[Python-Dev] Please reject or postpone PEP 526

[Steven D'Aprano]

On Fri, Sep 02, 2016 at 08:10:24PM +0300, Koos Zevenhoven wrote:

> A good checker should be able to infer that x is a union type at the
> point that it's passed to spam, even without the type annotation. For
> example:
> 
> def eggs(cond:bool):
>     if cond:
>         x = 1
>     else:
>         x = 1.5
>     spam(x)   # a good type checker infers that x is of type Union[int, float]

Oh I really hope not. I wouldn't call that a *good* type checker. I 
would call that a type checker that is overly permissive.

Maybe you think that it's okay because ints and floats are somewhat 
compatible. But suppose I wrote:

    if cond:
        x = HTTPServer(*args)
    else:
        x = 1.5

Would you want the checker to infer Union[HTTPServer, float]? I 
wouldn't. I would want the checker to complain that the two branches of 
the `if` result in different types for x. If I really mean it, then I 
can give a type-hint.

In any case, this PEP isn't about specifying when to declare variable 
types, it is for picking syntax. Do you have a better idea for variable 
syntax?



-- 
Steve