[Python-ideas] Generics Syntax

[אלעזר]

Yes the "class A[T]:" syntax requires on the ability to express variance as
an operator, but not the other way around.

It might be an argument in favor of switching to the + syntax: to make
possible future syntax change in class definition somewhat easier to
swallow.

~Elazar

On Thu, Sep 15, 2016 at 2:03 PM Nick Coghlan <ncoghlan at gmail.com> wrote:

> On 15 September 2016 at 19:53, Ivan Levkivskyi <levkivskyi at gmail.com>
> wrote:
> >
> >
> > On 15 September 2016 at 11:46, אלעזר <elazarg at gmail.com> wrote:
> >>
> >> And that thread is only about variance. What about the generic syntax?
> >
> >
> >  If you mean code like this:
> >
> >     class Container[+T]:
> >         @abstractmethod
> >         def __contains__(self, x: T) -> bool: ...
> >
> > then there is little chance that this will be accepted because it
> requires
> > changes to Python syntax.
>
> If the proposed spelling is tweaked to be "class
> Container(Generic[+T]):", then it doesn't require a syntax change, as
> that's merely a matter of implementing unary plus on type vars:
>
>     >>> +object()
>     Traceback (most recent call last):
>      File "<stdin>", line 1, in <module>
>     TypeError: bad operand type for unary +: 'object'
>     >>> class UnaryPlus:
>     ...     def __pos__(self):
>     ...         return self
>     ...
>     >>> +UnaryPlus()
>     <__main__.UnaryPlus object at 0x7f5e0fe91c50>
>
> (I have no opinion on the value of providing a simpler spelling for
> covariance, I'm just noting that if you keep the "Generic[T]" aspect
> of the current spelling it wouldn't require any changes to Python's
> syntax and will work as far back as you care to support it)
>
> Cheers,
> Nick.
>
> --
> Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
>
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