# Extending

The main direction for `asyncio` extending is writing custom event loop classes. Asyncio has helpers that could be used to simplify this task.

Observera

Third-parties should reuse existing asyncio code with caution, a new Python version is free to break backward compatibility in internal part of API.

## 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.

  Förändrat i 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.