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python-3.7.4-docs-html/_sources/library/asyncio-future.rst.txt

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+.. currentmodule:: asyncio
+
+
+.. _asyncio-futures:
+
+=======
+Futures
+=======
+
+*Future* objects are used to bridge **low-level callback-based code**
+with high-level async/await code.
+
+
+Future Functions
+================
+
+.. function:: isfuture(obj)
+
+   Return ``True`` if *obj* is either of:
+
+   * an instance of :class:`asyncio.Future`,
+   * an instance of :class:`asyncio.Task`,
+   * a Future-like object with a ``_asyncio_future_blocking``
+     attribute.
+
+   .. versionadded:: 3.5
+
+
+.. function:: ensure_future(obj, \*, loop=None)
+
+   Return:
+
+   * *obj* argument as is, if *obj* is a :class:`Future`,
+     a :class:`Task`, or a Future-like object (:func:`isfuture`
+     is used for the test.)
+
+   * a :class:`Task` object wrapping *obj*, if *obj* is a
+     coroutine (:func:`iscoroutine` is used for the test.)
+
+   * a :class:`Task` object that would await on *obj*, if *obj* is an
+     awaitable (:func:`inspect.isawaitable` is used for the test.)
+
+   If *obj* is neither of the above a :exc:`TypeError` is raised.
+
+   .. important::
+
+      See also the :func:`create_task` function which is the
+      preferred way for creating new Tasks.
+
+   .. versionchanged:: 3.5.1
+      The function accepts any :term:`awaitable` object.
+
+
+.. function:: wrap_future(future, \*, loop=None)
+
+   Wrap a :class:`concurrent.futures.Future` object in a
+   :class:`asyncio.Future` object.
+
+
+Future Object
+=============
+
+.. class:: Future(\*, loop=None)
+
+   A Future represents an eventual result of an asynchronous
+   operation.  Not thread-safe.
+
+   Future is an :term:`awaitable` object.  Coroutines can await on
+   Future objects until they either have a result or an exception
+   set, or until they are cancelled.
+
+   Typically Futures are used to enable low-level
+   callback-based code (e.g. in protocols implemented using asyncio
+   :ref:`transports <asyncio-transports-protocols>`)
+   to interoperate with high-level async/await code.
+
+   The rule of thumb is to never expose Future objects in user-facing
+   APIs, and the recommended way to create a Future object is to call
+   :meth:`loop.create_future`.  This way alternative event loop
+   implementations can inject their own optimized implementations
+   of a Future object.
+
+   .. versionchanged:: 3.7
+      Added support for the :mod:`contextvars` module.
+
+   .. method:: result()
+
+      Return the result of the Future.
+
+      If the Future is *done* and has a result set by the
+      :meth:`set_result` method, the result value is returned.
+
+      If the Future is *done* and has an exception set by the
+      :meth:`set_exception` method, this method raises the exception.
+
+      If the Future has been *cancelled*, this method raises
+      a :exc:`CancelledError` exception.
+
+      If the Future's result isn't yet available, this method raises
+      a :exc:`InvalidStateError` exception.
+
+   .. method:: set_result(result)
+
+      Mark the Future as *done* and set its result.
+
+      Raises a :exc:`InvalidStateError` error if the Future is
+      already *done*.
+
+   .. method:: set_exception(exception)
+
+      Mark the Future as *done* and set an exception.
+
+      Raises a :exc:`InvalidStateError` error if the Future is
+      already *done*.
+
+   .. method:: done()
+
+      Return ``True`` if the Future is *done*.
+
+      A Future is *done* if it was *cancelled* or if it has a result
+      or an exception set with :meth:`set_result` or
+      :meth:`set_exception` calls.
+
+   .. method:: cancelled()
+
+      Return ``True`` if the Future was *cancelled*.
+
+      The method is usually used to check if a Future is not
+      *cancelled* before setting a result or an exception for it::
+
+          if not fut.cancelled():
+              fut.set_result(42)
+
+   .. method:: add_done_callback(callback, *, context=None)
+
+      Add a callback to be run when the Future is *done*.
+
+      The *callback* is called with the Future object as its only
+      argument.
+
+      If the Future is already *done* when this method is called,
+      the callback is scheduled with :meth:`loop.call_soon`.
+
+      An optional keyword-only *context* argument allows specifying a
+      custom :class:`contextvars.Context` for the *callback* to run in.
+      The current context is used when no *context* is provided.
+
+      :func:`functools.partial` can be used to pass parameters
+      to the callback, e.g.::
+
+          # Call 'print("Future:", fut)' when "fut" is done.
+          fut.add_done_callback(
+              functools.partial(print, "Future:"))
+
+      .. versionchanged:: 3.7
+         The *context* keyword-only parameter was added.
+         See :pep:`567` for more details.
+
+   .. method:: remove_done_callback(callback)
+
+      Remove *callback* from the callbacks list.
+
+      Returns the number of callbacks removed, which is typically 1,
+      unless a callback was added more than once.
+
+   .. method:: cancel()
+
+      Cancel the Future and schedule callbacks.
+
+      If the Future is already *done* or *cancelled*, return ``False``.
+      Otherwise, change the Future's state to *cancelled*,
+      schedule the callbacks, and return ``True``.
+
+   .. method:: exception()
+
+      Return the exception that was set on this Future.
+
+      The exception (or ``None`` if no exception was set) is
+      returned only if the Future is *done*.
+
+      If the Future has been *cancelled*, this method raises a
+      :exc:`CancelledError` exception.
+
+      If the Future isn't *done* yet, this method raises an
+      :exc:`InvalidStateError` exception.
+
+   .. method:: get_loop()
+
+      Return the event loop the Future object is bound to.
+
+      .. versionadded:: 3.7
+
+
+.. _asyncio_example_future:
+
+This example creates a Future object, creates and schedules an
+asynchronous Task to set result for the Future, and waits until
+the Future has a result::
+
+    async def set_after(fut, delay, value):
+        # Sleep for *delay* seconds.
+        await asyncio.sleep(delay)
+
+        # Set *value* as a result of *fut* Future.
+        fut.set_result(value)
+
+    async def main():
+        # Get the current event loop.
+        loop = asyncio.get_running_loop()
+
+        # Create a new Future object.
+        fut = loop.create_future()
+
+        # Run "set_after()" coroutine in a parallel Task.
+        # We are using the low-level "loop.create_task()" API here because
+        # we already have a reference to the event loop at hand.
+        # Otherwise we could have just used "asyncio.create_task()".
+        loop.create_task(
+            set_after(fut, 1, '... world'))
+
+        print('hello ...')
+
+        # Wait until *fut* has a result (1 second) and print it.
+        print(await fut)
+
+    asyncio.run(main())
+
+
+.. important::
+
+   The Future object was designed to mimic
+   :class:`concurrent.futures.Future`.  Key differences include:
+
+   - unlike asyncio Futures, :class:`concurrent.futures.Future`
+     instances cannot be awaited.
+
+   - :meth:`asyncio.Future.result` and :meth:`asyncio.Future.exception`
+     do not accept the *timeout* argument.
+
+   - :meth:`asyncio.Future.result` and :meth:`asyncio.Future.exception`
+     raise an :exc:`InvalidStateError` exception when the Future is not
+     *done*.
+
+   - Callbacks registered with :meth:`asyncio.Future.add_done_callback`
+     are not called immediately.  They are scheduled with
+     :meth:`loop.call_soon` instead.
+
+   - asyncio Future is not compatible with the
+     :func:`concurrent.futures.wait` and
+     :func:`concurrent.futures.as_completed` functions.