Example of failing an HTTP proxy request in Twisted.

twisted_fail_http_proxy_request.py

"""

In one terminal
$ $ python3 twisted_fail_http_proxy_request.py

In another terminal test it with:
$ curl -v -x http://localhost:8000  http://httpforever.com/

"""

from twisted.web import proxy, http
from twisted.internet import reactor


class YourProxyRequestHandler(proxy.ProxyRequest):
    """
    This is the request handler for a proxy request.
    """

    def process(self):
        if True:
            return self._fail()

        # Do the real processing.
        return super().process()

    def _fail(self):
        """
        Return a failure.
        """
        s = self.content.read()
        self.setResponseCode(400)
        self.setHeader(b'connection', b'close')
        self.write(b'We decided to fail.')
        self.finish()  # or just lose the connection.


class ProxyFactory(http.HTTPFactory):
    def buildProtocol(self, addr):
        p = proxy.Proxy()
        p.requestFactory = YourProxyRequestHandler
        return p


reactor.listenTCP(8000, ProxyFactory())
reactor.run()