[Python-Dev] PEP 550 v4

[Yury Selivanov]

On Mon, Aug 28, 2017 at 7:19 AM, Stefan Krah <stefan at bytereef.org> wrote:
> On Sun, Aug 27, 2017 at 11:19:20AM -0400, Yury Selivanov wrote:
>> On Sun, Aug 27, 2017 at 6:08 AM, Stefan Krah <stefan at bytereef.org> wrote:
>> > On Sat, Aug 26, 2017 at 04:13:24PM -0700, Nathaniel Smith wrote:
>> >> It's perfectly reasonable to have a script where you call
>> >> decimal.setcontext or np.seterr somewhere at the top to set the
>> >> defaults for the rest of the script.
>> >
>> > +100.  The only thing that makes sense for decimal is to change localcontext()
>> > to be automatically async-safe while preserving the rest of the semantics.
>>
>> TBH Nathaniel's argument isn't entirely correct.
>>
>> With the semantics defined in PEP 550 v4, you still can set decimal
>> context on top of your file, in your async functions etc.
>>
>> and this:
>>
>>     def bar():
>>         decimal.setcontext(ctx)
>>
>>     def foo():
>>          bar()
>>          # use decimal with context=ctx
>
> Okay, so if I understand this correctly we actually will not have dynamic
> scoping for regular functions:  bar() has returned, so the new context
> would not be found on the stack with proper dynamic scoping.

Correct. Although I would avoid associating PEP 550 with dynamic
scoping entirely, as we never intended to implement it.

[..]
> What about this?
>
> async def bar():
>     setcontext(Context(prec=1))
>     for i in range(10):
>         await asyncio.sleep(1)
>         yield i
>
> async def foo():
>     async for i in bar():
>         # ctx.prec=1?
>         print(Decimal(100) / 3)
>
>
>
> I'm searching for some abstract model to reason about the scopes.

Whatever is set in coroutines, generators, and async generators does
not leak out.  In the above example, "prec=1" will only be set inside
"bar()", and "foo()" will not see that.  (Same will happen for a
regular function and a generator).

Yury