Hi Jeroen,

On 2016-10-10, Jeroen Demeyer <jdeme...@cage.ugent.be> wrote:
> Maybe I'm misunderstanding you, but what I wanted to say is: Python 3 
> doesn't have any place to hook a custom metaclass.

Is there a way to have a metaclass similar to our ClasscallMetaclass at
all, in Python3?? I just tested, having

class Classcall(type):
    def __call__(cls, *args, **opts):
            classcall = cls.__classcall__
        except AttributeError:
            return type.__call__(cls, *args, **opts)
        return classcall(cls, *args, **opts)

class MyUniqueRepresentation:
    __metaclass__ = Classcall
    def __classcall__(cls, *args, **opts):
        out = super(cls,cls).__new__(cls, *args, **opts)
        print("classcall got",type(out), cls)
        out._reduction = (type(out), args, opts)
        return out

Loading the above into python2, I get
>>> class Foo(MyUniqueRepresentation):
...     def __init__(self, a,b):
...         self.a = a
...         self.b = b
>>> A = Foo("A","B")
('classcall got', <class '__main__.Foo'>, <class '__main__.Foo'>)
>>> A._reduction
(<class '__main__.Foo'>, ('A', 'B'), {})

But doing the same in python3, it gives
>>> class Foo(MyUniqueRepresentation):
...     def __init__(self, a,b):
...         self.a = a
...         self.b = b
>>> A = Foo("A","B")
>>> A._reduction
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'Foo' object has no attribute '_reduction'

So, in python2, the __classcall__ is called, in python3 it isn't. What
needs to be done to make the above example work in python3?

It makes me wonder: IIRC there are people trying to build SageMath with
python3 (that's why I put sage-devel on Cc). Does UniqueRepresentation
actually work with python3, when the metaclass apparently is not called
during creation of a class?

Best regards,

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