noconflict.py

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import inspect, types, __builtin__

############## preliminary: two utility functions #####################

def skip_redundant(iterable, skipset=None):
    "Redundant items are repeated items or items in the original skipset."
    if skipset is None: skipset = set()
    for item in iterable:
        if item not in skipset:
            skipset.add(item)
            yield item


def remove_redundant(metaclasses):
    skipset = set([types.ClassType])
    for meta in metaclasses: # determines the metaclasses to be skipped
        skipset.update(inspect.getmro(meta)[1:])
    return tuple(skip_redundant(metaclasses, skipset))

##################################################################
## now the core of the module: two mutually recursive functions ##
##################################################################

memoized_metaclasses_map = {}

##
# Not intended to be used outside of this module, unless you know
#  what you are doing.
def get_noconflict_metaclass(bases, left_metas, right_metas):
    # make tuple of needed metaclasses in specified priority order
    metas = left_metas + tuple(map(type, bases)) + right_metas
    needed_metas = remove_redundant(metas)

    # return existing confict-solving meta, if any
    if needed_metas in memoized_metaclasses_map:
        return memoized_metaclasses_map[needed_metas]
    # nope: compute, memoize and return needed conflict-solving meta
    elif not needed_metas:         # wee, a trivial case, happy us
        meta = type
    elif len(needed_metas) == 1: # another trivial case
        meta = needed_metas[0]
    # check for recursion, can happen i.e. for Zope ExtensionClasses
    elif needed_metas == bases: 
        raise TypeError("Incompatible root metatypes", needed_metas)
    else: # gotta work ...
        metaname = '_' + ''.join([m.__name__ for m in needed_metas])
        meta = classmaker()(metaname, needed_metas, {})
    memoized_metaclasses_map[needed_metas] = meta
    return meta

def classmaker(left_metas=(), right_metas=()):
    def make_class(name, bases, adict):
        metaclass = get_noconflict_metaclass(bases, left_metas, right_metas)
        return metaclass(name, bases, adict)
    return make_class

if (__name__ == '__main__'):
    from vyperlogix.misc import ObjectTypeName

    class M_A(type):
        pass
    class M_B(type):
        pass
    class A(object):
        __metaclass__=M_A
    class B(object):
        __metaclass__=M_B
    #from noconflict import classmaker
    class C(A,B):
        __metaclass__=classmaker()
    print ObjectTypeName._typeName(C)