On Monday, May 28, 2018 at 7:24:31 AM UTC-7, Simon King wrote:
>
> However, I find your notion of immutability ("do not allow assigning
> attributes and make all existing attributes immutable") far too narrow.
> IIRC, immutable means that once the object is created, its equivalence
> class with respect to the "==" operator will be preserved no matter what
> of its (no-underscore?) methods are called.
>
> Thus, I believe it must be allowed to assign to an attribute of object X,
> as long as the behaviour of X==Y remains unchanged for all objects Y
> (and of course hash(X) doesn't change either).
>
That's a definition of "immutable" that suffices for using things as keys
in dictionaries. However, if the object you created might be referenced
elsewhere, in totally unrelated code (because that code happens to have
called PolynomialRing(Rationals(),names=['x','y']) as well) then you need
to be a *lot* more careful. I'm not sure that locking some objects
completely from mutation would be a feasible solution (it will be nearly
impossible to enforce in python, and we might not be able to get proper
performance with it), but if programmers don't take this very seriously
we'll end up with a code base that is very difficult to reason about.
The conflation of the two notions of immutability, and the different levels
of enforcement required for them, is what worries me about
UniqueRepresentation. Originally, the use of UniqueRepresentation was
required to ensure the system could *repeatedly* produce identical objects
automatically, based on input parameters, see
https://trac.sagemath.org/ticket/25388#comment:10 . It has the side-effect
of allowing quick equality-by-identity checking. So it may *look* like
UniqueRepresentation is a great way of speeding up "==" and "hash" for
things that are immutable, but it requires a much higher level of
immutability than required for hash.
(and in practice, the memory leaks caused by UniqueRepresentation bite us
with clockwork regularity)
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