If you do:

async with trio.open_nursery() as n:
    async with my_special_scope():
        n.start_soon(some_task)
    await something_else()

then it is actively unhelpful for some_task to see the new value, because that would make it try to communicate with a task that it will outlive

I want a try/except specifically to detect an error during entry into an async context manager. Due to syntactic nesting, I don't see how to do this with the usual async with syntax.

try:
    async with my_cm():
        raise SomeError  # I don't want an error within the block like this to count for this try/except, but it does.
except ...:
    ...

The obvious solution is to use an AsyncExitStack:

async with AsyncExitStack() as stack:
    try:
        await stack.enter_async_context(my_cm())
    except ...:
        ...
   raise SomeError  # There, now this is outside the try/except.

I know Trio goes to some great pains to prevent misnesting of CMs. I just wanted to double check that this try/except approach, which is similarly not possible with the normal nesting syntax, doesn't similarly violate any rules.

entered_cm = False
try:
    async with my_cm():
        entered_cm = True
        await foo()
except SomeError as e:
    if not entered_cm:
        raise
    # ...

In reply to@mikenerone:matrix.org

I want a try/except specifically to detect an error during entry into an async context manager. Due to syntactic nesting, I don't see how to do this with the usual async with syntax.

try:
    async with my_cm():
        raise SomeError  # I don't want an error within the block like this to count for this try/except, but it does.
except ...:
    ...

The obvious solution is to use an AsyncExitStack:

async with AsyncExitStack() as stack:
    try:
        await stack.enter_async_context(my_cm())
    except ...:
        ...
   raise SomeError  # There, now this is outside the try/except.

I know Trio goes to some great pains to prevent misnesting of CMs. I just wanted to double check that this try/except approach, which is similarly not possible with the normal nesting syntax, doesn't similarly violate any rules.

entered_cm = False
try:
    async with my_cm():
        entered_cm = True
        await foo()
except SomeError as e:
    if not entered_cm:
        raise
    # ...

In reply to@arthur-tacca:matrix.org
It's equivalent to this scope-respecting snippet (which you could equally use instead) so it doesn't violate any nesting rules. The reason it's equivalent boils down to what @TeamSpen210 (Spencer Brown) said: if the call to my_cm() or cm.__enter__() fails then .__exit__() does not get called regardless of whether using async with directly or AsyncExitStack like your snippet.

entered_cm = False
try:
    async with my_cm():
        entered_cm = True
        await foo()
except SomeError as e:
    if not entered_cm:
        raise
    # ...

In reply to@mikenerone:matrix.org
At least it's easy to work around (either of the above approaches, or by wrapping in a custom CM of your own (and not even that if the target CM is under your control)), but it feels like it should be able to be done syntactically.