[Python-Dev] __metaclass__ problem

Phillip J. Eby pje at telecommunity.com
Sat Mar 19 01:33:22 CET 2005

At 10:11 AM 3/19/05 +1000, Nick Coghlan wrote:
>Nick Coghlan wrote:
>>If you are not getting an exception when breaking this rule, my guess 
>>would be that your metaclasses are not inheriting from 'type', or else 
>>are not invoking type's __new__ method. The logic to trigger the 
>>exception lives in type's __new__ method - if that doesn't get invoked, 
>>you won't get the exception.
>OK, I actually read the bug report - I think the 'invalid metaclass' 
>exception should also be getting thrown in the case described there.
>Py> class Meta1(type): pass
>Py> class Meta2(Meta1): pass
>Py> class MetaA(type): pass
>Py> class C1(object):
>...   __metaclass__ = Meta1
>Py> class C2(C1):
>...   __metaclass__ = Meta2
>Py> class C3(C2):
>...   __metaclass__ = Meta1
>Py> type(C3)
><class '__main__.Meta2'>
>'Meta1' is NOT a subclass of 'Meta2', yet the exception is not thrown. 
>Instead, the explicitly requested metaclass has been silently replaced 
>with a subclass. I think the OP is justified in calling that 'suprising'.

This is precisely the documented (in Guido's essay) behavior.  That is, 
type.__new__ uses the "most derived" of the explicit metaclass and the 
__class__ attributes of the bases.

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