Before this patch the context manager's __aexit__() method would not be
executed if a return/break/continue statement was used to exit an async
with block. async with now has the same semantics as normal with.
The fix here applies purely to the compiler, and does not modify the
runtime at all. It might (eventually) be better to define new bytecode(s)
to handle async with (and maybe other async constructs) in a cleaner, more
efficient way.
One minor drawback with addressing this issue purely in the compiler is
that it wasn't possible to get 100% CPython semantics. The thing that is
different here to CPython is that the __aexit__ method is not looked up in
the context manager until it is needed, which is after the body of the
async with statement has executed. So if a context manager doesn't have
__aexit__ then CPython raises an exception before the async with is
executed, whereas uPy will raise it after it is executed. Note that
__aenter__ is looked up at the beginning in uPy because it needs to be
called straightaway, so if the context manager isn't a context manager then
it'll still raise an exception at the same location as CPython. The only
difference is if the context manager has the __aenter__ method but not the
__aexit__ method, then in that case uPy has different behaviour. But this
is a very minor, and acceptable, difference.
Damien George