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160 lines
5.4 KiB
ReStructuredText
160 lines
5.4 KiB
ReStructuredText
.. currentmodule:: asyncio
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=======
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Runners
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=======
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**Source code:** :source:`Lib/asyncio/runners.py`
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This section outlines high-level asyncio primitives to run asyncio code.
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They are built on top of an :ref:`event loop <asyncio-event-loop>` with the aim
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to simplify async code usage for common wide-spread scenarios.
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.. contents::
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:depth: 1
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:local:
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Running an asyncio Program
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==========================
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.. function:: run(coro, *, debug=None, loop_factory=None)
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Execute the :term:`coroutine` *coro* and return the result.
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This function runs the passed coroutine, taking care of
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managing the asyncio event loop, *finalizing asynchronous
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generators*, and closing the executor.
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This function cannot be called when another asyncio event loop is
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running in the same thread.
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If *debug* is ``True``, the event loop will be run in debug mode. ``False`` disables
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debug mode explicitly. ``None`` is used to respect the global
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:ref:`asyncio-debug-mode` settings.
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If *loop_factory* is not ``None``, it is used to create a new event loop;
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otherwise :func:`asyncio.new_event_loop` is used. The loop is closed at the end.
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This function should be used as a main entry point for asyncio programs,
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and should ideally only be called once. It is recommended to use
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*loop_factory* to configure the event loop instead of policies.
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The executor is given a timeout duration of 5 minutes to shutdown.
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If the executor hasn't finished within that duration, a warning is
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emitted and the executor is closed.
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Example::
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async def main():
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await asyncio.sleep(1)
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print('hello')
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asyncio.run(main())
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.. versionadded:: 3.7
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.. versionchanged:: 3.9
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Updated to use :meth:`loop.shutdown_default_executor`.
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.. versionchanged:: 3.10
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*debug* is ``None`` by default to respect the global debug mode settings.
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.. versionchanged:: 3.12
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Added *loop_factory* parameter.
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Runner context manager
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======================
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.. class:: Runner(*, debug=None, loop_factory=None)
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A context manager that simplifies *multiple* async function calls in the same
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context.
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Sometimes several top-level async functions should be called in the same :ref:`event
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loop <asyncio-event-loop>` and :class:`contextvars.Context`.
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If *debug* is ``True``, the event loop will be run in debug mode. ``False`` disables
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debug mode explicitly. ``None`` is used to respect the global
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:ref:`asyncio-debug-mode` settings.
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*loop_factory* could be used for overriding the loop creation.
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It is the responsibility of the *loop_factory* to set the created loop as the
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current one. By default :func:`asyncio.new_event_loop` is used and set as
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current event loop with :func:`asyncio.set_event_loop` if *loop_factory* is ``None``.
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Basically, :func:`asyncio.run()` example can be rewritten with the runner usage::
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async def main():
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await asyncio.sleep(1)
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print('hello')
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with asyncio.Runner() as runner:
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runner.run(main())
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.. versionadded:: 3.11
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.. method:: run(coro, *, context=None)
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Run a :term:`coroutine <coroutine>` *coro* in the embedded loop.
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Return the coroutine's result or raise its exception.
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An optional keyword-only *context* argument allows specifying a
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custom :class:`contextvars.Context` for the *coro* to run in.
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The runner's default context is used if ``None``.
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This function cannot be called when another asyncio event loop is
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running in the same thread.
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.. method:: close()
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Close the runner.
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Finalize asynchronous generators, shutdown default executor, close the event loop
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and release embedded :class:`contextvars.Context`.
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.. method:: get_loop()
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Return the event loop associated with the runner instance.
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.. note::
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:class:`Runner` uses the lazy initialization strategy, its constructor doesn't
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initialize underlying low-level structures.
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Embedded *loop* and *context* are created at the :keyword:`with` body entering
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or the first call of :meth:`run` or :meth:`get_loop`.
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Handling Keyboard Interruption
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==============================
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.. versionadded:: 3.11
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When :const:`signal.SIGINT` is raised by :kbd:`Ctrl-C`, :exc:`KeyboardInterrupt`
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exception is raised in the main thread by default. However this doesn't work with
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:mod:`asyncio` because it can interrupt asyncio internals and can hang the program from
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exiting.
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To mitigate this issue, :mod:`asyncio` handles :const:`signal.SIGINT` as follows:
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1. :meth:`asyncio.Runner.run` installs a custom :const:`signal.SIGINT` handler before
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any user code is executed and removes it when exiting from the function.
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2. The :class:`~asyncio.Runner` creates the main task for the passed coroutine for its
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execution.
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3. When :const:`signal.SIGINT` is raised by :kbd:`Ctrl-C`, the custom signal handler
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cancels the main task by calling :meth:`asyncio.Task.cancel` which raises
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:exc:`asyncio.CancelledError` inside the main task. This causes the Python stack
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to unwind, ``try/except`` and ``try/finally`` blocks can be used for resource
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cleanup. After the main task is cancelled, :meth:`asyncio.Runner.run` raises
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:exc:`KeyboardInterrupt`.
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4. A user could write a tight loop which cannot be interrupted by
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:meth:`asyncio.Task.cancel`, in which case the second following :kbd:`Ctrl-C`
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immediately raises the :exc:`KeyboardInterrupt` without cancelling the main task.
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