[Python-ideas] Trial balloon: adding variable type declarations in support of PEP 484

[Guido van Rossum]

Yeah I've come around to a similar position. I had ClassVar[] but your
ClassAttr[] is perhaps better.

On Saturday, August 6, 2016, Nick Coghlan <ncoghlan at gmail.com> wrote:

> On 6 August 2016 at 02:12, Guido van Rossum <guido at python.org
> <javascript:;>> wrote:
> > On Fri, Aug 5, 2016 at 12:40 AM, Nick Coghlan <ncoghlan at gmail.com
> <javascript:;>> wrote:
> >> That is:
> >>
> >>     class Starship:
> >>         stats class: Dict[str, int] = {}  # Pure class variable
> >>         damage class: int = 0  # Hybrid class/instance variable
> >>         captain: str  # Pure instance variable
> >>
> >> Pronounced as:
> >>
> >>     "stats is declared on the class as a dict mapping from strings to
> >> integers and is initialised as an empty dict"
> >>     "damage is declared on the class as an integer and is initialised
> as zero"
> >>     "captain is declared on instances as an integer"
> >>
> >> Just a minor thing, but the closer association with the name reads
> >> better to me since "Class attribute or instance attribute?" is really
> >> a property of the name binding, rather than of the permitted types
> >> that can be bound to that name
> >
> > Hmm... But the type is *also* a property of the name binding. And I
> > think the "class-var-ness" needs to be preserved in the
> > __annotations__ dict somehow, so that's another reason why it
> > "belongs" to the type rather than to the name binding (a nebulous
> > concept to begin with).
> >
> > Also, I like the idea that everything between the ':' and the '=' (or
> > the end of the line) belongs to the type checker. I expect that'll be
> > easier for people who aren't interested in the type checker.
>
> Fair point, although this and the __var_annotations__ discussion also
> raises the point that annotations have to date always been valid
> Python expressions. So perhaps rather than using the class keyword, it
> would make sense to riff off classmethod and generic types to propose:
>
>     class Starship:
>         stats: ClassAttr[Dict[str, int]] = {}  # Pure class variable
>         damage: DefaultAttr[int] = 0  # Hybrid class/instance variable
>         captain: str  # Pure instance variable
>
> Pronounced as:
>
>     "stats is a class attribute mapping from strings to integers and
> is initialised as an empty dict"
>     "damage is an instance attribute declared as an integer with a
> default value of zero defined on the class"
>     "captain is an instance attribute declared as a string"
>
> In addition to clearly distinguishing class-only attributes from
> class-attribute-as-default-value, this would also mean that the
> specific meaning of those annotations could be documented in
> typing.ClassAttr and typing.DefaultAttr, rather than needing to be
> covered in the documentation of the type annotation syntax itself.
>
> Cheers,
> Nick.
>
> --
> Nick Coghlan   |   ncoghlan at gmail.com <javascript:;>   |   Brisbane,
> Australia
>


-- 
--Guido (mobile)
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