[Python-Dev] Do PEP 526 type declarations define the types of variables or not?

[Ryan Gonzalez]

Maybe the PEP should just say it's for "annotating variables", and it would
mention "primarily for the purpose of types"?

--
Ryan
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On Sep 5, 2016 10:27 AM, "Mark Shannon" <mark at hotpy.org> wrote:

> Hi,
>
> PEP 526 states that "This PEP aims at adding syntax to Python for
> annotating the types of variables" and Guido seems quite insistent that the
> declarations are for the types of variables.
>
> However, I get the impression that most (all) of the authors and
> proponents of PEP 526 are quite keen to emphasise that the PEP in no way
> limits type checkers from doing what they want.
>
> This is rather contradictory. The behaviour of a typechecker is defined by
> the typesystem that it implements. Whether a type annotation determines the
> type of a variable or an expression alters changes what typesystems are
> feasible. So, stating that annotations define the type of variables *does*
> limit what a typechecker can or cannot do.
>
> Unless of course, others may have a different idea of what the "type of a
> variable" means.
> To me, it means it means that for all assignments `var = expr`
> the type of `expr` must be a subtype of the variable,
> and for all uses of var, the type of the use is the same as the type of
> the variable.
>
> In this example:
>
>     def bar()->Optional[int]: ...
>
>     def foo()->int:
>         x:Optional[int] = bar()
>         if x is None:
>             return -1
>         return x
>
> According to PEP 526 the annotation `x:Optional[int]`
> means that the *variable* `x` has the type `Optional[int]`.
> So what is the type of `x` in `return x`?
> If it is `Optional[int]`, then a type checker is obliged to reject this
> code. If it is `int` then what does "type of a variable" actually mean,
> and why aren't the other uses of `x` int as well?
>
> Cheers,
> Mark.
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