On Thu, Aug 06, 2020 at 11:22:38AM +1000, Chris Angelico wrote:
> On Thu, Aug 6, 2020 at 11:11 AM Steven D'Aprano <st...@pearwood.info> wrote:
> > [Dominik Vilsmeier]:
> > > > That should be possible by doing `fred = my_property(42)` and defining
> > > > `__set_name__` on the `my_property` class.
> >
> > Just because you define your own dunder method (which you shouldn't do,
> > since dunders are reserved for the interpreter's use) doesn't make
> > something which is a syntax error stop being a syntax error.
> >
>
> This isn't "defining your own dunder". The syntax as described already
> works inside a class:
>
> class my_property:
> def __init__(self, n):
> self.n = n
> def __set_name__(self, cls, name):
> print("I'm a property %r on class %s" % (name, cls.__name__))
>
> class X:
> fred = my_property(42)
>
> I'm a property 'fred' on class X
*blinks*
When did this happen?
I'm on Python-Ideas, Python-Dev, and I get announcements of new issues
on the bug tracker, and I don't recall ever seeing this feature
discussed.
[looks up the docs]
Okay, apparently it was added in 3.6. But the documentation says:
"""
When using the default metaclass type, or any metaclass that ultimately
calls type.__new__, the following additional customisation steps are
invoked after creating the class object:
first, type.__new__ collects all of the descriptors in the class
namespace that define a __set_name__() method;
"""
https://docs.python.org/3/reference/datamodel.html#class-object-creation
but that's not what is happening here, since my_property is not a
descriptor, it's just an arbitrary instance.
(To be a descriptor, it needs to have `__get__` and/or `__set__`
methods.)
Have I missed something or does this need a documentation fix?
--
Steven
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