def _update_bases(space, w_bases):
    bases_w = space.listview(w_bases)
    new_bases_w = []
    changed = False
    for w_base in bases_w:
        w_meth = space.lookup(w_base, '__mro_entries__')
        if w_meth is not None:
            new_base_w = space.get_and_call_function(w_meth, w_base, w_bases)
            if not space.isinstance_w(new_base_w, space.w_tuple):
                raise oefmt(space.w_TypeError, "__mro_entries__ must return a tuple")
            new_bases_w.extend(space.fixedview(new_base_w))
            changed = True
        else:
            new_bases_w.append(w_base)
    if not changed:
        return bases_w
    return new_bases_w

def build_class(space, w_func, w_name, __args__):
    # ...
    orig_bases_w, kwds_w = __args__.unpack()
    w_orig_bases = space.newtuple(orig_bases_w)
    bases_w = _update_bases(space, w_orig_bases)
    w_bases = space.newtuple(bases_w)
    # ...