[Python-ideas] PEP 550 v2

[Jelle Zijlstra]

2017-08-16 10:37 GMT+02:00 Nathaniel Smith <njs at pobox.com>:

> On Tue, Aug 15, 2017 at 11:53 PM, Jelle Zijlstra
> <jelle.zijlstra at gmail.com> wrote:
> > Minor suggestion: Could we allow something like
> > `sys.set_new_context_item(description='mylib.context',
> > initial_value='spam')`? That would make it easier for type checkers to
> infer
> > the type of a ContextItem, and it would save a line of code in the common
> > case.
>
> This is a really handy feature in general, actually! In fact all of
> asyncio's thread-locals define initial values (using a trick involving
> subclassing threading.local), and I recently added this feature to
> trio.TaskLocal as well just because it's so convenient.
>
> However, something that you realize almost immediately when trying to
> use this is that in many cases, what you actually want is an initial
> value *factory*. Like, if you write new_context_item(initial_value=[])
> then you're going to have a bad time. So, should we support something
> like new_context_item(initializer=lambda: [])?
>
> The semantics are a little bit subtle. I guess it would be something
> like: if ci.get() goes to find the value and fails at all levels, then
> we call the factory function and assign its return value to the
> *deepest* LC, EC[0]. The idea being that we're pretending that the
> value was there all along in the outermost scope, you just didn't
> notice before now.
>
> > With this modification, the type of new_context_item would be
> >
> > @overload
> > def new_context_item(*, description: str, initial_value: T) ->
> > ContextItem[T]: ...
> > @overload
> > def new_context_item(*, description: str) -> ContextItem[Any]: ...
> >
> > If we only allow the second variant, type checkers would need some sort
> of
> > special casing to figure out that after .set(), .get() will return the
> same
> > type.
>
> I'm not super familiar with PEP 484.
>
> Would using a factory function instead of an initial value break this
> type inference?
>
> If you want to automatically infer that whatever type I use to
> initialize the value is the only type it can ever have, is there a way
> for users to easily override that? Like could I write something like
>
> my_ci: ContextItem[int, str] = new_context_item(initial_value=0)
>
> It would be `ContextItem[Union[int, str]]`, but yes, that should work.


> ?
>
> -n
>
> --
> Nathaniel J. Smith -- https://vorpus.org
>
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