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

[Mark Shannon]

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.