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Filename : autoasync.py
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# Copyright 2014-2015 Nathan West
#
# This file is part of autocommand.
#
# autocommand is free software: you can redistribute it and/or modify
# it under the terms of the GNU Lesser General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# autocommand is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU Lesser General Public License for more details.
#
# You should have received a copy of the GNU Lesser General Public License
# along with autocommand.  If not, see <http://www.gnu.org/licenses/>.

from asyncio import get_event_loop, iscoroutine
from functools import wraps
from inspect import signature


async def _run_forever_coro(coro, args, kwargs, loop):
    '''
    This helper function launches an async main function that was tagged with
    forever=True. There are two possibilities:

    - The function is a normal function, which handles initializing the event
      loop, which is then run forever
    - The function is a coroutine, which needs to be scheduled in the event
      loop, which is then run forever
      - There is also the possibility that the function is a normal function
        wrapping a coroutine function

    The function is therefore called unconditionally and scheduled in the event
    loop if the return value is a coroutine object.

    The reason this is a separate function is to make absolutely sure that all
    the objects created are garbage collected after all is said and done; we
    do this to ensure that any exceptions raised in the tasks are collected
    ASAP.
    '''

    # Personal note: I consider this an antipattern, as it relies on the use of
    # unowned resources. The setup function dumps some stuff into the event
    # loop where it just whirls in the ether without a well defined owner or
    # lifetime. For this reason, there's a good chance I'll remove the
    # forever=True feature from autoasync at some point in the future.
    thing = coro(*args, **kwargs)
    if iscoroutine(thing):
        await thing


def autoasync(coro=None, *, loop=None, forever=False, pass_loop=False):
    '''
    Convert an asyncio coroutine into a function which, when called, is
    evaluted in an event loop, and the return value returned. This is intented
    to make it easy to write entry points into asyncio coroutines, which
    otherwise need to be explictly evaluted with an event loop's
    run_until_complete.

    If `loop` is given, it is used as the event loop to run the coro in. If it
    is None (the default), the loop is retreived using asyncio.get_event_loop.
    This call is defered until the decorated function is called, so that
    callers can install custom event loops or event loop policies after
    @autoasync is applied.

    If `forever` is True, the loop is run forever after the decorated coroutine
    is finished. Use this for servers created with asyncio.start_server and the
    like.

    If `pass_loop` is True, the event loop object is passed into the coroutine
    as the `loop` kwarg when the wrapper function is called. In this case, the
    wrapper function's __signature__ is updated to remove this parameter, so
    that autoparse can still be used on it without generating a parameter for
    `loop`.

    This coroutine can be called with ( @autoasync(...) ) or without
    ( @autoasync ) arguments.

    Examples:

    @autoasync
    def get_file(host, port):
        reader, writer = yield from asyncio.open_connection(host, port)
        data = reader.read()
        sys.stdout.write(data.decode())

    get_file(host, port)

    @autoasync(forever=True, pass_loop=True)
    def server(host, port, loop):
        yield_from loop.create_server(Proto, host, port)

    server('localhost', 8899)

    '''
    if coro is None:
        return lambda c: autoasync(
            c, loop=loop,
            forever=forever,
            pass_loop=pass_loop)

    # The old and new signatures are required to correctly bind the loop
    # parameter in 100% of cases, even if it's a positional parameter.
    # NOTE: A future release will probably require the loop parameter to be
    # a kwonly parameter.
    if pass_loop:
        old_sig = signature(coro)
        new_sig = old_sig.replace(parameters=(
            param for name, param in old_sig.parameters.items()
            if name != "loop"))

    @wraps(coro)
    def autoasync_wrapper(*args, **kwargs):
        # Defer the call to get_event_loop so that, if a custom policy is
        # installed after the autoasync decorator, it is respected at call time
        local_loop = get_event_loop() if loop is None else loop

        # Inject the 'loop' argument. We have to use this signature binding to
        # ensure it's injected in the correct place (positional, keyword, etc)
        if pass_loop:
            bound_args = old_sig.bind_partial()
            bound_args.arguments.update(
                loop=local_loop,
                **new_sig.bind(*args, **kwargs).arguments)
            args, kwargs = bound_args.args, bound_args.kwargs

        if forever:
            local_loop.create_task(_run_forever_coro(
                coro, args, kwargs, local_loop
            ))
            local_loop.run_forever()
        else:
            return local_loop.run_until_complete(coro(*args, **kwargs))

    # Attach the updated signature. This allows 'pass_loop' to be used with
    # autoparse
    if pass_loop:
        autoasync_wrapper.__signature__ = new_sig

    return autoasync_wrapper