Run a trio example mixing cancellation and exceptions

trio_cancel_and_raise.py

"""
Run a trio example mixing cancellation and exceptions.

Shows that the behavior can be hard to predict since it depends on factors like:
- whether a nursery is involved
- whether a checkpoint is performed after the raise

Produces the following output:

    =========== cancel_and_raise ===========
    > RuntimeError caught inside
    > No exception outside

    ===== cancel_and_raise_in_nursery ======
    > MultiError raised inside
    > RuntimeError caught outside

    == cancel_raise_and_await_in_nursery ===
    > MultiError raised inside
    > No exception outside

    == cancel_raise_in_nursery_and_await ===
    > Cancelled raised inside
    > No exception outside

"""


import trio


async def main(coro):
    try:
        with trio.CancelScope() as cancel_scope:
            try:
                await coro(cancel_scope)
            except RuntimeError:
                print("> RuntimeError caught inside")
                pass
            except trio.MultiError:
                print("> MultiError raised inside")
                raise
            except trio.Cancelled:
                print("> Cancelled raised inside")
                raise
    except RuntimeError:
        print("> RuntimeError caught outside")
    else:
        print("> No exception outside")


async def cancel_and_raise(cancel_scope):
    cancel_scope.cancel()
    raise RuntimeError


async def cancel_and_raise_in_nursery(cancel_scope):
    async with trio.open_nursery():
        cancel_scope.cancel()
        raise RuntimeError


async def cancel_raise_and_await_in_nursery(cancel_scope):
    async with trio.open_nursery():
        try:
            cancel_scope.cancel()
            raise RuntimeError
        finally:
            await trio.sleep(0)


async def cancel_raise_in_nursery_and_await(cancel_scope):
    try:
        async with trio.open_nursery():
            cancel_scope.cancel()
            raise RuntimeError
    finally:
        await trio.sleep(0)


coros = [
    cancel_and_raise,
    cancel_and_raise_in_nursery,
    cancel_raise_and_await_in_nursery,
    cancel_raise_in_nursery_and_await]
for coro in coros:
    print(f" {coro.__name__} ".center(40, "="))
    trio.run(main, coro)
    print()