twisted.internet.defer module documentation

Log and return failure.

This method can be used as an errback that passes the failure on to the next errback unmodified. Note that if this is the last errback, and the deferred gets garbage collected after being this errback has been called, the clean up code logs it again.

Return a Deferred that has already had .callback(result) called.

This is useful when you're writing synchronous code to an asynchronous interface: i.e., some code is calling you expecting a Deferred result, but you don't actually need to do anything asynchronous. Just return defer.succeed(theResult).

See fail for a version of this function that uses a failing Deferred rather than a successful one.

Return a Deferred that has already had .errback(result) called.

See succeed's docstring for rationale.

Create a Deferred from a callable and arguments.

Call the given function with the given arguments. Return a Deferred which has been fired with its callback as the result of that invocation or its errback with a Failure for the exception thrown.

Invoke a function that may or may not return a Deferred.

Call the given function with the given arguments. If the returned object is a Deferred, return it. If the returned object is a Failure, wrap it with fail and return it. Otherwise, wrap it in succeed and return it. If an exception is raised, convert it to a Failure, wrap it in fail, and then return it.

Enable or disable Deferred debugging.

When debugging is on, the call stacks from creation and invocation are recorded, and added to any AlreadyCalledErrors we raise.

Determine whether Deferred debugging is enabled.

Schedule the execution of a coroutine that awaits/yields from Deferreds, wrapping it in a Deferred that will fire on success/failure of the coroutine. If a Deferred is passed to this function, it will be returned directly (mimicing asyncio's ensure_future function).

Coroutine functions return a coroutine object, similar to how generators work. This function turns that coroutine into a Deferred, meaning that it can be used in regular Twisted code. For example:

    import treq
    from twisted.internet.defer import ensureDeferred
    from twisted.internet.task import react

    async def crawl(pages):
        results = {}
        for page in pages:
            results[page] = await treq.content(await treq.get(page))
        return results

    def main(reactor):
        pages = [
            "http://localhost:8080"
        ]
        d = ensureDeferred(crawl(pages))
        d.addCallback(print)
        return d

    react(main)

Returns, via a Deferred, a list with the results of the given Deferreds - in effect, a "join" of multiple deferred operations.

The returned Deferred will fire when all of the provided Deferreds have fired, or when any one of them has failed.

This method can be cancelled by calling the cancel method of the Deferred, all the Deferreds in the list will be cancelled.

This differs from DeferredList in that you don't need to parse the result for success/failure.

deferredGenerator and waitForDeferred help you write Deferred-using code that looks like a regular sequential function. Consider the use of inlineCallbacks instead, which can accomplish the same thing in a more concise manner.

There are two important functions involved: waitForDeferred, and deferredGenerator. They are used together, like this:

    @deferredGenerator
    def thingummy():
        thing = waitForDeferred(makeSomeRequestResultingInDeferred())
        yield thing
        thing = thing.getResult()
        print(thing) #the result! hoorj!

waitForDeferred returns something that you should immediately yield; when your generator is resumed, calling thing.getResult() will either give you the result of the Deferred if it was a success, or raise an exception if it was a failure. Calling getResult is absolutely mandatory. If you do not call it, your program will not work.

deferredGenerator takes one of these waitForDeferred-using generator functions and converts it into a function that returns a Deferred. The result of the Deferred will be the last value that your generator yielded unless the last value is a waitForDeferred instance, in which case the result will be None. If the function raises an unhandled exception, the Deferred will errback instead. Remember that return result won't work; use yield result; return in place of that.

Note that not yielding anything from your generator will make the Deferred result in None. Yielding a Deferred from your generator is also an error condition; always yield waitForDeferred(d) instead.

The Deferred returned from your deferred generator may also errback if your generator raised an exception. For example:

    @deferredGenerator
    def thingummy():
        thing = waitForDeferred(makeSomeRequestResultingInDeferred())
        yield thing
        thing = thing.getResult()
        if thing == 'I love Twisted':
            # will become the result of the Deferred
            yield 'TWISTED IS GREAT!'
            return
        else:
            # will trigger an errback
            raise Exception('DESTROY ALL LIFE')

Put succinctly, these functions connect deferred-using code with this 'fake blocking' style in both directions: waitForDeferred converts from a Deferred to the 'blocking' style, and deferredGenerator converts from the 'blocking' style to a Deferred.

Return val from a inlineCallbacks generator.

Note: this is currently implemented by raising an exception derived from BaseException. You might want to change any 'except:' clauses to an 'except Exception:' clause so as not to catch this exception.

Also: while this function currently will work when called from within arbitrary functions called from within the generator, do not rely upon this behavior.

inlineCallbacks helps you write Deferred-using code that looks like a regular sequential function. For example:

    @inlineCallbacks
    def thingummy():
        thing = yield makeSomeRequestResultingInDeferred()
        print(thing)  # the result! hoorj!

When you call anything that results in a Deferred, you can simply yield it; your generator will automatically be resumed when the Deferred's result is available. The generator will be sent the result of the Deferred with the 'send' method on generators, or if the result was a failure, 'throw'.

Things that are not Deferreds may also be yielded, and your generator will be resumed with the same object sent back. This means yield performs an operation roughly equivalent to maybeDeferred.

Your inlineCallbacks-enabled generator will return a Deferred object, which will result in the return value of the generator (or will fail with a failure object if your generator raises an unhandled exception). Note that you can't use return result to return a value; use returnValue(result) instead. Falling off the end of the generator, or simply using return will cause the Deferred to have a result of None.

Be aware that returnValue will not accept a Deferred as a parameter. If you believe the thing you'd like to return could be a Deferred, do this:

    result = yield result
    returnValue(result)

The Deferred returned from your deferred generator may errback if your generator raised an exception:

    @inlineCallbacks
    def thingummy():
        thing = yield makeSomeRequestResultingInDeferred()
        if thing == 'I love Twisted':
            # will become the result of the Deferred
            returnValue('TWISTED IS GREAT!')
        else:
            # will trigger an errback
            raise Exception('DESTROY ALL LIFE')

It is possible to use the return statement instead of returnValue:

    @inlineCallbacks
    def loadData(url):
        response = yield makeRequest(url)
        return json.loads(response)

You can cancel the Deferred returned from your inlineCallbacks generator before it is fired by your generator completing (either by reaching its end, a return statement, or by calling returnValue). A CancelledError will be raised from the yielded Deferred that has been cancelled if that Deferred does not otherwise suppress it.