Descriptors

A descriptor is an object that controls attribute access on another class. Any class with __get__ (and optionally __set__) placed as a class attribute intercepts reads and writes of that attribute on every instance. Properties, methods and @classmethod are all built on descriptors. __set_name__ tells the descriptor which attribute name it was given.

How it reads

• fuel = Positive() lives on the class, but guards every instance
• self.fuel = fuel inside __init__ actually calls Positive.__set__
• The real value is stored under a private name like _fuel

Cloud tip: One descriptor class can guard many attributes. That is what makes it better than writing a property for each one.