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Sun Sep 4 07:30:18 EDT 2016 · https://mail.python.org/pipermail/python-dev/2016-September/146163.html

On 04/09/16 11:57, Chris Angelico wrote:
> On Sun, Sep 4, 2016 at 8:52 PM, 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.
>
> So what you have is actually a change in *implication* (since the PEP
> doesn't stipulate semantics); and the old way (the comment) implies
> something contrary to the semantics of at least one of the type
> checkers that uses it (MyPy).
Do we really want to make a major, irrevocable change to the language 
just because MyPy does something? MyPy is very far from complete (it 
doesn't even support Optional types yet).

Are there any current type checkers that
> actually do associate that with the value? That is, to have "variable
> = func() # str" indicate the same type check as "def func() -> str"?
> If not, this is a strong argument in favour of the PEP, since it would
> synchronize the syntax with the current best-of-breed checkers.
I believe pytype uses value, rather the variable, tracking. It is thus 
more precise. Of course, it is more of an inferencer than a checker.
We (semmle.com) do precise value tracking to infer a lot of "type" 
errors in unannotated code (as the vast majority of Python code is).

It would be a real shame if PEP 526 mandates against checkers doing as 
good as job as possible. Forcing all uses of a variable to have the same 
type is a major and, IMO crippling, limitation.

E.g.
def foo(x:Optional[int])->int:
     if x is None:
        return -1
     return x + 1

If the type of the *variable* 'x' is Optional[int] then 'return x + 1' 
doesn't type check. If the type of the *parameter* 'x' is Optional[int] 
then a checker can readily verify the above code.

I want a checker to check my code and, with minimal annotations, give me 
confidence that my code is correct.

Cheers,
Mark.

>
> ChrisA
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[Python-Dev] Please reject or postpone PEP 526