[Python-Dev] Please reject or postpone PEP 526

[Ivan Levkivskyi]

On 4 September 2016 at 12:52, Mark Shannon <mark at hotpy.org> wrote:

> The key difference is in placement.
> PEP 484 style
> variable = value # annotation
>
> Which reads to me as if the annotation refers to the value.
> PEP 526
> variable: annotation = value
>
> Which reads very much as if the annotation refers to the variable.
> That is a change in terms of semantics and a change for the worse, in
> terms of expressibility.


I still think it is better to leave the decision to type checkers.
The proposed syntax allows two forms:

variable: annotation = value

and

variable: annotation

The first form still could be interpreted by type checkers
as annotation for value (a cast to more precise type):

variable = cast(annotation, value) # visually also looks similar

and PEP says that annotations "are intended to help with type inference in
complex cases".
Such interpretation could be useful for function local variables
(the implementation is also optimised for such use case).
While the second form (without value)
indeed looks like annotation of variable, and is equivalent to:

__annotations__['variable'] = annotation

This form could be useful for annotating instance/class variables.
Or just for documentation (I really like this).

In addition, expression annotations are allowed

expression: annotation [= value]

This form is not used by 3rd party tools (as far as I know) but one of the
possible use cases
could be "check-points" for values:

somedict[somefunc(somevar)]: annotation # type checker could flag this if
something went wrong.

Finally, I would like to reiterate, both interpretations (annotating value
vs annotating variable)
are possible and we (OK at least me, but it looks like Guido also agree)
don't want to force 3rd party tools to use only one of those.

--
Ivan
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