Nick Coghlan added the comment:
Why should it trip the PEP 3115 namespace preparation exception? We only check
for __prepare__ (and hence need to do an early most-derived metaclass
resolution) on instances of "type", not on arbitrary callables used as a
metaclass hint
The checks are different in the two places because the rules are different in
the two places. (One case that *can* be made is that we should be throwing
different exceptions or chaining them to show which metaclass resolution
failed, rather than re-using the same message in both places).
This means that when you define a non-type metaclass hint, you're bypassing
*all* of the PEP 3115 machinery, including the early metaclass resolution.
However, if your metaclass hint still creates a type instance, you're not
bypassing tp_new.
If you're asking "Where is the bug in the presented example code?", it's here,
in the definition of your metaclass hint:
def metaclass_callable(name, bases, namespace):
return OtherMetaclass(name, bases, namespace)
Instantiating instances of "type" directly in Python 3 bypasses the PEP 3115
machinery - that's the entire reason that types.new_class was added. So if you
want that metaclass hint to be PEP 3115 compliant, you need to explicitly write
it that way:
def metaclass_callable(name, bases, namespace):
return types.new_class(name, bases, dict(metaclass=OtherMetaclass)
----------
resolution: not a bug ->
stage: resolved ->
status: closed -> open
title: Class definition is not consistent with types.new_class -> Documentation
for handling of non-type metaclass hints is unclear
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Python tracker <[email protected]>
<http://bugs.python.org/issue28437>
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