Async ExitStack

async_exit_stack.py

from inspect import iscoroutinefunction, isawaitable
import sys
from collections import deque


# NOTE: this follows the contextlib.ExitStack implementation


class _BaseExitStack:
    def __init__(self):
        self._exit_callbacks = deque()

    def pop_all(self):
        """Preserve the context stack by transferring it to a new instance"""
        new_stack = type(self)()
        new_stack._exit_callbacks = self._exit_callbacks
        self._exit_callbacks = deque()
        return new_stack

    def push(self, exit_obj):
        """Registers a callback with the standard __exit__ method signature

        Can suppress exceptions the same way __exit__ methods can.

        Also accepts any object with an __exit__ method (registering a call
        to the method instead of the object itself)
        """
        # We use an unbound method rather than a bound method to follow
        # the standard lookup behaviour for special methods
        _cb_type = type(exit_obj)
        try:
            exit_method = getattr(_cb_type, '__aexit__', None)
            if exit_method is None:
                exit_method = _cb_type.__exit__
        except AttributeError:
            # Not a context manager, so assume its a callable
            self._exit_callbacks.append(exit_obj)
        else:
            self._push_cm_exit(exit_obj, exit_method)
        return exit_obj  # Allow use as a decorator

    def enter_context(self, cm):
        """Enters the supplied context manager

        If successful, also pushes its __exit__ method as a callback and
        returns the result of the __enter__ method.
        """
        # We look up the special methods on the type to match the with statement
        _cm_type = type(cm)
        _exit = _cm_type.__exit__
        result = _cm_type.__enter__(cm)
        self._push_cm_exit(cm, _exit)
        return result

    @staticmethod
    def _create_exit_wrapper(cm, cm_exit):
        def _exit_wrapper(exc_type, exc, tb):
            return cm_exit(cm, exc_type, exc, tb)
        return _exit_wrapper

    def _push_cm_exit(self, cm, cm_exit):
        """Helper to correctly register callbacks to __exit__ methods"""
        _exit_wrapper = self._create_exit_wrapper(cm, cm_exit)
        _exit_wrapper.__self__ = cm
        self.push(_exit_wrapper)

    @staticmethod
    def _create_cb_wrapper(callback, *args, **kwds):
        def _exit_wrapper(exc_type, exc, tb):
            callback(*args, **kwds)
        return _exit_wrapper

    def callback(self, callback, *args, **kwds):
        """Registers an arbitrary callback and arguments.

        Cannot suppress exceptions.
        """
        _exit_wrapper = self._create_cb_wrapper(callback, *args, **kwds)

        # We changed the signature, so using @wraps is not appropriate, but
        # setting __wrapped__ may still help with introspection
        _exit_wrapper.__wrapped__ = callback
        self.push(_exit_wrapper)
        return callback  # Allow use as a decorator

    def _shutdown_loop(self, *exc_details):
        # Will yield each exit callback and expect result back
        received_exc = exc_details[0] is not None

        # We manipulate the exception state so it behaves as though
        # we were actually nesting multiple with statements
        frame_exc = sys.exc_info()[1]

        def _fix_exception_context(new_exc, old_exc):
            # Context may not be correct, so find the end of the chain
            while 1:
                exc_context = new_exc.__context__
                if exc_context is old_exc:
                    # Context is already set correctly (see issue 20317)
                    return
                if exc_context is None or exc_context is frame_exc:
                    break
                new_exc = exc_context
            # Change the end of the chain to point to the exception
            # we expect it to reference
            new_exc.__context__ = old_exc

        # Callbacks are invoked in LIFO order to match the behaviour of
        # nested context managers
        suppressed_exc = False
        pending_raise = False
        while self._exit_callbacks:
            cb = self._exit_callbacks.pop()
            try:
                cb_result = yield cb(*exc_details)
                if cb_result:
                    suppressed_exc = True
                    pending_raise = False
                    exc_details = (None, None, None)
            except:
                new_exc_details = sys.exc_info()
                # simulate the stack of exceptions by setting the context
                _fix_exception_context(new_exc_details[1], exc_details[1])
                pending_raise = True
                exc_details = new_exc_details

        if pending_raise:
            try:
                # bare "raise exc_details[1]" replaces our carefully
                # set-up context
                fixed_ctx = exc_details[1].__context__
                raise exc_details[1]
            except BaseException:
                exc_details[1].__context__ = fixed_ctx
                raise
        return received_exc and suppressed_exc


class ExitStack(_BaseExitStack):
    """Context manager for dynamic management of a stack of exit callbacks

    For example:

        with ExitStack() as stack:
            files = [stack.enter_context(open(fname)) for fname in filenames]
            # All opened files will automatically be closed at the end of
            # the with statement, even if attempts to open files later
            # in the list raise an exception
    """

    def __enter__(self):
        return self

    def __exit__(self, *exc_details):
        gen = self._shutdown_loop(*exc_details)
        try:
            result = next(gen)
            while 1:
                result = gen.send(result)
        except StopIteration as e:
            return e.value

    def close(self):
        """Immediately unwind the context stack"""
        self.__exit__(None, None, None)


# Inspired by discussions on https://bugs.python.org/issue29302
class AsyncExitStack(_BaseExitStack):
    """Async Context manager for dynamic management of a stack of exit callbacks

    also maps __enter__ and __exit__ to outer __aenter__, __aexit__

    For example:

        async with AsyncExitStack() as stack:
            files = [await stack.enter_context(open(fname)) for fname in filenames]
            # All opened files will automatically be closed at the end of
            # the with statement, even if attempts to open files later
            # in the list raise an exception
    """

    @staticmethod
    def _create_exit_wrapper(cm, cm_exit):
        if iscoroutinefunction(cm_exit):
            async def _exit_wrapper(exc_type, exc, tb):
                return await cm_exit(cm, exc_type, exc, tb)
            return _exit_wrapper

        return _BaseExitStack._create_exit_wrapper(cm, cm_exit)

    @staticmethod
    def _create_cb_wrapper(callback, *args, **kwds):
        if iscoroutinefunction(callback):
            async def _exit_wrapper(exc_type, exc, tb):
                await callback(*args, **kwds)
            return _exit_wrapper

        return _BaseExitStack._create_cb_wrapper(callback, *args, **kwds)

    async def enter_context(self, cm):
        """Enters the supplied context manager

        If successful, also pushes its __exit__ method as a callback and
        returns the result of the __enter__ method.
        """
        # We look up the special methods on the type to match the with statement
        _cm_type = type(cm)

        # if you have both aenter + enter, aenter will 'win'
        _exit = getattr(_cm_type, '__aexit__', None)
        if _exit is None:
            return super().enter_context(cm)

        result = await _cm_type.__aenter__(cm)
        self._push_cm_exit(cm, _exit)
        return result

    async def close(self):
        """Immediately unwind the context stack"""
        await self.__aexit__(None, None, None)

    async def __aenter__(self):
        return self

    async def __aexit__(self, *exc_details):
        gen = self._shutdown_loop(*exc_details)
        try:
            result = next(gen)
            while 1:
                try:
                    if isawaitable(result):
                        result = await result
                    result = gen.send(result)
                except StopIteration:
                    raise
                except BaseException as e:
                    result = gen.throw(e)
        except StopIteration as e:
            return e.value

for now this is untested, I'll be testing and using it soon in prod