Background: I asked a stackoverflow question here <http://stackoverflow.com/questions/40029807/why-does-the-class-definitions-metaclass-keyword-argument-accept-a-callable> .
The Python documentation <https://docs.python.org/3.6/reference/datamodel.html#determining-the-appropriate-metaclass> 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()."
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