[Python-ideas] PEP 563 and expensive backwards compatibility

[Jelle Zijlstra]

2017-09-13 13:01 GMT-07:00 Jim J. Jewett <jimjjewett at gmail.com>:

> On Wed, Sep 13, 2017 at 3:12 PM, Lukasz Langa <lukasz at langa.pl> wrote:
> > On Sep 13, 2017, at 2:56 PM, Jim J. Jewett <jimjjewett at gmail.com> wrote:
>
> >> I am generally supportive of leaving the type annotations
> >> unprocessed by default, but there are use cases where
> >> they should be processed (and even cases where doing it
> >> at the right time matters, because of a side effect).
>
> > What is the "right time" you're speaking of?
>
> The "right time" is whenever they are currently evaluated.
> (Definition time, I think, but won't swear.)
>
> For example, the "annotation" might really be a call to a logger,
> showing the current environment, including names that will be rebound
> before the module finishes loading.
>
> I'm perfectly willing to agree that even needing this much control
> over timing is a code smell, but it is currently possible, and I would
> rather it not become impossible.
>

Is this just a theoretical concern? Unless there is significant real-world
code doing this sort of thing, I don't see much of a problem in deprecating
such code using the normal __future__-based deprecation cycle.


>
> At a minimum, it seems like "just run this typing function that you
> should already be using" should either save the right context, or the
> PEP should state explicitly that this functionality is being
> withdrawn.  (And go ahead and suggest a workaround, such as running
> the code before the method definition, or as a decorator.)
>
>
> >> (1)  The PEP suggests opting out with @typing.no_type_hints ...
>
> > This is already possible. PEP 484 specifies that
>
> > "A # type: ignore comment on a line by itself is equivalent to adding an
> > inline # type: ignore to each line until the end of the current indented
> > block. At top indentation level this has effect of disabling type
> checking
> > until the end of file."
>
> Great!  Please mention this as well as (or perhaps instead of)
> typing.no_type_check.
>
>
> >> It would be a bit messy (like the old coding cookie),
> >> but recognizing a module-wide
>
> >> # typing.no_type_hints
>
> >> comment and then falling back to the current behavior
> >> would be enough for me.
>
> > Do you know of any other per-module feature toggle of this kind?
>
> No, thus the comment about it being messy.  But it does offer one way
> to ensure that annotations are evaluated within the proper
> environment, even without having to save those environments.
>
> -jJ
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