[Python-Dev] A small step to removing the type/class split

[Barry A. Warsaw]

>>>>> "NS" == Neil Schemenauer <nas@arctrix.com> writes:

    NS> I've run into a problem with ExtensionClass which I believe
    NS> can only be fixed by modifying Python.  I will try to explain.

    NS> I have an ExtensionClass which defines __cmp__.  Depending on
    NS> the objects being compared, the __cmp__ method may never be
    NS> called.  This is due to code in object.c that looks like this:

    |     if (PyInstance_Check(v) || PyInstance_Check(w)) {
    |         try to use use __cmp__ method
    |     }
    |     else {
    |         try number coerce and fall back on type name comparison
    |     }

I hit a similar wall when I tried (a long time ago) to take a boolean
class I'd written in Python and make it a built-in type.  The problem
was that in Python I could compare a Boolean instance against any
other object for truth equivalence, but because of this hack, I
couldn't do the same with the built-in type.

    |   * Define a new type flag Py_TPFLAGS_INSTANCE.
    |   * Create a new predicate Py_IsInstance which checks for this
    |     flag.
    |   * Set this flag on PyInstance_Type.
    |   * Replace most occurances of PyInstance_Check with
    |     Py_IsInstance.

    NS> Extension types (like ExtensionClass) can then define the type
    NS> flag Py_TPLAGS_INSTANCE and be treated as an instance type by
    NS> the Python interpreter.  This should make it quite a bit
    NS> easier to make extension types behave like "real" classes.

I'm not sure how well this addresses what I ran into.  Would
PyBooleanObject then have to have it's Py_TPFLAGS_INSTANCE flag set?
Does that actually make sense?  How does this interact with the rich
comparisons idea?  Seems like this is a hack that doesn't quite get at
the heart of the matter, worthwhile as it may be given the current
implementation.

-Barry