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