twisted.internet.defer.Deferred : API documentation

This is a callback which will be put off until later.

Why do we want this? Well, in cases where a function in a threaded program would block until it gets a result, for Twisted it should not block. Instead, it should return a Deferred.

This can be implemented for protocols that run over the network by writing an asynchronous protocol for twisted.internet. For methods that come from outside packages that are not under our control, we use threads (see for example twisted.enterprise.adbapi).

For more information about Deferreds, see doc/core/howto/defer.html or http://twistedmatrix.com/documents/current/core/howto/defer.html

When creating a Deferred, you may provide a canceller function, which will be called by d.cancel() to let you do any clean-up necessary if the user decides not to wait for the deferred to complete.

Instance Variable	called	A flag which is `False` until either `callback` or `errback` is called and afterwards always `True`. (type: `bool`)
Instance Variable	paused	A counter of how many unmatched `pause` calls have been made on this instance. (type: `int`)
Method	__init__	Initialize a `Deferred`.
Method	addCallbacks	Add a pair of callbacks (success and error) to this `Deferred`.
Method	addCallback	Convenience method for adding just a callback.
Method	addErrback	Convenience method for adding just an errback.
Method	addBoth	Convenience method for adding a single callable as both a callback and an errback.
Method	chainDeferred	Chain another `Deferred` to this `Deferred`.
Method	callback	Run all success callbacks that have been added to this `Deferred`.
Method	errback	Run all error callbacks that have been added to this `Deferred`.
Method	pause	Stop processing on a `Deferred` until `unpause`() is called.
Method	unpause	Process all callbacks made since `pause`() was called.
Method	cancel	Cancel this `Deferred`.
Method	__str__	Return a string representation of this `Deferred`.
Instance Variable	_suppressAlreadyCalled	A flag used by the cancellation mechanism which is `True` if the Deferred has no canceller and has been cancelled, `False` otherwise. If `True`, it can be expected that `callback` or `errback` will eventually be called and the result should be silently discarded. (type: `bool`)
Instance Variable	_runningCallbacks	A flag which is `True` while this instance is executing its callback chain, used to stop recursive execution of `_runCallbacks` (type: `bool`)
Instance Variable	_chainedTo	If this Deferred is waiting for the result of another Deferred, this is a reference to the other Deferred. Otherwise, `None`.
Method	_startRunCallbacks	Undocumented
Method	_continuation	Build a tuple of callback and errback with `_continue` to be used by `_addContinue` and `_removeContinue` on another Deferred.
Method	_runCallbacks	Run the chain of callbacks once a result is available.

def __init__(self, canceller=None): (source)

overridden in twisted.internet.defer.DeferredList, twisted.persisted.crefutil._Defer

Initialize a Deferred.

Parameters

canceller

a callable used to stop the pending operation scheduled by this Deferred when Deferred.cancel is invoked. The canceller will be passed the deferred whose cancelation is requested (i.e., self).

If a canceller is not given, or does not invoke its argument's callback or errback method, Deferred.cancel will invoke Deferred.errback with a CancelledError.

Note that if a canceller is not given, callback or errback may still be invoked exactly once, even though defer.py will have already invoked errback, as described above. This allows clients of code which returns a Deferred to cancel it without requiring the Deferred instantiator to provide any specific implementation support for cancellation. New in 10.1.

(type: a 1-argument callable which takes a Deferred. The return result is ignored.)

def chainDeferred(self, d): (source)

Chain another Deferred to this Deferred.

This method adds callbacks to this Deferred to call d's callback or errback, as appropriate. It is merely a shorthand way of performing the following:

   self.addCallbacks(d.callback, d.errback)

When you chain a deferred d2 to another deferred d1 with d1.chainDeferred(d2), you are making d2 participate in the callback chain of d1. Thus any event that fires d1 will also fire d2. However, the converse is not true; if d2 is fired d1 will not be affected.

Note that unlike the case where chaining is caused by a Deferred being returned from a callback, it is possible to cause the call stack size limit to be exceeded by chaining many Deferreds together with chainDeferred.

Returns self. (type: a Deferred)

def callback(self, result): (source)

Run all success callbacks that have been added to this Deferred.

Each callback will have its result passed as the first argument to the next; this way, the callbacks act as a 'processing chain'. If the success-callback returns a Failure or raises an Exception, processing will continue on the *error* callback chain. If a callback (or errback) returns another Deferred, this Deferred will be chained to it (and further callbacks will not run until that Deferred has a result).

An instance of Deferred may only have either callback or errback called on it, and only once.

Parameters	result	The object which will be passed to the first callback added to this `Deferred` (via `addCallback`).
Raises	AlreadyCalledError	If `callback` or `errback` has already been called on this `Deferred`.

def errback(self, fail=None): (source)

Run all error callbacks that have been added to this Deferred.

Each callback will have its result passed as the first argument to the next; this way, the callbacks act as a 'processing chain'. Also, if the error-callback returns a non-Failure or doesn't raise an Exception, processing will continue on the *success*-callback chain.

If the argument that's passed to me is not a failure.Failure instance, it will be embedded in one. If no argument is passed, a failure.Failure instance will be created based on the current traceback stack.

Passing a string as `fail' is deprecated, and will be punished with a warning message.

An instance of Deferred may only have either callback or errback called on it, and only once.

Parameters	fail	The `Failure` object which will be passed to the first errback added to this `Deferred` (via `addErrback`). Alternatively, a `Exception` instance from which a `Failure` will be constructed (with no traceback) or `None` to create a `Failure` instance from the current exception state (with a traceback).
Raises	AlreadyCalledError	If `callback` or `errback` has already been called on this `Deferred`.
	NoCurrentExceptionError	If `fail` is `None` but there is no current exception state.

def cancel(self): (source)

Cancel this Deferred.

If the Deferred has not yet had its errback or callback method invoked, call the canceller function provided to the constructor. If that function does not invoke callback or errback, or if no canceller function was provided, errback with CancelledError.

If this Deferred is waiting on another Deferred, forward the cancellation to the other Deferred.

def _runCallbacks(self): (source)

Run the chain of callbacks once a result is available.

This consists of a simple loop over all of the callbacks, calling each with the current result and making the current result equal to the return value (or raised exception) of that call.

If self._runningCallbacks is true, this loop won't run at all, since it is already running above us on the call stack. If self.paused is true, the loop also won't run, because that's what it means to be paused.

The loop will terminate before processing all of the callbacks if a Deferred without a result is encountered.

If a Deferred with a result is encountered, that result is taken and the loop proceeds.

Note The implementation is complicated slightly by the fact that chaining (associating two Deferreds with each other such that one will wait for the result of the other, as happens when a Deferred is returned from a callback on another Deferred) is supported iteratively rather than recursively, to avoid running out of stack frames when processing long chains.

t.i.d.Deferred : class documentation