Support for results that aren't immediately available.

Maintainer: Glyph Lefkowitz

Class AlreadyCalledError Undocumented
Class CancelledError This error is raised by default when a Deferred is cancelled.
Class TimeoutError This error is raised by default when a Deferred times out.
Function logError Log and return failure.
Function succeed Return a Deferred that has already had .callback(result) called.
Function fail Return a Deferred that has already had .errback(result) called.
Function execute Create a Deferred from a callable and arguments.
Function maybeDeferred Invoke a function that may or may not return a Deferred.
Function timeout Undocumented
Function passthru Undocumented
Function setDebugging Enable or disable Deferred debugging.
Function getDebugging Determine whether Deferred debugging is enabled.
Class Deferred This is a callback which will be put off until later.
Function ensureDeferred No summary
Class DebugInfo Deferred debug helper.
Class FirstError First error to occur in a DeferredList if fireOnOneErrback is set.
Class DeferredList DeferredList is a tool for collecting the results of several Deferreds.
Function gatherResults Returns, via a Deferred, a list with the results of the given Deferreds - in effect, a "join" of multiple deferred operations.
Class waitForDeferred See deferredGenerator.
Function deferredGenerator No summary
Function returnValue Return val from a inlineCallbacks generator.
Function inlineCallbacks

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


Class DeferredLock A lock for event driven systems.
Class DeferredSemaphore A semaphore for event driven systems.
Class QueueOverflow Undocumented
Class QueueUnderflow Undocumented
Class DeferredQueue An event driven queue.
Class AlreadyTryingToLockError Raised when DeferredFilesystemLock.deferUntilLocked is called twice on a single DeferredFilesystemLock.
Class DeferredFilesystemLock A FilesystemLock that allows for a Deferred to be fired when the lock is acquired.
Variable _NO_RESULT The result used to represent the fact that there is no result. Never ever ever use this as an actual result for a Deferred. You have been warned.
Variable _CONTINUE A marker left in Deferred.callbacks to indicate a Deferred chain. Always accompanied by a Deferred instance in the args tuple pointing at the Deferred which is chained to the Deferred which has this marker.
Function _cancelledToTimedOutError A default translation function that translates Failures that are CancelledErrors to TimeoutErrors.
Function _parseDListResult Undocumented
Function _deferGenerator See deferredGenerator.
Class _DefGen_Return Undocumented
Class _CancellationStatus Cancellation status of an inlineCallbacks invocation.
Function _inlineCallbacks Carry out the work of inlineCallbacks.
Function _cancellableInlineCallbacks Make an @inlineCallbacks cancellable.
Class _InternalInlineCallbacksCancelledError A unique exception used only in _cancellableInlineCallbacks to verify that an inlineCallbacks is being cancelled as expected.
Class _ConcurrencyPrimitive No class docstring; 1/3 methods documented
_NO_RESULT =
The result used to represent the fact that there is no result. Never ever ever use this as an actual result for a Deferred. You have been warned.
_CONTINUE =
A marker left in Deferred.callbacks to indicate a Deferred chain. Always accompanied by a Deferred instance in the args tuple pointing at the Deferred which is chained to the Deferred which has this marker.
def logError(err): (source)

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.

def succeed(result): (source)

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.

ParametersresultThe result to give to the Deferred's 'callback' method.
Returns (type: Deferred)
def fail(result=None): (source)

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

See succeed's docstring for rationale.

ParametersresultThe same argument that Deferred.errback takes.
Returns (type: Deferred)
RaisesNoCurrentExceptionErrorIf result is None but there is no current exception state.
def execute(callable, *args, **kw): (source)

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.

def maybeDeferred(f, *args, **kw): (source)

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.

ParametersfThe callable to invoke (type: Any callable)
argsThe arguments to pass to f
kwThe keyword arguments to pass to f
ReturnsThe result of the function call, wrapped in a Deferred if necessary. (type: Deferred)
def timeout(deferred): (source)
Undocumented
def passthru(arg): (source)
Undocumented
def setDebugging(on): (source)

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.

def getDebugging(): (source)

Determine whether Deferred debugging is enabled.

def _cancelledToTimedOutError(value, timeout): (source)

A default translation function that translates Failures that are CancelledErrors to TimeoutErrors.

ParametersvalueAnything (type: Anything)
timeoutThe timeout (type: int)
Returns (type: value)
RaisesTimeoutError
Present Since16.5
def ensureDeferred(coro): (source)

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)
ParameterscoroThe coroutine object to schedule, or a Deferred. (type: A Python 3.5+ async def coroutine, a Python 3.4+ yield from using types.GeneratorType, or a Deferred.)
Returns (type: Deferred)
def _parseDListResult(l, fireOnOneErrback=False): (source)
Undocumented
def gatherResults(deferredList, consumeErrors=False): (source)

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.

ParametersconsumeErrors(keyword param) a flag, defaulting to False, indicating that failures in any of the given Deferreds should not be propagated to errbacks added to the individual Deferreds after this gatherResults invocation. Any such errors in the individual Deferreds will be converted to a callback result of None. This is useful to prevent spurious 'Unhandled error in Deferred' messages from being logged. This parameter is available since 11.1.0. (type: bool)
def _deferGenerator(g, deferred): (source)
def deferredGenerator(f): (source)

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.

def returnValue(val): (source)

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.

@failure._extraneous
def _inlineCallbacks(result, g, status): (source)

Carry out the work of inlineCallbacks.

Iterate the generator produced by an @inlineCallbacks-decorated function, g, send()ing it the results of each value yielded by that generator, until a Deferred is yielded, at which point a callback is added to that Deferred to call this function again.

ParametersresultThe last result seen by this generator. Note that this is never a Deferred - by the time this function is invoked, the Deferred has been called back and this will be a particular result at a point in its callback chain.
ga generator object returned by calling a function or method decorated with @inlineCallbacks
statusa _CancellationStatus tracking the current status of g
def _cancellableInlineCallbacks(g): (source)

Make an @inlineCallbacks cancellable.

Parametersga generator object returned by calling a function or method decorated with @inlineCallbacks
ReturnsDeferred for the @inlineCallbacks that is cancellable.
def inlineCallbacks(f): (source)

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 yieldeed Deferred that has been cancelled if that Deferred does not otherwise suppress it.

API Documentation for Twisted, generated by pydoctor at 2018-10-15 19:29:43.