[ python-Bugs-963246 ] Multiple Metaclass inheritance limitation

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Bugs item #963246, was opened at 2004-05-30 11:52
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Category: Python Interpreter Core
Group: Python 2.3
Status: Open
Resolution: None
Priority: 5
Submitted By: Greg Chapman (glchapman)
Assigned to: Nobody/Anonymous (nobody)
Summary: Multiple Metaclass inheritance limitation

Initial Comment:
I'm not sure if this constitutes a bug or a limitation 
(which should be documented?), but the following 
doesn't work with Python 2.3.4.

Assume CMeta is a type defined in C, with
  tp_base = PyType_Type
  tp_new = some C function which uses the mro to 
      call the inherited tp_new
  tp_flags includes Py_TPFLAGS_BASETYPE

class PyMeta(type):
    def __new__(meta, name, bases, attrs):
        return super(PyMeta, meta).__new__(meta,
                       name, bases, attrs)

class MetaTest(CMeta, PyMeta):
    pass

class Test:
    __metaclass__ = MetaTest

The attempt to define Test generates a 
TypeError: "type.__new__(MetaTest) is not safe, use 
CMeta.__new__()".  

The above error is generated (in tp_new_wrapper) by 
the super call in PyMeta.__new__, but this is only 
reached as a result of an initial call to CMeta.tp_new 
(which, using the mro to find the next "__new__" 
method, finds and calls PyMeta.__new__).  

It may be there is no good way to allow the above 
scenario, but I just thought I'd point it out in case 
someone can think of a workaround.



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