Thanks you very much for the explanations. I am sorry for bothering you, actually my question concerned the use of identity operator. I suggest that '=='  means equal not identity. Therefore, it concerns the instance not the ontology and makes a comparison between the values not the types. In my modest opinion this is a bug. Anyway, you have explain very well the class None and the role of print as function in Python. My little remark is that a communication act should not change the type of object that shows. Instead it transforms the object in Noneclass. This is a peculiar of Python. Warm regards. Rocco