Django内置的server基本包括两部分：django.core.servers和django.core.handlers
相识
servers.basehttp是Django自身提供的一个用于开发测试的server模块，其中提供的WSGIServer、ServerHandler、WSGIRequestHandler其实都是属于WSGI server，django只不过是对python内置的WSGI模块simple_server做的一层包装。
handlers package包括base.py和wsgi.py两个模块。
base.py中只定义了一个BaseHandler类，它复杂一些基础的功能，比如加载中间件，处理异常，获取响应数据等
wsgi.py才是主角，其中最重要的就是WSGIHandler类，它继承了base.py中的BaseHandler，只添加了一个__call__方法，那为什么添加这个方法呢？
django.core.wsgi
import django
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.
"""
django.setup()
return WSGIHandler()
可见我们启动server时传入的application其实是WSGIHandler的一个实例，而根据WSGI规范，这个application必须要是callable，python中的callable包括函数、方法以及任何定义了__call__方法的对象
回到handlers.wsgi
class WSGIHandler(base.BaseHandler):
initLock = Lock()
request_class = WSGIRequest
def __call__(self, environ, start_response):
# Set up middleware if needed. We couldn't do this earlier, because
# settings weren't available.
if self._request_middleware is None:
with self.initLock:
try:
# Check that middleware is still uninitialized.
if self._request_middleware is None:
self.load_middleware()
except:
# Unload whatever middleware we got
self._request_middleware = None
raise
set_script_prefix(get_script_name(environ))
signals.request_started.send(sender=self.__class__, environ=environ)
try:
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:
response = self.get_response(request)
response._handler_class = self.__class__
status = '%s %s' % (response.status_code, response.reason_phrase)
response_headers = [(str(k), str(v)) for k, v in response.items()]
for c in response.cookies.values():
response_headers.append((str('Set-Cookie'), str(c.output(header=''))))
start_response(force_str(status), response_headers)
if getattr(response, 'file_to_stream', None) is not None and environ.get('wsgi.file_wrapper'):
response = environ['wsgi.file_wrapper'](response.file_to_stream)
return response
由于__call__是一个请求的入口，它需要调用BaseHandler中定义的方法去执行加载中间件等一系列操作和异常处理，除此之外，WSGIHandler还会处理cookie、触发signal等。
至于如何返回响应，具体可看handlers.base模块，大概就是找出请求的path，通过匹配路由，找到并执行用户定义的view方法，执行中间件处理方法，最后返回响应。当然，还有一系列的异常处理。