2017-08-16 10:37 GMT+02:00 Nathaniel Smith <n...@pobox.com>: > On Tue, Aug 15, 2017 at 11:53 PM, Jelle Zijlstra > <jelle.zijls...@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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