On Mon, Aug 28, 2017 at 1:33 PM, Stefan Krah <[email protected]> wrote:
[..]
>> * Context managers like decimal contexts, numpy.errstate, and
>> warnings.catch_warnings.
>
> The decimal context works like this:
>
> 1) There is a default context template (user settable).
>
> 2) Whenever the first operation *in a new thread* occurs, the
> thread-local context is initialized with a copy of the
> template.
>
>
> I don't find it very intuitive if setcontext() is somewhat local in
> coroutines but they don't participate in some form of CLS.
>
> You have to think about things like "what happens in a fresh thread
> when a coroutine calls setcontext() before any other decimal operation
> has taken place".
I'm sorry, I don't follow you here.
PEP 550 semantics:
setcontext() in a regular code would set the context for the whole thread.
setcontext() in a coroutine/generator/async generator would set the
context for all the code it calls.
> So perhaps Nathaniel is right that the PEP is not so useful for numpy
> and decimal backwards compat.
Nathaniel's argument is pretty weak as I see it. He argues that some
people would take the following code:
def bar():
# set decimal context
def foo():
bar()
# use the decimal context set in bar()
and blindly convert it to async/await:
async def bar():
# set decimal context
async def foo():
await bar()
# use the decimal context set in bar()
And that it's a problem that it will stop working.
But almost nobody converts the code by simply slapping async/await on
top of it -- things don't work this way. It was never a goal for
async/await or asyncio, or even trio/curio. Porting code to
async/await almost always requires a thoughtful rewrite.
In async/await, the above code is an *anti-pattern*. It's super
fragile and can break by adding a timeout around "await bar". There's
no workaround here.
Asynchronous code is fundamentally non-local and a more complex topic
on its own, with its own concepts: Asynchronous Tasks, timeouts,
cancellation, etc. Fundamentally: "(synchronous code) !=
(asynchronous code) - (async/await)".
Yury
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