On Mon, Mar 17, 2014 at 11:48 AM, Nathaniel Smith <njs@pobox.com> wrote:
> One more question that I think should be answered by the PEP and may
> influence the associativity decision is what happens if in an A @ B @ C
> expression, each operand has its own type that defines __matmul__ and
> __rmatmul__?  For example, A can be an ndarray, B a sympy expression and C a
> pyoperator.

The general rule in Python is that in a binary operation A # B, then
first we try A.__special__, and if that doesn't exist or it returns
NotImplemented, then we try B.__rspecial__. (The exception is that if
B.__class__ is a proper subclass of A.__class__, then we do it in the
reverse order.)

This is the simple case.  My question was: "what happens if in an A @ B @ C expression, each operand has its own type that defines __matmul__ and __rmatmul__?"

Are we going to recommend that other projects adopt numpy's __array_priority__?

In mixed-type expressions, do you expect A @ B @ C to have type of A, B, or C?

Does __matmul__ first then __rmatmul__ rule makes sense if @ becomes right-associative or should the order be reversed?