[Python-Dev] Please reject or postpone PEP 526

[Steven D'Aprano]

On Fri, Sep 02, 2016 at 10:47:41AM -0700, Steve Dower wrote:
> "I'm not seeing what distinction you think you are making here. What 
> distinction do you see between:
> 
>     x: int = func(value)
> 
> and
> 
>     x = func(value)  #type: int"
> 
> Not sure whether I agree with Mark on this particular point, but the 
> difference I see here is that the first describes what types x may 
> ever contain, while the latter describes what type of being assigned 
> to x right here. So one is a variable annotation while the other is an 
> expression annotation.

Ultimately Python is a dynamically typed language, and that's not 
changing. This means types are fundamentally associated with *values*, 
not *variables* (names). But in practice, you can go a long way by 
pretending that it is the variable that carries the type. That's the 
point of the static type checker: if you see that x holds an int here, 
then assume (unless told differently) that x should always be an int. 
Because in practice, most exceptions to that are due to bugs, or at 
least sloppy code.

Of course, it is up to the type checker to decide how strict it wants to 
be, whether to treat violations as a warning or a error, whether to 
offer the user a flag to set the behaviour, etc. None of this is 
relevant to the PEP. The PEP only specifies the syntax, leaving 
enforcement or non-enforcement to the checker, and it says:

    PEP 484 introduced type hints, a.k.a. type annotations. While its 
    main focus was function annotations, it also introduced the notion
    of type comments to annotate VARIABLES [emphasis added]

not expressions. And:

    This PEP aims at adding syntax to Python for annotating the types
    of variables and attributes, instead of expressing them through
    comments

which to me obviously implies that the two ways (type comment, and 
variable type hint) are intended to be absolutely identical in 
semantics, at least as far as the type-checker is concerned.

(They will have different semantics at runtime: the comment is just a 
comment, while the type hint will set an __annotations__ mapping.)

But perhaps the PEP needs to make it explicit that they are to be 
treated exactly the same.


-- 
Steve