Suggestion: Deprecate metaclasses that are not instances of type

[Neil Girdhar]

Background: I asked a stackoverflow question here.

The Python documentation is very confusing to me. It says that:

if an explicit metaclass is given and it is not an instance of type, then it is used directly as the metaclass

This seems to suggest that in this case, the "explicit metaclass" does not need to be "subtype of all of these candidate metaclasses" as it would in the third case.  (This is not true.)

Also, providing a callable as a metaclass doesn't seem to be any more flexible, readable, or powerful than providing an instance of type.  Therefore, I suggest that we deprecate the second case and replace the entire section (3.3.3.2) of the documentation to say:

"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 an instance of type()."

[Steven D'Aprano]

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.



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Steve
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[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

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[Neil Girdhar]

Bug: http://bugs.python.org/issue28437