Extending¶

Writing a Custom Event Loop¶

asyncio.AbstractEventLoop declares very many methods. Implementing all them from scratch is a tedious job.

A loop can get many common methods implementation for free by inheriting from asyncio.BaseEventLoop.

In turn, the successor should implement a bunch of private methods declared but not implemented in asyncio.BaseEventLoop.

For example, loop.create_connection() checks arguments, resolves DNS addresses, and calls loop._make_socket_transport() that should be implemented by inherited class. The _make_socket_transport() method is not documented and is considered as an internal API.

Future and Task private constructors¶

asyncio.Future and asyncio.Task should be never created directly, please use corresponding loop.create_future() and loop.create_task(), or asyncio.create_task() factories instead.

However, third-party event loops may reuse built-in future and task implementations for the sake of getting a complex and highly optimized code for free.

For this purpose the following, private constructors are listed:

Future.__init__(*, loop=None)¶

Create a built-in future instance.

loop is an optional event loop instance.

Task.__init__(coro, *, loop=None, name=None, context=None)¶

Create a built-in task instance.

loop is an optional event loop instance. The rest of arguments are described in loop.create_task() description.

Changed in version 3.11: context argument is added.

Task lifetime support¶

A third party task implementation should call the following functions to keep a task visible by asyncio.all_tasks() and asyncio.current_task():

asyncio._register_task(task)¶

Register a new task as managed by asyncio.

Call the function from a task constructor.

asyncio._unregister_task(task)¶

Unregister a task from asyncio internal structures.

The function should be called when a task is about to finish.

asyncio._enter_task(loop, task)¶

Switch the current task to the task argument.

Call the function just before executing a portion of embedded coroutine (coroutine.send() or coroutine.throw()).

asyncio._leave_task(loop, task)¶

Switch the current task back from task to None.

Call the function just after coroutine.send() or coroutine.throw() execution.