I thank all of you for your answers. And sorry for taking a bit long to reply!
After many trys and errors, here is something that works for me:
```
from sage.structure.sage_object import SageObject
from sage.structure.unique_representation import CachedRepresentation
class MyInteger(CachedRepresentation, SageObject):
@staticmethod
def __classcall_private__(cls, n):
if n % 2 == 0:
return MyInteger_even(n)
else:
return cls.__classcall__(cls, n)
def __init__(self, n):
self.n = n
class MyInteger_even(MyInteger):
def __init__(self, n):
self.n = n
self.half = n // 2
```
However, this has a major drawback, as all instances that have same data are
references to one another. And I don't find any way to bypass that:
```
sage: from copy import deepcopy
sage: s1 = MyInteger(2)
sage: s2 = MyInteger(2)
sage: s3 = deepcopy(s2)
sage: s1 == s2
True
sage: s1 is s2
True
sage: s1 == s3
True
sage: s1 is s3
True
sage: s1.n
2
sage: s3.n = 5
sage: s1.n
5
sage:
```
I appreciate that this may not be a problem for classes representing sets and
categories (like `EuclideanSpace`). But for classes representing children
elements, this may cause problems. And this is the case for me, as I need this
for `FiniteDrinfeldModule` and `FiniteDrinfeldModule_rank_two`
(https://trac.sagemath.org/ticket/33713).
Is there a way to avoid this?
Kindest regards,
Antoine
On Sat, 2022-05-14 at 21:07 -0700, Nils Bruin wrote:
> It's probably worth pointing out that __classcall_private__ is not a standard
> python facility. It looks like you need
>
> class Foo(metaclass=ClasscallMetaclass):
>
> to make it work on Foo.
>
> On Saturday, 14 May 2022 at 20:21:48 UTC-7 Travis Scrimshaw wrote:
> > For this you want to use __classcall_private__ as otherwise you would likely
> > end up in an infinite loop when you try to construct the subclass. There are
> > lots of examples of this in the Sage library code.
> >
> > Best,
> > Travis
> >
> >
> > On Sunday, May 15, 2022 at 5:42:49 AM UTC+9 Eric Gourgoulhon wrote:
> > > Hi,
> > >
> > > Le samedi 14 mai 2022 à 00:07:02 UTC+2, David Roe a écrit :
> > > > I think the following should work:
> > > >
> > > > class MyObject:
> > > > def __classcall__(cls, arg):
> > > > if isinstance(arg, special):
> > > > return typecall(MyObject_specific_case, arg)
> > > > else:
> > > > return typecall(MyObject, arg)
> > > >
> > > > plus the same __init__ you had before. I haven't checked it though....
> > > > David
> > > >
> > > >
> > >
> > >
> > > An alternative is to use __classcall_private__
> > > For an example, see the class EuclideanSpace in
> > > src/sage/manifolds/differentiable/examples/euclidean.py
> > >
> > > EuclideanSpace(n) actually returns an instance of the subclass
> > > EuclideanPlane if n = 2 or of the subclass Euclidean3dimSpace if n = 3.
> > >
> > > Best wishes,
> > >
> > > Eric.
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