.. currentmodule:: asyncio Tasks and coroutines ==================== .. _coroutine: Coroutines ---------- A coroutine is a generator that follows certain conventions. For documentation purposes, all coroutines should be decorated with ``@asyncio.coroutine``, but this cannot be strictly enforced. Coroutines use the ``yield from`` syntax introduced in :pep:`380`, instead of the original ``yield`` syntax. The word "coroutine", like the word "generator", is used for two different (though related) concepts: - The function that defines a coroutine (a function definition decorated with ``@asyncio.coroutine``). If disambiguation is needed we will call this a *coroutine function* (:func:`iscoroutinefunction` returns ``True``). - The object obtained by calling a coroutine function. This object represents a computation or an I/O operation (usually a combination) that will complete eventually. If disambiguation is needed we will call it a *coroutine object* (:func:`iscoroutine` returns ``True``). Things a coroutine can do: - ``result = yield from future`` -- suspends the coroutine until the future is done, then returns the future's result, or raises an exception, which will be propagated. (If the future is cancelled, it will raise a ``CancelledError`` exception.) Note that tasks are futures, and everything said about futures also applies to tasks. - ``result = yield from coroutine`` -- wait for another coroutine to produce a result (or raise an exception, which will be propagated). The ``coroutine`` expression must be a *call* to another coroutine. - ``return expression`` -- produce a result to the coroutine that is waiting for this one using ``yield from``. - ``raise exception`` -- raise an exception in the coroutine that is waiting for this one using ``yield from``. Calling a coroutine does not start its code running -- it is just a generator, and the coroutine object returned by the call is really a generator object, which doesn't do anything until you iterate over it. In the case of a coroutine object, there are two basic ways to start it running: call ``yield from coroutine`` from another coroutine (assuming the other coroutine is already running!), or schedule its execution using the :meth:`BaseEventLoop.create_task` method. Coroutines (and tasks) can only run when the event loop is running. .. decorator:: coroutine Decorator to mark coroutines. If the coroutine is not yielded from before it is destroyed, an error message is logged. See :ref:`Detect coroutines never scheduled `. .. note:: In this documentation, some methods are documented as coroutines, even if they are plain Python functions returning a :class:`Future`. This is intentional to have a freedom of tweaking the implementation of these functions in the future. If such a function is needed to be used in a callback-style code, wrap its result with :func:`async`. .. _asyncio-hello-world-coroutine: Example: "Hello World" coroutine ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Print ``"Hello World"`` every two seconds using a coroutine:: import asyncio @asyncio.coroutine def greet_every_two_seconds(): while True: print('Hello World') yield from asyncio.sleep(2) loop = asyncio.get_event_loop() try: loop.run_until_complete(greet_every_two_seconds()) finally: loop.close() .. seealso:: :ref:`Hello World example using a callback `. Example: Chain coroutines ^^^^^^^^^^^^^^^^^^^^^^^^^ Example chaining coroutines:: import asyncio @asyncio.coroutine def compute(x, y): print("Compute %s + %s ..." % (x, y)) yield from asyncio.sleep(1.0) return x + y @asyncio.coroutine def print_sum(x, y): result = yield from compute(x, y) print("%s + %s = %s" % (x, y, result)) loop = asyncio.get_event_loop() loop.run_until_complete(print_sum(1, 2)) loop.close() ``compute()`` is chained to ``print_sum()``: ``print_sum()`` coroutine waits until ``compute()`` is completed before returning its result. Sequence diagram of the example: .. image:: tulip_coro.png :align: center The "Task" is created by the :meth:`BaseEventLoop.run_until_complete` method when it gets a coroutine object instead of a task. The diagram shows the control flow, it does not describe exactly how things work internally. For example, the sleep coroutine creates an internal future which uses :meth:`BaseEventLoop.call_later` to wake up the task in 1 second. InvalidStateError ----------------- .. exception:: InvalidStateError The operation is not allowed in this state. TimeoutError ------------ .. exception:: TimeoutError The operation exceeded the given deadline. .. note:: This exception is different from the builtin :exc:`TimeoutError` exception! Future ------ .. class:: Future(\*, loop=None) This class is *almost* compatible with :class:`concurrent.futures.Future`. Differences: - :meth:`result` and :meth:`exception` do not take a timeout argument and raise an exception when the future isn't done yet. - Callbacks registered with :meth:`add_done_callback` are always called via the event loop's :meth:`~BaseEventLoop.call_soon_threadsafe`. - This class is not compatible with the :func:`~concurrent.futures.wait` and :func:`~concurrent.futures.as_completed` functions in the :mod:`concurrent.futures` package. .. 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:: cancelled() Return ``True`` if the future was cancelled. .. method:: done() Return True if the future is done. Done means either that a result / exception are available, or that the future was cancelled. .. method:: result() Return the result this future represents. If the future has been cancelled, raises :exc:`CancelledError`. If the future's result isn't yet available, raises :exc:`InvalidStateError`. If the future is done and has an exception set, this exception is raised. .. 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, raises :exc:`CancelledError`. If the future isn't done yet, raises :exc:`InvalidStateError`. .. method:: add_done_callback(fn) Add a callback to be run when the future becomes done. The callback is called with a single argument - the future object. If the future is already done when this is called, the callback is scheduled with :meth:`~BaseEventLoop.call_soon`. .. method:: remove_done_callback(fn) Remove all instances of a callback from the "call when done" list. Returns the number of callbacks removed. .. method:: set_result(result) Mark the future done and set its result. If the future is already done when this method is called, raises :exc:`InvalidStateError`. .. method:: set_exception(exception) Mark the future done and set an exception. If the future is already done when this method is called, raises :exc:`InvalidStateError`. Example: Future with run_until_complete() ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Example combining a :class:`Future` and a :ref:`coroutine function `:: import asyncio @asyncio.coroutine def slow_operation(future): yield from asyncio.sleep(1) future.set_result('Future is done!') loop = asyncio.get_event_loop() future = asyncio.Future() loop.create_task(slow_operation(future)) loop.run_until_complete(future) print(future.result()) loop.close() The coroutine function is responsible of the computation (which takes 1 second) and it stores the result into the future. The :meth:`~BaseEventLoop.run_until_complete` method waits for the completion of the future. .. note:: The :meth:`~BaseEventLoop.run_until_complete` method uses internally the :meth:`~Future.add_done_callback` method to be notified when the future is done. Example: Future with run_forever() ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ The previous example can be written differently using the :meth:`Future.add_done_callback` method to describe explicitly the control flow:: import asyncio @asyncio.coroutine def slow_operation(future): yield from asyncio.sleep(1) future.set_result('Future is done!') def got_result(future): print(future.result()) loop.stop() loop = asyncio.get_event_loop() future = asyncio.Future() loop.create_task(slow_operation(future)) future.add_done_callback(got_result) try: loop.run_forever() finally: loop.close() In this example, the future is responsible to display the result and to stop the loop. .. note:: The "slow_operation" coroutine object is only executed when the event loop starts running, so it is possible to add a "done callback" to the future after creating the task scheduling the coroutine object. Task ---- .. class:: Task(coro, \*, loop=None) Schedule the execution of a :ref:`coroutine `: wrap it in a future. A task is a subclass of :class:`Future`. A task is responsible to execute a coroutine object in an event loop. If the wrapped coroutine yields from a future, the task suspends the execution of the wrapped coroutine and waits for the completition of the future. When the future is done, the execution of the wrapped coroutine restarts with the result or the exception of the future. Event loops use cooperative scheduling: an event loop only runs one task at the same time. Other tasks may run in parallel if other event loops are running in different threads. While a task waits for the completion of a future, the event loop executes a new task. The cancellation of a task is different than cancelling a future. Calling :meth:`cancel` will throw a :exc:`~concurrent.futures.CancelledError` to the wrapped coroutine. :meth:`~Future.cancelled` only returns ``True`` if the wrapped coroutine did not catch the :exc:`~concurrent.futures.CancelledError` exception, or raised a :exc:`~concurrent.futures.CancelledError` exception. If a pending task is destroyed, the execution of its wrapped :ref:`coroutine ` did not complete. It is probably a bug and a warning is logged: see :ref:`Pending task destroyed `. Don't create directly :class:`Task` instances: use the :meth:`BaseEventLoop.create_task` method. .. classmethod:: all_tasks(loop=None) Return a set of all tasks for an event loop. By default all tasks for the current event loop are returned. .. classmethod:: current_task(loop=None) Return the currently running task in an event loop or ``None``. By default the current task for the current event loop is returned. ``None`` is returned when called not in the context of a :class:`Task`. .. method:: cancel() Request this task to cancel itself. This arranges for a :exc:`~concurrent.futures.CancelledError` to be thrown into the wrapped coroutine on the next cycle through the event loop. The coroutine then has a chance to clean up or even deny the request using try/except/finally. Contrary to :meth:`Future.cancel`, this does not guarantee that the task will be cancelled: the exception might be caught and acted upon, delaying cancellation of the task or preventing it completely. The task may also return a value or raise a different exception. Immediately after this method is called, :meth:`~Future.cancelled` will not return ``True`` (unless the task was already cancelled). A task will be marked as cancelled when the wrapped coroutine terminates with a :exc:`~concurrent.futures.CancelledError` exception (even if :meth:`cancel` was not called). .. method:: get_stack(\*, limit=None) Return the list of stack frames for this task's coroutine. If the coroutine is active, this returns the stack where it is suspended. If the coroutine has completed successfully or was cancelled, this returns an empty list. If the coroutine was terminated by an exception, this returns the list of traceback frames. The frames are always ordered from oldest to newest. The optional limit gives the maximum number of frames to return; by default all available frames are returned. Its meaning differs depending on whether a stack or a traceback is returned: the newest frames of a stack are returned, but the oldest frames of a traceback are returned. (This matches the behavior of the traceback module.) For reasons beyond our control, only one stack frame is returned for a suspended coroutine. .. method:: print_stack(\*, limit=None, file=None) Print the stack or traceback for this task's coroutine. This produces output similar to that of the traceback module, for the frames retrieved by get_stack(). The limit argument is passed to get_stack(). The file argument is an I/O stream to which the output goes; by default it goes to sys.stderr. Example: Parallel execution of tasks ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Example executing 3 tasks (A, B, C) in parallel:: import asyncio @asyncio.coroutine def factorial(name, number): f = 1 for i in range(2, number+1): print("Task %s: Compute factorial(%s)..." % (name, i)) yield from asyncio.sleep(1) f *= i print("Task %s: factorial(%s) = %s" % (name, number, f)) loop = asyncio.get_event_loop() tasks = [ loop.create_task(factorial("A", 2)), loop.create_task(factorial("B", 3)), loop.create_task(factorial("C", 4))] loop.run_until_complete(asyncio.wait(tasks)) loop.close() Output:: Task A: Compute factorial(2)... Task B: Compute factorial(2)... Task C: Compute factorial(2)... Task A: factorial(2) = 2 Task B: Compute factorial(3)... Task C: Compute factorial(3)... Task B: factorial(3) = 6 Task C: Compute factorial(4)... Task C: factorial(4) = 24 A task is automatically scheduled for execution when it is created. The event loop stops when all tasks are done. Task functions -------------- .. note:: In the functions below, the optional *loop* argument allows to explicitly set the event loop object used by the underlying task or coroutine. If it's not provided, the default event loop is used. .. function:: as_completed(fs, \*, loop=None, timeout=None) Return an iterator whose values, when waited for, are :class:`Future` instances. Raises :exc:`asyncio.TimeoutError` if the timeout occurs before all Futures are done. Example:: for f in as_completed(fs): result = yield from f # The 'yield from' may raise # Use result .. note:: The futures ``f`` are not necessarily members of fs. .. function:: async(coro_or_future, \*, loop=None) Wrap a :ref:`coroutine object ` in a future using the :meth:`BaseEventLoop.create_task` method. If the argument is a :class:`Future`, it is returned directly. .. function:: gather(\*coros_or_futures, loop=None, return_exceptions=False) Return a future aggregating results from the given coroutine objects or futures. All futures must share the same event loop. If all the tasks are done successfully, the returned future's result is the list of results (in the order of the original sequence, not necessarily the order of results arrival). If *return_exceptions* is True, exceptions in the tasks are treated the same as successful results, and gathered in the result list; otherwise, the first raised exception will be immediately propagated to the returned future. Cancellation: if the outer Future is cancelled, all children (that have not completed yet) are also cancelled. If any child is cancelled, this is treated as if it raised :exc:`~concurrent.futures.CancelledError` -- the outer Future is *not* cancelled in this case. (This is to prevent the cancellation of one child to cause other children to be cancelled.) .. function:: iscoroutine(obj) Return ``True`` if *obj* is a :ref:`coroutine object `. .. function:: iscoroutinefunction(obj) Return ``True`` if *func* is a decorated :ref:`coroutine function `. .. function:: sleep(delay, result=None, \*, loop=None) Create a :ref:`coroutine ` that completes after a given time (in seconds). If *result* is provided, it is produced to the caller when the coroutine completes. The resolution of the sleep depends on the :ref:`granularity of the event loop `. .. function:: shield(arg, \*, loop=None) Wait for a future, shielding it from cancellation. The statement:: res = yield from shield(something()) is exactly equivalent to the statement:: res = yield from something() *except* that if the coroutine containing it is cancelled, the task running in ``something()`` is not cancelled. From the point of view of ``something()``, the cancellation did not happen. But its caller is still cancelled, so the yield-from expression still raises :exc:`~concurrent.futures.CancelledError`. Note: If ``something()`` is cancelled by other means this will still cancel ``shield()``. If you want to completely ignore cancellation (not recommended) you can combine ``shield()`` with a try/except clause, as follows:: try: res = yield from shield(something()) except CancelledError: res = None .. function:: wait(futures, \*, loop=None, timeout=None, return_when=ALL_COMPLETED) Wait for the Futures and coroutine objects given by the sequence *futures* to complete. Coroutines will be wrapped in Tasks. Returns two sets of :class:`Future`: (done, pending). The sequence *futures* must not be empty. *timeout* can be used to control the maximum number of seconds to wait before returning. *timeout* can be an int or float. If *timeout* is not specified or ``None``, there is no limit to the wait time. *return_when* indicates when this function should return. It must be one of the following constants of the :mod:`concurrent.futures` module: .. tabularcolumns:: |l|L| +-----------------------------+----------------------------------------+ | Constant | Description | +=============================+========================================+ | :const:`FIRST_COMPLETED` | The function will return when any | | | future finishes or is cancelled. | +-----------------------------+----------------------------------------+ | :const:`FIRST_EXCEPTION` | The function will return when any | | | future finishes by raising an | | | exception. If no future raises an | | | exception then it is equivalent to | | | :const:`ALL_COMPLETED`. | +-----------------------------+----------------------------------------+ | :const:`ALL_COMPLETED` | The function will return when all | | | futures finish or are cancelled. | +-----------------------------+----------------------------------------+ This function is a :ref:`coroutine `. Usage:: done, pending = yield from asyncio.wait(fs) .. note:: This does not raise :exc:`asyncio.TimeoutError`! Futures that aren't done when the timeout occurs are returned in the second set. .. function:: wait_for(fut, timeout, \*, loop=None) Wait for the single :class:`Future` or :ref:`coroutine object ` to complete with timeout. If *timeout* is ``None``, block until the future completes. Coroutine objects are wrapped in a future using the :meth:`BaseEventLoop.create_task` method. Returns result of the Future or coroutine. When a timeout occurs, it cancels the task and raises :exc:`asyncio.TimeoutError`. To avoid the task cancellation, wrap it in :func:`shield`. This function is a :ref:`coroutine `, usage:: result = yield from asyncio.wait_for(fut, 60.0)