[Python-Dev] PEP 550 v4

Ethan Furman ethan at stoneleaf.us
Thu Sep 7 10:06:14 EDT 2017

On 09/07/2017 06:41 AM, Elvis Pranskevichus wrote:
> On Thursday, September 7, 2017 9:05:58 AM EDT Ethan Furman wrote:

>> The disagreement seems to be whether a LogicalContext should be
>> created implicitly vs explicitly (or opt-out vs opt-in). As a user
>> trying to track down a decimal context change not propagating, I
>> would not suspect the above code of automatically creating a
>> LogicalContext and isolating the change, whereas Greg's context
>> manager version is abundantly clear.
>> The implicit vs explicit argument comes down, I think, to resource
>> management: some resources in Python are automatically managed
>> (memory), and some are not (files) -- which type should LCs be?
> You are confusing resource management with the isolation mechanism.  PEP
> 550 contextvars are analogous to threading.local(), which the PEP makes
> very clear from the outset.

I might be, and I wouldn't be surprised.  :)  On the other hand, one can look at isolation as being a resource.

> threading.local(), the isolation mechanism, is *implicit*.

I don't think so.  You don't get threading.local() unless you call it -- that makes it explicit.

> decimal.localcontext() is an  *explicit* resource manager that relies on
> threading.local() magic.  PEP 550 simply provides a threading.local()
> alternative that works in tasks and generators.  That's it!

The concern is *how* PEP 550 provides it:

- explicitly, like threading.local(): has to be set up manually,
   preferably with a context manager

- implicitly: it just happens under certain conditions


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