trivial __copy__ and __deepcopy__ methods for Family and Set

[mkoeppe]

The constructors Family and Set are intended to construct an immutable Parent.

We equip the implementation classes with trivial __copy__ and __deepcopy__ methods that just return self.

Parents that subclass CachedRepresentation/UniqueRepresentation already have these methods, but the implementation classes of Family and Set do not.

Slightly tricky is the question how to deal with mutable elements. Set documents that:

    Sets with unhashable objects work, but with less functionality::

        sage: B = Set([QQ, [3, 1], 5])  # unhashable

Should a copy be made? Should it be documented that by creating the Set, a mutable element belongs to the set object in the sense of object ownership and it is an error to mutate it?

CC: @tscrim @nbruin

Component: refactoring

Issue created by migration from https://trac.sagemath.org/ticket/32462

[tscrim]

comment:1

Probably the easiest way to do this would be to have a finite, e.g., Set try to make a copy of each member. If it returns the same object each time, then return self. For an infinite Family, we are safe as members are created on demand each time

[mkoeppe]

comment:2

What you are describing is deepcopy semantics and is what I implemented for __deepcopy__ of immutable vectors and sequences in the branch of #13811

[tscrim]

comment:3

Ah, right. I think hashability would be a good guide for this. If hashable, then return self otherwise play it safe and return a copy.