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Recently I have been learning about managed attributes in Python and a common theme with properties and descriptors is, that they have to be assigned as class attributes. But nowhere can I find an explanation of why and especially why they cannot be assigned as instance attributes. So my question has actually two parts:
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A class attribute is a Python variable that belongs to a class rather than a particular object. It is shared between all the objects of this class and it is defined outside the constructor function, __init__(self,...) , of the class.
In python, everything is an object. And every object has attributes and methods or functions. Attributes are described by data variables for example like name, age, height etc. Properties are special kind of attributes which have getter, setter and delete methods like __get__, __set__ and __delete__ methods.
Attributes of a class are function objects that define corresponding methods of its instances. They are used to implement access controls of the classes. Attributes of a class can also be accessed using the following built-in methods and functions : getattr() – This function is used to access the attribute of object.
A private attribute provides you a level of protection from the users of your class, for that attribute. If you use a public attribute, you will need to add in more logic to test for invalid values up front, which can be more work, as well as more computationally expensive.
It is because of the way Python tries to resolve attributes:
Voila ;-)
I just found this link which explains it better than me.
Another nice illustration.
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