twisted.internet.defer.Deferred

class documentation

class Deferred: (source)

Known subclasses: twisted.internet.defer.DeferredList, twisted.persisted.crefutil._Defer

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.

Class Method	`fromCoroutine`	Schedule the execution of a coroutine that awaits on `Deferred`s, wrapping it in a `Deferred` that will fire on success/failure of the coroutine.
Class Method	`fromFuture`	Adapt an `asyncio.Future` to a `Deferred`.
Method	`__init__`	Initialize a `Deferred`.
Method	`__iter__`	Undocumented
Method	`__str__`	Return a string representation of this `Deferred`.
Method	`addBoth`	Convenience method for adding a single callable as both a callback and an errback.
Method	`addCallback`	Convenience method for adding just a callback.
Method	`addCallbacks`	Add a pair of callbacks (success and error) to this `Deferred`.
Method	`addErrback`	Convenience method for adding just an errback.
Method	`addTimeout`	Time out this `Deferred` by scheduling it to be cancelled after `timeout` seconds.
Method	`asFuture`	Adapt this `Deferred` into a `asyncio.Future` which is bound to `loop`.
Method	`callback`	Run all success callbacks that have been added to this `Deferred`.
Method	`cancel`	Cancel this `Deferred`.
Method	`chainDeferred`	Chain another `Deferred` 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	`send`	Undocumented
Method	`unpause`	Process all callbacks made since `pause`() was called.
Class Variable	`debug`	Undocumented
Instance Variable	`callbacks`	Undocumented
Instance Variable	`called`	A flag which is `False` until either `callback` or `errback` is called and afterwards always `True`.
Instance Variable	`paused`	A counter of how many unmatched `pause` calls have been made on this instance.
Instance Variable	`result`	Undocumented
Method	`_continuation`	Build a tuple of callback and errback with `_CONTINUE`.
Method	`_runCallbacks`	Run the chain of callbacks once a result is available.
Method	`_startRunCallbacks`	Undocumented
Instance Variable	`_canceller`	Undocumented
Instance Variable	`_chainedTo`	If this `Deferred` is waiting for the result of another `Deferred`, this is a reference to the other Deferred. Otherwise, `None`.
Instance Variable	`_debugInfo`	Undocumented
Instance Variable	`_runningCallbacks`	A flag which is `True` while this instance is executing its callback chain, used to stop recursive execution of `_runCallbacks`
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.

@classmethod
def fromCoroutine(cls, coro): (source)

Schedule the execution of a coroutine that awaits on Deferreds, wrapping it in a Deferred that will fire on success/failure of the coroutine.

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 Deferred
    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 = Deferred.fromCoroutine(crawl(pages))
        d.addCallback(print)
        return d

    react(main)

Parameters
coro:A Python 3.5+ `async def` coroutine or a Python 3.4+ `yield from` coroutine.	The coroutine object to schedule.
Returns
`Deferred`	Undocumented
Raises
`ValueError`	If `coro` is not a coroutine or generator.
Present Since
Twisted 21.2.0

@classmethod
def fromFuture(cls, future): (source)

Adapt an asyncio.Future to a Deferred.

Parameters
future:`asyncio.Future`	The Future to adapt.
Returns
`Deferred`	A Deferred which will fire when the Future fires.
Present Since
Twisted 17.5.0
Note
This creates a `Deferred` from a `asyncio.Future`, not from a `coroutine`; in other words, you will need to call `asyncio.ensure_future`, `asyncio.loop.create_task` or create an `asyncio.Task` yourself to get from a `coroutine` to a `asyncio.Future` if what you have is an awaitable coroutine and not a `asyncio.Future`. (The length of this list of techniques is exactly why we have left it to the caller!)

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

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

Initialize a Deferred.

Parameters

canceller:a 1-argument callable which takes a Deferred. The return result is ignored.

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.

def __iter__(self): (source)

Undocumented

def __str__(self): (source)

Return a string representation of this Deferred.

Returns
`str`	Undocumented

def addBoth(self, callback, *args, **kw): (source)

Convenience method for adding a single callable as both a callback and an errback.

See addCallbacks.

def addCallback(self, callback, *args, **kw): (source)

Convenience method for adding just a callback.

See addCallbacks.

def addCallbacks(self, callback, errback=None, callbackArgs=None, callbackKeywords=None, errbackArgs=None, errbackKeywords=None): (source)

Add a pair of callbacks (success and error) to this Deferred.

These will be executed when the 'master' callback is run.

Returns
a `Deferred`	`self`.

def addErrback(self, errback, *args, **kw): (source)

Convenience method for adding just an errback.

See addCallbacks.

def addTimeout(self, timeout, clock, onTimeoutCancel=None): (source)

Time out this Deferred by scheduling it to be cancelled after timeout seconds.

The timeout encompasses all the callbacks and errbacks added to this defer.Deferred before the call to addTimeout, and none added after the call.

If this Deferred gets timed out, it errbacks with a TimeoutError, unless a cancelable function was passed to its initialization or unless a different onTimeoutCancel callable is provided.

Parameters
timeout:`int`	number of seconds to wait before timing out this `Deferred`
clock:`twisted.internet.interfaces.IReactorTime`	The object which will be used to schedule the timeout.
onTimeoutCancel:`callable`	A callable which is called immediately after this `Deferred` times out, and not if this `Deferred` is otherwise cancelled before the timeout. It takes an arbitrary value, which is the value of this `Deferred` at that exact point in time (probably a `CancelledError` `Failure`), and the `timeout`. The default callable (if none is provided) will translate a `CancelledError` `Failure` into a `TimeoutError`.
Returns
a `Deferred`	`self`.
Present Since
16.5

def asFuture(self, loop): (source)

Adapt this Deferred into a asyncio.Future which is bound to loop.

Parameters
loop:`asyncio.AbstractEventLoop` or similar	The asyncio event loop to bind the `asyncio.Future` to.
Returns
`asyncio.Future`	A Future which will fire when the Deferred fires.
Present Since
Twisted 17.5.0
Note
converting a `Deferred` to an `asyncio.Future` consumes both its result and its errors, so this method implicitly converts `self` into a `Deferred` firing with `None`, regardless of what its result previously would have been.

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 cancel(self): (source)

overridden in twisted.internet.defer.DeferredList

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 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
a `Deferred`	`self`.

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 pause(self): (source)

Stop processing on a Deferred until unpause() is called.

@failure._extraneous
def send(self, value=None): (source)

Undocumented

def unpause(self): (source)

Process all callbacks made since pause() was called.

debug: bool = (source)

Undocumented

callbacks: list = (source)

Undocumented

called: bool = (source)

A flag which is False until either callback or errback is called and afterwards always True.

paused: int = (source)

A counter of how many unmatched pause calls have been made on this instance.

result = (source)

Undocumented

def _continuation(self): (source)

Build a tuple of callback and errback with _CONTINUE.

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 _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 `Deferred`s 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.

def _startRunCallbacks(self, result): (source)

Undocumented

_canceller = (source)

Undocumented

_chainedTo: Optional[Deferred] = (source)

If this Deferred is waiting for the result of another Deferred, this is a reference to the other Deferred. Otherwise, None.

_debugInfo = (source)

Undocumented

_runningCallbacks: bool = (source)

A flag which is True while this instance is executing its callback chain, used to stop recursive execution of _runCallbacks

_suppressAlreadyCalled: bool = (source)

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.