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Fri Sep 2 14:37:30 EDT 2016 · https://mail.python.org/pipermail/python-dev/2016-September/146079.html

[Replying to  Steve Dower]
On Sat, Sep 03, 2016 at 04:19:13AM +1000, Steven D'Aprano wrote:
> 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.

I see it differently, but I'm quite used to OCaml:

# let f () =
    let x : int = 10 in
    let x : float = 320.0 in
      x;;
Warning 26: unused variable x.
val f : unit -> float = <fun>
# f();;
- : float = 320.

Like in Python, in OCaml variables can be rebound and indeed have different
types with different explicit type constraints.

Expressions can also be annotated, but require parentheses (silly example):

# let x = (10 * 20 : int);;
val x : int = 200

So I'm quite happy with the proposed syntax in the PEP, perhaps the
parenthesized expression annotations could also be added.  But these
are only very rarely needed.

Stefan Krah

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[Python-Dev] Please reject or postpone PEP 526