Django WSGI 架构解析：从入口配置到 WSGIHandler 源码

# Python dotted path to the WSGI application used by Django's runserver.
WSGI_APPLICATION = 'tomato.wsgi.application'

import os 
os.environ.setdefault("DJANGO_SETTINGS_MODULE", "tomato.settings")
from django.core.wsgi import get_wsgi_application
application = get_wsgi_application()

from django.core.handlers.wsgi import WSGIHandler
def get_wsgi_application():
  """
  The public interface to Django's WSGI support. Should return a WSGI
  callable. 
  Allows us to avoid making django.core.handlers.WSGIHandler public API, in
  case the internal WSGI implementation changes or moves in the future.
 
  """
  """
  # 继承, 但只实现了 __call__ 方法, 方便使用
  class WSGIHandler(base.BaseHandler):
  """
  return WSGIHandler()

# 继承, 但只实现了 __call__ 方法, 方便使用
class WSGIHandler(base.BaseHandler):
  initLock = Lock() 
  # 关于此, 日后展开, 可以将其视为一个代表 http 请求的类
  request_class = WSGIRequest 
  # WSGIHandler 也可以作为函数来调用
  def __call__(self, environ, start_response):
    # Set up middleware if needed. We couldn't do this earlier, because
    # settings weren't available. 
    # 这里的检测: 因为 self._request_middleware 是最后才设定的, 所以如果为空,
    # 很可能是因为 self.load_middleware() 没有调用成功.
    if self._request_middleware is None:
      with self.initLock:
        try:
          # Check that middleware is still uninitialised.
          if self._request_middleware is None:
            因为 load_middleware() 可能没有调用, 调用一次.
            self.load_middleware()
        except:
          # Unload whatever middleware we got
          self._request_middleware = None
          raise 
    set_script_prefix(base.get_script_name(environ))
    signls.request_started.send(sender=self.__class__) # __class__ 代表自己的类 
    try:
      # 实例化 request_class = WSGIRequest, 将在日后文章中展开, 可以将其视为一个代表 http 请求的类
      request = self.request_class(environ)
 
    except UnicodeDecodeError:
      logger.warning('Bad Request (UnicodeDecodeError)',
        exc_info=sys.exc_info(),
        extra={
          'status_code': 400,
        }
      )
      response = http.HttpResponseBadRequest()
    else:
      # 调用 self.get_response(), 将会返回一个相应对象 response<br>      ############# 关键的操作, self.response() 可以获取响应数据.     
      response = self.get_response(request)
 
    # 将 self 挂钩到 response 对象
    response._handler_class = self.__class__ 
    try:
      status_text = STATUS_CODE_TEXT[response.status_code]
    except KeyError:
      status_text = 'UNKNOWN STATUS CODE'
     # 状态码
    status = '%s %s' % (response.status_code, status_text) 
    response_headers = [(str(k), str(v)) for k, v in response.items()] 
    # 对于每个一个 cookie, 都在 header 中设置: Set-cookie xxx=yyy
    for c in response.cookies.values():
      response_headers.append((str('Set-Cookie'), str(c.output(header=''))))
 
    # start_response() 操作已经在上节中介绍了
    start_response(force_str(status), response_headers) 
    # 成功返回相应对象
    return response