[Python-Dev] PEP 550 v4 (original) (raw)

Stefan Krah stefan at bytereef.org
Mon Aug 28 07:19:03 EDT 2017

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.

and this:

async def bar(): # use decimal with context=ctx async def foo(): decimal.setcontext(ctx) await bar() The only thing that will not work, is this (ex1): async def bar(): decimal.setcontext(ctx) async def foo(): await bar() # use decimal with context=ctx

Here we do have dynamic scoping.

Speaking of (ex1), there's an example that didn't work in any PEP 550 version:

def bar(): decimal.setcontext(ctx) yield async def foo(): list(bar()) # use decimal with context=ctx

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.

Stefan Krah

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