[Python-ideas] Suggestion: Deprecate metaclasses that are not instances of type

[Neil Girdhar]

That's fair.  However, the documentation should at least be repaired by
replacing section 3.3.3.2 with:

"The metaclass of a class definition is selected from the explicitly
specified metaclass (if any) and the metaclasses (i.e. type(cls)) of all
specified base classes. The most derived metaclass is one which is a
subtype of all of these candidate metaclasses. If none of the candidate
metaclasses meets that criterion, then the class definition will fail with
TypeError. If provided, the explicit metaclass must be a callable accepting
the positional arguments (name, bases, _dict)."

This is because something happened along the way and Objects/typeobject.c:
type_new no longer coincides with Lib/types.py:new_class.  The Python
version conditionally calls _calculate_meta whereas the C version calls it
unconditionally.  I consider the C implementation to be the "correct"
version.

Best,

Neil

On Thu, Oct 13, 2016 at 5:41 PM Steven D'Aprano <steve at pearwood.info> wrote:

> On Thu, Oct 13, 2016 at 01:46:34PM -0700, Neil Girdhar wrote:
>
> > If provided, the explicit metaclass must be an instance of
> > type()."
>
> -1 for pointless breakage.
>
> The metaclass has always been permitted to be any callable. You haven't
> given any good reason for gratuitously changing this.
>
>
>
> --
> Steve
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