[Python-ideas] PEP 550 v2

[Nick Coghlan]

TLDR: I really like this version, and the tweaks I suggest below are
just cosmetic. I figure if there are any major technical traps
lurking, you'll find them as you work through updating the reference
implementation.

On 16 August 2017 at 09:55, Yury Selivanov <yselivanov.ml at gmail.com> wrote:
> Context Item Object
> -------------------
>
> The ``sys.new_context_item(description)`` function creates a
> new ``ContextItem`` object.  The ``description`` parameter is a
> ``str``, explaining the nature of the context key for introspection
> and debugging purposes.
>
> ``ContextItem`` objects have the following methods and attributes:
>
> * ``.description``: read-only description;

It may be worth having separate "name" and "description" attributes,
similar to __name__ and __doc__ being separate on things like
functions. That way, error messages can just show "name", while
debuggers and other introspection tools can include a more detailed
description.

> Coroutine Object Modifications
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>
> To achieve this, a small set of modifications to the coroutine object
> is needed:
>
> * New ``cr_local_context`` attribute.  This attribute is readable
>   and writable for Python code.

For ease of introspection, it's probably worth using a common
`__local_context__` attribute name across all the different types that
support one, and encouraging other object implementations to do the
same.

This isn't like cr_await and gi_yieldfrom, where we wanted to use
different names because they refer to different kinds of objects.

> Acknowledgments
> ===============
[snip]

> Thanks to Nick Coghlan for numerous suggestions and ideas on the
> mailing list, and for coming up with a case that cause the complete
> rewrite of the initial PEP version [19]_.
[snip]

> .. [19] https://mail.python.org/pipermail/python-ideas/2017-August/046780.html

The threading in pipermail makes it difficult to get from your reply
back to my original comment, so it may be better to link directly to
the latter: https://mail.python.org/pipermail/python-ideas/2017-August/046775.html

And to be completely explicit about: I like your proposed approach of
leaving it up to iterator developers to decide whether or not to run
with a local context or not. If they don't manipulate any context
items, it won't matter, and if they do, it's straightforward to add a
suitable call to sys.run_in_local_context().

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia