def _wait(self, d, running=_wait_is_running):
        """Take a Deferred that only ever callbacks. Block until it happens.
        """
        if running:
            raise RuntimeError("_wait is not reentrant")

        from twisted.internet import reactor
        results = []
        def append(any):
            if results is not None:
                results.append(any)
        def crash(ign):
            if results is not None:
                reactor.crash()
        crash = utils.suppressWarnings(
            crash, util.suppress(message=r'reactor\.crash cannot be used.*',
                                 category=DeprecationWarning))
        def stop():
            reactor.crash()
        stop = utils.suppressWarnings(
            stop, util.suppress(message=r'reactor\.crash cannot be used.*',
                                category=DeprecationWarning))

        running.append(None)
        try:
            d.addBoth(append)
            if results:
                # d might have already been fired, in which case append is
                # called synchronously. Avoid any reactor stuff.
                return
            d.addBoth(crash)
            reactor.stop = stop
            try:
                reactor.run()
            finally:
                del reactor.stop

            # If the reactor was crashed elsewhere due to a timeout, hopefully
            # that crasher also reported an error. Just return.
            # _timedOut is most likely to be set when d has fired but hasn't
            # completed its callback chain (see self._run)
            if results or self._timedOut: #defined in run() and _run()
                return

            # If the timeout didn't happen, and we didn't get a result or
            # a failure, then the user probably aborted the test, so let's
            # just raise KeyboardInterrupt.

            # FIXME: imagine this:
            # web/test/test_webclient.py:
            # exc = self.assertRaises(error.Error, wait, method(url))
            #
            # wait() will raise KeyboardInterrupt, and assertRaises will
            # swallow it. Therefore, wait() raising KeyboardInterrupt is
            # insufficient to stop trial. A suggested solution is to have
            # this code set a "stop trial" flag, or otherwise notify trial
            # that it should really try to stop as soon as possible.
            raise KeyboardInterrupt()
        finally:
            results = None
            running.pop()
Take a Deferred that only ever callbacks. Block until it happens.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/trial/_asynctest.py