PEP 526 - var annotations and the spirit of python

Ian Kelly ian.g.kelly at gmail.com
Wed Jul 4 15:26:03 EDT 2018


On Wed, Jul 4, 2018, 9:36 AM Steven D'Aprano <
steve+comp.lang.python at pearwood.info> wrote:

> On Wed, 04 Jul 2018 13:48:26 +0100, Bart wrote:
> >> A better example would be:
> >>
> >>      x: int = None
> >>
> >> which ought to be read as "x is an int, or None, and it's currently
> >> None".
> >
> > In that case the type hint is lying.
>
> "Practicality beats purity."
>
> "This type, or None" is such a common pattern that any half-way decent
> type checker ought to be able to recognise it. You can, of course,
> explicitly annotate it:
>
>     x: Optional[int] = None
>
> but the type checker should infer that if you assign None to a variable
> which is declared int, you must have meant Optional[int] rather than just
> int. If it doesn't, get a better type checker.
>
> Note that None is a special case (because sometimes special cases *are*
> special enough to break the rules).


I don't think this case is special enough. As a person coming along later
and trying to read the code, I should be able to trust that the type hint
means what it says. I should not have to go look up the rules of the linter
to infer that in this case, type A actually means type B.

Optimize for readability, not writability.


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