On Mon, Aug 28, 2017 at 6:07 PM, Nathaniel Smith <njs@pobox.com> wrote:
The important difference between generators/coroutines and normal
function calls is that with normal function calls, the link between
the caller and callee is fixed for the entire lifetime of the inner
frame, so there's no way for the context to shift under your feet. If
all we had were normal function calls, then (green-) thread locals
using the save/restore trick would be enough to handle all the use
cases above -- it's only for generators/coroutines where the
save/restore trick breaks down. This means that pushing/popping LCs
when crossing into/out of a generator frame is the minimum needed to
get the desired semantics, and it keeps the LC stack small (important
since lookups can be O(n) in the worst case), and it minimizes the
backcompat breakage for operations like decimal.setcontext() where
people *do* expect to call it in a subroutine and have the effects be
visible in the caller.

I like this way of looking at things. Does this have any bearing on asyncio.Task? To me those look more like threads than like generators. Or possibly they should inherit the lookup chain from the point when the Task was created, but not be affected at all by the lookup chain in place when they are executed. FWIW we *could* have a policy that OS threads also inherit the lookup chain from their creator, but I doubt that's going to fly with backwards compatibility.

I guess my general (hurried, sorry) view is that we're at a good point where we have a small number of mechanisms but are still debating policies on how those mechanisms should be used. (The basic mechanism is chained lookup and the policies are about how the chains are fit together for various language/library constructs.)

--
--Guido van Rossum (python.org/~guido)