How to define what a class is ?

[ast]

Hi

Since a class is an object, I ask myself 
how to define rigorously what a class is.

classes are instances from type, but not all, since
a class may be an instance of a metaclass

A class is always callable

A class inherit from some others classes, so they
have a bases attribute

any thing else ?

Suppose I provide to you an object and that I
ask to you to tell me if it is a class or not. How
would you proceed ?

thx