[Python-Dev] PEP 550 v4
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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