The previous example created a server that dealt with response errors by aborting response generation, potentially avoiding pointless work. However, it did this silently for any error. In this example, we’ll modify the previous example so that it logs each failed response.
This example will use the Twisted API for logging errors. As was
mentioned in the first example covering Deferreds , errbacks are passed an error. In the previous
example, the _responseFailed errback accepted this error
as a parameter but ignored it. The only way this example will differ
is that this _responseFailed will use that error
parameter to log a message.
This example will require all of the imports required by the previous example plus one new import:
from twisted.python.log import err
The only other part of the previous example which changes is
the _responseFailed callback, which will now log the
error passed to it:
...
def _responseFailed(self, failure, call):
call.cancel()
err(failure, b"Async response demo interrupted response")
We’re passing two arguments to err here. The first is the error which is being
passed in to the callback. This is always an object of type Failure , a class which represents an
exception and (sometimes, but not always) a traceback. err will
format this nicely for the log. The second argument is a descriptive string that
tells someone reading the log what the source of the error was.
Here’s the full example with the two above modifications:
from twisted.web.resource import Resource
from twisted.web.server import NOT_DONE_YET
from twisted.internet import reactor
from twisted.python.log import err
class DelayedResource(Resource):
def _delayedRender(self, request):
request.write(b"<html><body>Sorry to keep you waiting.</body></html>")
request.finish()
def _responseFailed(self, failure, call):
call.cancel()
err(failure, b"Async response demo interrupted response")
def render_GET(self, request):
call = reactor.callLater(5, self._delayedRender, request)
request.notifyFinish().addErrback(self._responseFailed, call)
return NOT_DONE_YET
resource = DelayedResource()
Run this server as in the previous example and interrupt a request. Unlike the previous example, where the server gave no indication that this had happened, you’ll see a message in the log output with this version.