The previous example had a Resource that generates its response
asynchronously rather than immediately upon the call to its render
method. Though it was a useful demonstration of the NOT_DONE_YET
feature of Twisted Web, the example didn’t reflect what a realistic application
might want to do. This example introduces Deferred , the Twisted class which is used
to provide a uniform interface to many asynchronous events, and shows you an
example of using a Deferred -returning API to generate an
asynchronous response to a request in Twisted Web.
Deferred is the result of two consequences of the
asynchronous programming approach. First, asynchronous code is
frequently (if not always) concerned with some data (in Python, an
object) which is not yet available but which probably will be
soon. Asynchronous code needs a way to define what will be done to the
object once it does exist. It also needs a way to define how to handle
errors in the creation or acquisition of that object. These two needs
are satisfied by the callbacks and errbacks of
a Deferred . Callbacks are added to
a Deferred with Deferred.addCallback ; errbacks
are added with Deferred.addErrback . When the
object finally does exist, it is passed to Deferred.callback which passes it
on to the callback added with addCallback . Similarly, if
an error occurs, Deferred.errback is called and
the error is passed along to the errback added
with addErrback . Second, the events that make
asynchronous code actually work often take many different,
incompatible forms. Deferred acts as the uniform
interface which lets different parts of an asynchronous application
interact and isolates them from implementation details they shouldn’t
be concerned with.
That’s almost all there is to Deferred . To solidify your new
understanding, now consider this rewritten version
of DelayedResource which uses a Deferred -based delay
API. It does exactly the same thing as the previous example . Only the implementation is different.
First, the example must import that new API that was just mentioned, deferLater :
from twisted.internet.task import deferLater
Next, all the other imports (these are the same as last time):
from twisted.web.resource import Resource
from twisted.web.server import NOT_DONE_YET
from twisted.internet import reactor
With the imports done, here’s the first part of
the DelayedResource implementation. Again, this part of
the code is identical to the previous version:
class DelayedResource(Resource):
def _delayedRender(self, request):
request.write(b"<html><body>Sorry to keep you waiting.</body></html>")
request.finish()
Next we need to define the render method. Here’s where things
change a bit. Instead of using callLater ,
We’re going to use deferLater this
time. deferLater accepts a reactor, delay (in seconds, as
with callLater ), and a function to call after the delay
to produce that elusive object discussed in the description
of Deferred s. We’re also going to
use _delayedRender as the callback to add to
the Deferred returned by deferLater . Since
it expects the request object as an argument, we’re going to set up
the deferLater call to return a Deferred
which has the request object as its result.
...
def render_GET(self, request):
d = deferLater(reactor, 5, lambda: request)
The Deferred referenced by d now needs to
have the _delayedRender callback added to it. Once this
is done, _delayedRender will be called with the result
of d (which will be request , of course — the
result of (lambda: request)() ).
...
d.addCallback(self._delayedRender)
Finally, the render method still needs to return NOT_DONE_YET ,
for exactly the same reasons as it did in the previous version of the
example.
...
return NOT_DONE_YET
And with that, DelayedResource is now implemented
based on a Deferred . The example still isn’t very
realistic, but remember that since Deferred s offer a
uniform interface to many different asynchronous event sources, this
code now resembles a real application even more closely; you could
easily replace deferLater with
another Deferred -returning API and suddenly you might
have a resource that does something useful.
Finally, here’s the complete, uninterrupted example source, as an rpy script:
from twisted.internet.task import deferLater
from twisted.web.resource import Resource
from twisted.web.server import NOT_DONE_YET
from twisted.internet import reactor
class DelayedResource(Resource):
def _delayedRender(self, request):
request.write(b"<html><body>Sorry to keep you waiting.</body></html>")
request.finish()
def render_GET(self, request):
d = deferLater(reactor, 5, lambda: request)
d.addCallback(self._delayedRender)
return NOT_DONE_YET
resource = DelayedResource()