"""
@asynccontextmanager code, copied from Python 3.7's contextlib.
For usage in Python 3.6.
"""
import abc
from functools import wraps

import _collections_abc

__all__ = ["asynccontextmanager"]


class AbstractAsyncContextManager(abc.ABC):

    """An abstract base class for asynchronous context managers."""

    async def __aenter__(self):
        """Return `self` upon entering the runtime context."""
        return self

    @abc.abstractmethod
    async def __aexit__(self, exc_type, exc_value, traceback):
        """Raise any exception triggered within the runtime context."""
        return None

    @classmethod
    def __subclasshook__(cls, C):
        if cls is AbstractAsyncContextManager:
            return _collections_abc._check_methods(C, "__aenter__", "__aexit__")
        return NotImplemented


class _GeneratorContextManagerBase:
    """Shared functionality for @contextmanager and @asynccontextmanager."""

    def __init__(self, func, args, kwds):
        self.gen = func(*args, **kwds)
        self.func, self.args, self.kwds = func, args, kwds
        # Issue 19330: ensure context manager instances have good docstrings
        doc = getattr(func, "__doc__", None)
        if doc is None:
            doc = type(self).__doc__
        self.__doc__ = doc
        # Unfortunately, this still doesn't provide good help output when
        # inspecting the created context manager instances, since pydoc
        # currently bypasses the instance docstring and shows the docstring
        # for the class instead.
        # See http://bugs.python.org/issue19404 for more details.


class _AsyncGeneratorContextManager(
    _GeneratorContextManagerBase, AbstractAsyncContextManager
):
    """Helper for @asynccontextmanager."""

    async def __aenter__(self):
        try:
            return await self.gen.__anext__()
        except StopAsyncIteration:
            raise RuntimeError("generator didn't yield") from None

    async def __aexit__(self, typ, value, traceback):
        if typ is None:
            try:
                await self.gen.__anext__()
            except StopAsyncIteration:
                return
            else:
                raise RuntimeError("generator didn't stop")
        else:
            if value is None:
                value = typ()
            # See _GeneratorContextManager.__exit__ for comments on subtleties
            # in this implementation
            try:
                await self.gen.athrow(typ, value, traceback)
                raise RuntimeError("generator didn't stop after athrow()")
            except StopAsyncIteration as exc:
                return exc is not value
            except RuntimeError as exc:
                if exc is value:
                    return False
                # Avoid suppressing if a StopIteration exception
                # was passed to throw() and later wrapped into a RuntimeError
                # (see PEP 479 for sync generators; async generators also
                # have this behavior). But do this only if the exception wrapped
                # by the RuntimeError is actully Stop(Async)Iteration (see
                # issue29692).
                if isinstance(value, (StopIteration, StopAsyncIteration)):
                    if exc.__cause__ is value:
                        return False
                raise
            except BaseException as exc:
                if exc is not value:
                    raise


def asynccontextmanager(func):
    """@asynccontextmanager decorator.
    Typical usage:
        @asynccontextmanager
        async def some_async_generator(<arguments>):
            <setup>
            try:
                yield <value>
            finally:
                <cleanup>
    This makes this:
        async with some_async_generator(<arguments>) as <variable>:
            <body>
    equivalent to this:
        <setup>
        try:
            <variable> = <value>
            <body>
        finally:
            <cleanup>
    """

    @wraps(func)
    def helper(*args, **kwds):
        return _AsyncGeneratorContextManager(func, args, kwds)

    return helper
