[Python-ideas] PEP 563 and expensive backwards compatibility

[Jim J. Jewett]

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.

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withdrawn.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