[Python-Dev] PEP 208 and __coerce__

[Neil Schemenauer]

While working on the implementation of PEP 208, I discovered that
__coerce__ has some surprising properties.  Initially I
implemented __coerce__ so that the numberic operation currently
being performed was called on the values returned by __coerce__.
This caused test_class to blow up due to code like this:

    class Test:
        def __coerce__(self, other):
            return (self, other)

The 2.0 "solves" this by not calling __coerce__ again if the
objects returned by __coerce__ are instances.  This has the
effect of making code like:

    class A:
        def __coerce__(self, other):
            return B(), other

    class B:
        def __coerce__(self, other):
            return 1, other

    A() + 1

fail to work in the expected way.  The question is: how should
__coerce__ work?  One option is to leave it work the way it does
in 2.0.  Alternatively, I could change it so that if coerce
returns (self, *) then __coerce__ is not called again.


  Neil