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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def fetch( self, request: Union[str, "HTTPRequest"], raise_error: bool = True, **kwargs: Any ) -> Awaitable["HTTPResponse"]: """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. .. versionchanged:: 6.0 The ``callback`` argument was removed. Use the returned `.Future` instead. The ``raise_error=False`` argument only affects the `HTTPError` raised when a non-200 response code is used, instead of suppressing all errors. """

        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_proxy = _RequestProxy(request, self.defaults)
        future = Future()  # type: Future[HTTPResponse]

        def handle_response(response: "HTTPResponse") -> None:
            if response.error:
                if raise_error or not response._error_is_response_code:
                    future_set_exception_unless_cancelled(future, response.error)
                    return
            future_set_result_unless_cancelled(future, response)

        self.fetch_impl(cast(HTTPRequest, request_proxy), handle_response)
        return future