Equality using Coercion considered harmful?

Currently, Sage specifies that, upon comparing with a==b two objects a and b that do not have the same parent, a coercion is attempted to put them in the same parent. To prepare a discussion on whether that specification is viable in the long run, we are gathering here (borderline?) use cases where this behaviour is harmful. (Note that there is also the related problem of broken notions of equality for elements with the same parent which is discussed at EqualityUnhashable.)

sage: bool(pi == 3.14159265358979323)
sage: hash(pi)
sage: hash(3.14159265358979323)

However Python 2.7 documentation specifies:

Called by built-in function hash() and for operations on members of hashed collections including set,
frozenset, and dict. __hash__() should return an integer. The only required property is that objects
which compare equal have the same hash value;

As a first consequence of the above behavior:

sage: {3.1415926535897932: 'approx', pi: 'exact'}
{3.1415926535897932: 'approx', pi: 'exact'}
sage: {0:"exact", 0.0000000000000000000:"approx"}
{0: 'approx'}

And also:

sage: pii = 3.14159265358979323
sage: bool(pii == pi)
sage: dd = {pi: "exact"}
sage: pi in dd
sage: pii in dd
sage: pii in dd.keys()

pi is an integer (and transitivity of equality)

sage: R1 = RealField(2)
sage: R2 = RealField(56)
sage: pi1 = R1(pi); pi1
sage: pi2 = R2(pi); pi2
sage: pi1 == pi2
sage: bool(pi == pi2)
sage: pi1 == 3

And we get a nice proof that pi = 3.

More strange consequences when using UniqueRepresentation

sage: F1 = FiniteEnumeratedSet([0.000000])
sage: F2 = FiniteEnumeratedSet([0])
sage: F1 is F2
sage: F2.list()

An example showing that Sage's specifications clashes with Python's specifications

sage: S = SymmetricFunctions(QQ)
sage: x = S.s()[5]
sage: y = S.p()(x)
sage: x == y
sage: hash(x), hash(y)
(-1840429907820881728, 5178019317311573726)

It's surely syntactically nice to have x == y evaluate True after a coercion. However enforcing that the two hash functions be the same would be simply impossible: this would force to systematically coerce any symmetric function to some fixed base for computing the hash function, and we just can't afford that.

Yet another example (#22594)

sage: bring=ZZ
sage: for a in cartesian_product([[bring.one()]]):
....:     print a[0].parent()
Integer Ring
sage: bring=QQ
sage: for a in cartesian_product([[bring.one()]]):
....:     print a[0].parent()
Integer Ring

EqualityCoercion (last edited 2017-03-13 22:32:31 by hivertwiki)