阅读 Tornado 源码过程中的一个疑惑，求解答

def fetch(self, request, callback=None, raise_error=True, **kwargs): """Executes a request, asynchronously returning an `HTTPResponse`.  The request may be either a string URL or an `HTTPRequest` object.  If it is a string, we construct an `HTTPRequest` using any additional  kwargs: ``HTTPRequest(request, **kwargs)``  This method returns a `.Future` whose result is an  `HTTPResponse`. By default, the ``Future`` will raise an  `HTTPError` if the request returned a non-200 response code  (other errors may also be raised if the server could not be  contacted). Instead, if ``raise_error`` is set to False, the  response will always be returned regardless of the response  code.  If a ``callback`` is given, it will be invoked with the `HTTPResponse`.  In the callback interface, `HTTPError` is not automatically raised.  Instead, you must check the response's ``error`` attribute or  call its `~HTTPResponse.rethrow` method.  """ if self._closed: raise RuntimeError("fetch() called on closed AsyncHTTPClient") if not isinstance(request, HTTPRequest): request = HTTPRequest(url=request, **kwargs) else: if kwargs: raise ValueError("kwargs can't be used if request is an HTTPRequest object") # We may modify this (to add Host, Accept-Encoding, etc), # so make sure we don't modify the caller's object. This is also # where normal dicts get converted to HTTPHeaders objects. request.headers = httputil.HTTPHeaders(request.headers) request = _RequestProxy(request, self.defaults) future = TracebackFuture() if callback is not None: callback = stack_context.wrap(callback) def handle_future(future): exc = future.exception() if isinstance(exc, HTTPError) and exc.response is not None: response = exc.response elif exc is not None: response = HTTPResponse( request, 599, error=exc, request_time=time.time() - request.start_time) else: response = future.result() self.io_loop.add_callback(callback, response) future.add_done_callack(handle_future) def handle_response(response): if raise_error and response.error: future.set_exception(response.error) else: future.set_result(response) self.fetch_impl(request, handle_response) return future def fetch_impl(self, request, callback): raise NotImplementedError()