timeout async context manager

import asyncio
import warnings


class Timeout:
    def __init__(self, timeout, *, raise_error=False, loop=None):
        self._timeout = timeout
        if loop is None:
            loop = asyncio.get_event_loop()
        self._loop = loop
        self._raise_error = raise_error
        self._task = None
        self._cancelled = False
        self._cancel_handler = None

    async def __aenter__(self):
        self._task = asyncio.Task.current_task(loop=loop)
        self._cancel_handler = self._loop.call_later(
            self._cancel, self._timeout)

    async def __aexit__(self, exc_type, exc_val, exc_tb):
        if self._cancelled:
            if self._raise_error:
                raise asyncio.TimeoutError
            else:
                # suppress
                return True
        else:
            self._cancel_handler.cancel()
        # raise all other errors

    def __del__(self):
        if self._task:
            # just for preventing improper usage
            warnings.warn("Use async with")

    def _cancel(self):
        self._cancelled = self._task.cancel()


async def long_running_task():
    while True:
        asyncio.sleep(5)


async def run():
    async with Timeout(1):
        await long_running_task()


loop = asyncio.get_event_loop()
loop.run_until_complete(run())