On Wed, May 9, 2012 at 8:37 AM, Tres Seaver <[email protected]> wrote:
>> No, the "mcl" in the call is just the designated metaclass - the
>> *actual* metaclass of the resulting class definition may be something
>> different. That's why this is a separate method from mcl.__new__.
>
> Why not make it a static method, if there is no notion of a useful 'cls'
> argument?
We need the explicitly declared metaclass as well as the bases in
order to determine the correct metaclass.
As a static method, the invocation would look like:
type.build_class(mcl, bases, keywords, exec_body)
Since mcl will always be an instance of type in 3.x (due to all
classes being subtypes of object), it makes more sense to just make it
a class method and invoke the method on the declared metaclass:
mcl.build_class(bases, keywords, exec_body)
The following assertion *does not* hold reliably:
cls = mcl.build_class(bases, keywords, exec_body)
assert type(cls) == mcl # Not guaranteed
Instead, the invariant that holds is the weaker assertion:
cls = mcl.build_class(bases, keywords, exec_body)
assert isinstance(cls, mcl)
This is due to the fact that one of the bases may specify that the
actual metaclass is a subtype of mcl rather than mcl itself.
Cheers,
Nick.
--
Nick Coghlan | [email protected] | Brisbane, Australia
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