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

[Joao S. O. Bueno]

> The other combination that feels a bit confusing when adding "class" tags is
> saying "attr: class T", without an initializer.... in what case would
> someone do that? what does it mean if I find some code saying that about the
> class, that it might get that attribute set somewhere else?

Actually, for the  __init_subclass__ class initializer (PEP 487) ,
discussed in depth on a recently active thread on Python-dev, such
attributes would make sense

On 4 August 2016 at 17:32, Guido van Rossum <guido at python.org> wrote:
> On Thu, Aug 4, 2016 at 12:11 PM, Daniel Moisset <dmoisset at machinalis.com> wrote:
> [...]
>> I follow the example perfectly. Now suppose a reader finds the following
>> piece of code:
>>
>> class Starship:
>>     stats: class Dict[str, int] = {}  # Pure class variable
>>     damage: class int = 0  # Hybrid class/instance variable
>>     captain: str  # Pure instance variable
>>     speed: float = 0
>>
>> I added a new attribute (similar to b in your original example). Given that
>> the type declaration doesn't say "class",the reader might be inclined to
>> think it's an instance variable. But in runtime (if I got you right), that
>> variable will be stored in "Starship.__dict__" and writing "Starship.speed =
>> 3" will change the speed of those starship instances that still haven't set
>> the attribute. So in the end both "damage" and "speed" have "class variable"
>> runtime semantics, even when one is flagged as "class" and the other isn't.
>
> We may have to debate more what the checker should allow here. (Since
> it's okay for a type checker to disallow things that might actually
> work at runtime.) I'm inclined to allow it, as long as the value
> assigned to Starship.speed is compatible with float.
>
>> The other combination that feels a bit confusing when adding "class" tags is
>> saying "attr: class T", without an initializer.... in what case would
>> someone do that? what does it mean if I find some code saying that about the
>> class, that it might get that attribute set somewhere else?
>
> Agreed that looks silly. We probably should make that a syntax error.
>
> --
> --Guido van Rossum (python.org/~guido)
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