AsyncFuture

class AsyncFuture

Bases: TypedReferenceCount

This class represents a thread-safe handle to a promised future result of an asynchronous operation, providing methods to query its status and result as well as register callbacks for this future’s completion.

An AsyncFuture can be awaited from within a coroutine or task. It keeps track of tasks waiting for this future and automatically reactivates them upon this future’s completion.

A task itself is also a subclass of AsyncFuture. Other subclasses are not generally necessary, except to override the function of cancel().

Until the future is done, it is “owned” by the resolver thread, though it’s still legal for other threads to query its state. When the resolver thread resolves this future using set_result(), or any thread calls cancel(), it instantly enters the “done” state, after which the result becomes a read-only field that all threads can access.

When the future returns true for done(), a thread can use cancelled() to determine whether the future was cancelled or get_result() to access the result of the operation. Not all operations define a meaningful result value, so some will always return nullptr.

In Python, the cancelled(), wait() and get_result() methods are wrapped up into a single result() method which waits for the future to complete before either returning the result or throwing an exception if the future was cancelled. However, it is preferable to use the await keyword when running from a coroutine, which only suspends the current task and not the entire thread.

This API aims to mirror and be compatible with Python’s Future class.

New in version 1.10.0.

Inheritance diagram

AsyncFuture(void)

Initializes the future in the pending state.

PyObject *add_done_callback(PyObject *self, PyObject *fn)

virtual bool cancel(void)

Cancels the future. Returns true if it was cancelled, or false if the future was already done. Either way, done() will return true after this call returns.

Please note that calling this is not a guarantee that the operation corresponding this future does not run. It could already be in the process of running, or perhaps not respond to a cancel signal. All this guarantees is that the future is marked as done when this call returns.

In the case of a task, this is equivalent to remove().

bool cancelled(void) const

Returns true if the future was cancelled. It is always safe to call this.

bool done(void) const

Returns true if the future is done or has been cancelled. It is always safe to call this.

static PyObject *gather(PyObject *args)

Creates a new future that returns done() when all of the contained futures are done.

Calling cancel() on the returned future will result in all contained futures that have not yet finished to be cancelled.

static TypeHandle get_class_type(void)

std::string const &get_done_event(void) const

Returns the event name that will be triggered when the future finishes. See set_done_event().

virtual void output(std::ostream &out) const

PyObject *result(PyObject *self, PyObject *timeout = (&::_Py_NoneStruct)) const

void set_done_event(std::string const &done_event)

Sets the event name that will be triggered when the future finishes. Will not be triggered if the future is cancelled, but it will be triggered for a coroutine task that exits with an exception.

void set_result(PyObject*)

Sets this future’s result. Can only be called if done() returns false.

Sets this future’s result as a generic TypedObject.

Sets this future’s result. Can only be done while the future is not done. Calling this marks the future as done and schedules the done callbacks.

This variant takes two pointers; the second one is only set if this object inherits from ReferenceCount, so that a reference can be held.

Assumes the manager’s lock is not held.

PointerTo<AsyncFuture> shield(PointerTo<AsyncFuture> future)

Creates a new future that shields the given future from cancellation. Calling cancel() on the returned future will not affect the given future.

void wait(void)

void wait(double timeout)

Waits until the future is done.

Waits until the future is done, or until the timeout is reached. Note that this can be considerably less efficient than wait() without a timeout, so it’s generally not a good idea to use this unless you really need to.