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   .. versionchanged:: 3.5.1
58      The function accepts any :term:`awaitable` object.
59
60
61.. function:: wrap_future(future, *, loop=None)
62
63   Wrap a :class:`concurrent.futures.Future` object in a
64   :class:`asyncio.Future` object.
65
66
67Future Object
68=============
69
70.. class:: Future(*, loop=None)
71
72   A Future represents an eventual result of an asynchronous
73   operation.  Not thread-safe.
74
75   Future is an :term:`awaitable` object.  Coroutines can await on
76   Future objects until they either have a result or an exception
77   set, or until they are cancelled.
78
79   Typically Futures are used to enable low-level
80   callback-based code (e.g. in protocols implemented using asyncio
81   :ref:`transports <asyncio-transports-protocols>`)
82   to interoperate with high-level async/await code.
83
84   The rule of thumb is to never expose Future objects in user-facing
85   APIs, and the recommended way to create a Future object is to call
86   :meth:`loop.create_future`.  This way alternative event loop
87   implementations can inject their own optimized implementations
88   of a Future object.
89
90   .. versionchanged:: 3.7
91      Added support for the :mod:`contextvars` module.
92
93   .. method:: result()
94
95      Return the result of the Future.
96
97      If the Future is *done* and has a result set by the
98      :meth:`set_result` method, the result value is returned.
99
100      If the Future is *done* and has an exception set by the
101      :meth:`set_exception` method, this method raises the exception.
102
103      If the Future has been *cancelled*, this method raises
104      a :exc:`CancelledError` exception.
105
106      If the Future's result isn't yet available, this method raises
107      a :exc:`InvalidStateError` exception.
108
109   .. method:: set_result(result)
110
111      Mark the Future as *done* and set its result.
112
113      Raises a :exc:`InvalidStateError` error if the Future is
114      already *done*.
115
116   .. method:: set_exception(exception)
117
118      Mark the Future as *done* and set an exception.
119
120      Raises a :exc:`InvalidStateError` error if the Future is
121      already *done*.
122
123   .. method:: done()
124
125      Return ``True`` if the Future is *done*.
126
127      A Future is *done* if it was *cancelled* or if it has a result
128      or an exception set with :meth:`set_result` or
129      :meth:`set_exception` calls.
130
131   .. method:: cancelled()
132
133      Return ``True`` if the Future was *cancelled*.
134
135      The method is usually used to check if a Future is not
136      *cancelled* before setting a result or an exception for it::
137
138          if not fut.cancelled():
139              fut.set_result(42)
140
141   .. method:: add_done_callback(callback, *, context=None)
142
143      Add a callback to be run when the Future is *done*.
144
145      The *callback* is called with the Future object as its only
146      argument.
147
148      If the Future is already *done* when this method is called,
149      the callback is scheduled with :meth:`loop.call_soon`.
150
151      An optional keyword-only *context* argument allows specifying a
152      custom :class:`contextvars.Context` for the *callback* to run in.
153      The current context is used when no *context* is provided.
154
155      :func:`functools.partial` can be used to pass parameters
156      to the callback, e.g.::
157
158          # Call 'print("Future:", fut)' when "fut" is done.
159          fut.add_done_callback(
160              functools.partial(print, "Future:"))
161
162      .. versionchanged:: 3.7
163         The *context* keyword-only parameter was added.
164         See :pep:`567` for more details.
165
166   .. method:: remove_done_callback(callback)
167
168      Remove *callback* from the callbacks list.
169
170      Returns the number of callbacks removed, which is typically 1,
171      unless a callback was added more than once.
172
173   .. method:: cancel()
174
175      Cancel the Future and schedule callbacks.
176
177      If the Future is already *done* or *cancelled*, return ``False``.
178      Otherwise, change the Future's state to *cancelled*,
179      schedule the callbacks, and return ``True``.
180
181   .. method:: exception()
182
183      Return the exception that was set on this Future.
184
185      The exception (or ``None`` if no exception was set) is
186      returned only if the Future is *done*.
187
188      If the Future has been *cancelled*, this method raises a
189      :exc:`CancelledError` exception.
190
191      If the Future isn't *done* yet, this method raises an
192      :exc:`InvalidStateError` exception.
193
194   .. method:: get_loop()
195
196      Return the event loop the Future object is bound to.
197
198      .. versionadded:: 3.7
199
200
201.. _asyncio_example_future:
202
203This example creates a Future object, creates and schedules an
204asynchronous Task to set result for the Future, and waits until
205the Future has a result::
206
207    async def set_after(fut, delay, value):
208        # Sleep for *delay* seconds.
209        await asyncio.sleep(delay)
210
211        # Set *value* as a result of *fut* Future.
212        fut.set_result(value)
213
214    async def main():
215        # Get the current event loop.
216        loop = asyncio.get_running_loop()
217
218        # Create a new Future object.
219        fut = loop.create_future()
220
221        # Run "set_after()" coroutine in a parallel Task.
222        # We are using the low-level "loop.create_task()" API here because
223        # we already have a reference to the event loop at hand.
224        # Otherwise we could have just used "asyncio.create_task()".
225        loop.create_task(
226            set_after(fut, 1, '... world'))
227
228        print('hello ...')
229
230        # Wait until *fut* has a result (1 second) and print it.
231        print(await fut)
232
233    asyncio.run(main())
234
235
236.. important::
237
238   The Future object was designed to mimic
239   :class:`concurrent.futures.Future`.  Key differences include:
240
241   - unlike asyncio Futures, :class:`concurrent.futures.Future`
242     instances cannot be awaited.
243
244   - :meth:`asyncio.Future.result` and :meth:`asyncio.Future.exception`
245     do not accept the *timeout* argument.
246
247   - :meth:`asyncio.Future.result` and :meth:`asyncio.Future.exception`
248     raise an :exc:`InvalidStateError` exception when the Future is not
249     *done*.
250
251   - Callbacks registered with :meth:`asyncio.Future.add_done_callback`
252     are not called immediately.  They are scheduled with
253     :meth:`loop.call_soon` instead.
254
255   - asyncio Future is not compatible with the
256     :func:`concurrent.futures.wait` and
257     :func:`concurrent.futures.as_completed` functions.
258