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Mon Sep 5 18:49:45 EDT 2016 · https://mail.python.org/pipermail/python-dev/2016-September/146235.html

Didn't Koos say this works more like an expression annotation?

IMO, the type of the expression is what is specified but the type of the 
variable can change over time (as you demonstrated).

Sven

PS: thinking this way, the new syntax is actually confusing as it 
annotates the variable not the expression. :-/

On 05.09.2016 17:26, Mark Shannon 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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[Python-Dev] Do PEP 526 type declarations define the types of variables or not?