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__ = MetaA
...
Traceback (most recent call last):
File "<stdin>", line 1, in ?
TypeError: Error when calling the metaclass bases
metaclass conflict: the metaclass of a derived class must be a (non-strict)
subclass of the metaclasses of all its bases
Py> class C2(C1):
... __metaclass__ = Meta2
...
Py> class C3(C2):
... __metaclass__ = Meta1
...
Py> type(C3)
<class '__main__.Meta2'>
Py>
'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'.
Cheers,
Nick.
--
Nick Coghlan | ncoghlan at email.com | Brisbane, Australia
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