1.. currentmodule:: asyncio
2
3
4.. _asyncio-futures:
5
6=======
7Futures
8=======
9
10**Source code:** :source:`Lib/asyncio/futures.py`,
11:source:`Lib/asyncio/base_futures.py`
12
13-------------------------------------
14
15*Future* objects are used to bridge **low-level callback-based code**
16with high-level async/await code.
17
18
19Future Functions
20================
21
22.. function:: isfuture(obj)
23
24   Return ``True`` if *obj* is either of:
25
26   * an instance of :class:`asyncio.Future`,
27   * an instance of :class:`asyncio.Task`,
28   * a Future-like object with a ``_asyncio_future_blocking``
29     attribute.
30
31   .. versionadded:: 3.5
32
33
34.. function:: ensure_future(obj, *, loop=None)
35
36   Return:
37
38   * *obj* argument as is, if *obj* is a :class:`Future`,
39     a :class:`Task`, or a Future-like object (:func:`isfuture`
40     is used for the test.)
41
42   * a :class:`Task` object wrapping *obj*, if *obj* is a
43     coroutine (:func:`iscoroutine` is used for the test);
44     in this case the coroutine will be scheduled by
45     ``ensure_future()``.
46
47   * a :class:`Task` object that would await on *obj*, if *obj* is an
48     awaitable (:func:`inspect.isawaitable` is used for the test.)
49
50   If *obj* is neither of the above a :exc:`TypeError` is raised.
51
52   .. important::
53
54      See also the :func:`create_task` function which is the
55      preferred way for creating new Tasks.
56
57      Save a reference to the result of this function, to avoid
58      a task disappearing mid-execution.
59
60   .. versionchanged:: 3.5.1
61      The function accepts any :term:`awaitable` object.
62
63   .. deprecated:: 3.10
64      Deprecation warning is emitted if *obj* is not a Future-like object
65      and *loop* is not specified and there is no running event loop.
66
67
68.. function:: wrap_future(future, *, loop=None)
69
70   Wrap a :class:`concurrent.futures.Future` object in a
71   :class:`asyncio.Future` object.
72
73   .. deprecated:: 3.10
74      Deprecation warning is emitted if *future* is not a Future-like object
75      and *loop* is not specified and there is no running event loop.
76
77
78Future Object
79=============
80
81.. class:: Future(*, loop=None)
82
83   A Future represents an eventual result of an asynchronous
84   operation.  Not thread-safe.
85
86   Future is an :term:`awaitable` object.  Coroutines can await on
87   Future objects until they either have a result or an exception
88   set, or until they are cancelled. A Future can be awaited multiple
89   times and the result is same.
90
91   Typically Futures are used to enable low-level
92   callback-based code (e.g. in protocols implemented using asyncio
93   :ref:`transports <asyncio-transports-protocols>`)
94   to interoperate with high-level async/await code.
95
96   The rule of thumb is to never expose Future objects in user-facing
97   APIs, and the recommended way to create a Future object is to call
98   :meth:`loop.create_future`.  This way alternative event loop
99   implementations can inject their own optimized implementations
100   of a Future object.
101
102   .. versionchanged:: 3.7
103      Added support for the :mod:`contextvars` module.
104
105   .. deprecated:: 3.10
106      Deprecation warning is emitted if *loop* is not specified
107      and there is no running event loop.
108
109   .. method:: result()
110
111      Return the result of the Future.
112
113      If the Future is *done* and has a result set by the
114      :meth:`set_result` method, the result value is returned.
115
116      If the Future is *done* and has an exception set by the
117      :meth:`set_exception` method, this method raises the exception.
118
119      If the Future has been *cancelled*, this method raises
120      a :exc:`CancelledError` exception.
121
122      If the Future's result isn't yet available, this method raises
123      a :exc:`InvalidStateError` exception.
124
125   .. method:: set_result(result)
126
127      Mark the Future as *done* and set its result.
128
129      Raises a :exc:`InvalidStateError` error if the Future is
130      already *done*.
131
132   .. method:: set_exception(exception)
133
134      Mark the Future as *done* and set an exception.
135
136      Raises a :exc:`InvalidStateError` error if the Future is
137      already *done*.
138
139   .. method:: done()
140
141      Return ``True`` if the Future is *done*.
142
143      A Future is *done* if it was *cancelled* or if it has a result
144      or an exception set with :meth:`set_result` or
145      :meth:`set_exception` calls.
146
147   .. method:: cancelled()
148
149      Return ``True`` if the Future was *cancelled*.
150
151      The method is usually used to check if a Future is not
152      *cancelled* before setting a result or an exception for it::
153
154          if not fut.cancelled():
155              fut.set_result(42)
156
157   .. method:: add_done_callback(callback, *, context=None)
158
159      Add a callback to be run when the Future is *done*.
160
161      The *callback* is called with the Future object as its only
162      argument.
163
164      If the Future is already *done* when this method is called,
165      the callback is scheduled with :meth:`loop.call_soon`.
166
167      An optional keyword-only *context* argument allows specifying a
168      custom :class:`contextvars.Context` for the *callback* to run in.
169      The current context is used when no *context* is provided.
170
171      :func:`functools.partial` can be used to pass parameters
172      to the callback, e.g.::
173
174          # Call 'print("Future:", fut)' when "fut" is done.
175          fut.add_done_callback(
176              functools.partial(print, "Future:"))
177
178      .. versionchanged:: 3.7
179         The *context* keyword-only parameter was added.
180         See :pep:`567` for more details.
181
182   .. method:: remove_done_callback(callback)
183
184      Remove *callback* from the callbacks list.
185
186      Returns the number of callbacks removed, which is typically 1,
187      unless a callback was added more than once.
188
189   .. method:: cancel(msg=None)
190
191      Cancel the Future and schedule callbacks.
192
193      If the Future is already *done* or *cancelled*, return ``False``.
194      Otherwise, change the Future's state to *cancelled*,
195      schedule the callbacks, and return ``True``.
196
197      .. versionchanged:: 3.9
198         Added the *msg* parameter.
199
200   .. method:: exception()
201
202      Return the exception that was set on this Future.
203
204      The exception (or ``None`` if no exception was set) is
205      returned only if the Future is *done*.
206
207      If the Future has been *cancelled*, this method raises a
208      :exc:`CancelledError` exception.
209
210      If the Future isn't *done* yet, this method raises an
211      :exc:`InvalidStateError` exception.
212
213   .. method:: get_loop()
214
215      Return the event loop the Future object is bound to.
216
217      .. versionadded:: 3.7
218
219
220.. _asyncio_example_future:
221
222This example creates a Future object, creates and schedules an
223asynchronous Task to set result for the Future, and waits until
224the Future has a result::
225
226    async def set_after(fut, delay, value):
227        # Sleep for *delay* seconds.
228        await asyncio.sleep(delay)
229
230        # Set *value* as a result of *fut* Future.
231        fut.set_result(value)
232
233    async def main():
234        # Get the current event loop.
235        loop = asyncio.get_running_loop()
236
237        # Create a new Future object.
238        fut = loop.create_future()
239
240        # Run "set_after()" coroutine in a parallel Task.
241        # We are using the low-level "loop.create_task()" API here because
242        # we already have a reference to the event loop at hand.
243        # Otherwise we could have just used "asyncio.create_task()".
244        loop.create_task(
245            set_after(fut, 1, '... world'))
246
247        print('hello ...')
248
249        # Wait until *fut* has a result (1 second) and print it.
250        print(await fut)
251
252    asyncio.run(main())
253
254
255.. important::
256
257   The Future object was designed to mimic
258   :class:`concurrent.futures.Future`.  Key differences include:
259
260   - unlike asyncio Futures, :class:`concurrent.futures.Future`
261     instances cannot be awaited.
262
263   - :meth:`asyncio.Future.result` and :meth:`asyncio.Future.exception`
264     do not accept the *timeout* argument.
265
266   - :meth:`asyncio.Future.result` and :meth:`asyncio.Future.exception`
267     raise an :exc:`InvalidStateError` exception when the Future is not
268     *done*.
269
270   - Callbacks registered with :meth:`asyncio.Future.add_done_callback`
271     are not called immediately.  They are scheduled with
272     :meth:`loop.call_soon` instead.
273
274   - asyncio Future is not compatible with the
275     :func:`concurrent.futures.wait` and
276     :func:`concurrent.futures.as_completed` functions.
277
278   - :meth:`asyncio.Future.cancel` accepts an optional ``msg`` argument,
279     but :func:`concurrent.futures.cancel` does not.
280