[Python-3000] Ambiguity in PEP 3115 and the args to __prepare__

[Guido van Rossum]

On 9/2/07, Brett Cannon <brett at python.org> wrote:
> PEP 3115 says a metaclass' __prepare__ takes two positional arguments,
> name and bases.  But the example has it actually accept an arbitrary
> number of arguments: name and then everything else is bound to bases.
>
> Which happens to be true?  I'm too tired to even fully trust that I am
> reading the PEP correctly, so I am not about to try to write an
> example to see which is correct and come up with a coherent rewording
> if I am right about what is wrong.  =)

I think you're misreading what you think is an example. I'm assuming
you're referring to this code:

       def prepare_class(name, *bases, metaclass=None, **kwargs):
          if metaclass is None:
             metaclass = compute_default_metaclass(bases)
          prepare = getattr(metaclass, '__prepare__', None)
          if prepare is not None:
             return prepare(name, bases, **kwargs)
          else:
             return dict()

This indeed *defines* a function with a *bases argument, but it is not
called __prepare__! It *calls* __prepare__ passing it name and bases,
i.e. the 2nd argument to prepare is a tuple of bases.

The only example defining __prepare__ later in the PEP takes two
positional arguments (name and bases again).


-- 
--Guido van Rossum (home page: http://www.python.org/~guido/)