 else:
  env['CONTENT_TYPE'] = self.headers['content-type']

 length = self.headers.get('content-length')
 if length:
  env['CONTENT_LENGTH'] = length

 for k, v in self.headers.items():
  k=k.replace('-','_').upper(); v=v.strip()
  if k in env:
   continue     # skip content length, type,etc.
  if 'HTTP_'+k in env:
   env['HTTP_'+k] += ','+v  # comma-separate multiple headers
  else:
   env['HTTP_'+k] = v
 return env

class ServerHandler(BaseHandler):

 def __init__(self,stdin,stdout,stderr,environ,
  multithread=True, multiprocess=False
 ):
  self.stdin = stdin
  self.stdout = stdout
  self.stderr = stderr
  self.base_env = environ
  self.wsgi_multithread = multithread
  self.wsgi_multiprocess = multiprocess
 ...

class BaseHandler:
 def run(self, application):
 """Invoke the application"""
 # Note to self: don't move the close()! Asynchronous servers shouldn't
 # call close() from finish_response(), so if you close() anywhere but
 # the double-error branch here, you'll break asynchronous servers by
 # prematurely closing. Async servers must return from 'run()' without
 # closing if there might still be output to iterate over.
  ...
  self.setup_environ()
  self.result = application(self.environ, self.start_response)
  self.finish_response()
  ...

def setup_environ(self):
 """Set up the environment for one request"""

 env = self.environ = self.os_environ.copy()
 self.add_cgi_vars() # 子類實現 self.environ.update(self.base_env)

 env['wsgi.input']  = self.get_stdin() # 注意沒有stdout
 env['wsgi.errors']  = self.get_stderr()
 env['wsgi.version']  = self.wsgi_version
 env['wsgi.run_once']  = self.wsgi_run_once
 env['wsgi.url_scheme'] = self.get_scheme()
 env['wsgi.multithread'] = self.wsgi_multithread
 env['wsgi.multiprocess'] = self.wsgi_multiprocess

 if self.wsgi_file_wrapper is not None:
  env['wsgi.file_wrapper'] = self.wsgi_file_wrapper

 if self.origin_server and self.server_software:
  env.setdefault('SERVER_SOFTWARE',self.server_software)

def start_response(self, status, headers,exc_info=None):
 """'start_response()' callable as specified by PEP 3333"""

 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"

 return self.write

def demo_app(environ,start_response):
 from io import StringIO
 stdout = StringIO()
 print("Hello world!", file=stdout)
 print(file=stdout)
 # http請求及環境
 h = sorted(environ.items())
 for k,v in h:
  print(k,'=',repr(v), file=stdout)
 # 回調寫入http_status, response_headers
 start_response("200 OK", [('Content-Type','text/plain; charset=utf-8')])
 # 返回處理結果response_body
 return [stdout.getvalue().encode("utf-8")]

def finish_response(self):
 if not self.result_is_file() or not self.sendfile():
  for data in self.result:
   self.write(data)
  self.finish_content()

python -m wsgiref.simple_server
Serving HTTP on 0.0.0.0 port 8000 ...
127.0.0.1 - - [31/Jan/2021 21:43:05] "GET /xyz?abc HTTP/1.1" 200 3338

def setup_environ(self):
 # Set up base environment
 env = self.base_environ = {}
 env['SERVER_NAME'] = self.server_name
 env['GATEWAY_INTERFACE'] = 'CGI/1.1'
 ...

def setup_environ(self):
 self.base_environ = {}
 self.base_environ['SERVER_NAME'] = self.server_name
 self.base_environ['GATEWAY_INTERFACE'] = 'CGI/1.1'

def _write(self,data):
 result = self.stdout.write(data)
 if result is None or result == len(data):
  return
 from warnings import warn
 warn("SimpleHandler.stdout.write() should not do partial writes",
  DeprecationWarning)
 while True:
  data = data[result:] # 持續的寫入，直到完成
  if not data:
   break
  result = self.stdout.write(data)

class Headers:
 """Manage a collection of HTTP response headers"""

 def __init__(self, headers=None):
  headers = headers if headers is not None else []
  self._headers = headers # 內部存儲使用數組
 
 def __setitem__(self, name, val):
  """Set the value of a header."""
  del self[name]
  self._headers.append(
   (self._convert_string_type(name), self._convert_string_type(val)))

 ....

 def __getitem__(self,name):
  """Get the first header value for 'name'

  Return None if the header is missing instead of raising an exception.

  Note that if the header appeared multiple times, the first exactly which
  occurrence gets returned is undefined. Use getall() to get all
  the values matching a header field name.
  """
  return self.get(name)

 def get(self,name,default=None):
  """Get the first header value for 'name', or return 'default'"""
  name = self._convert_string_type(name.lower())
  for k,v in self._headers:
   if k.lower()==name:
    return v
  return default

if self.headers.get('content-type') is None:
 env['CONTENT_TYPE'] = self.headers.get_content_type()
else:
 env['CONTENT_TYPE'] = self.headers['content-type']