Why is class variable accessible via class instance?

Question

I have created class Circle with no instance variables. I've added a class-method from_diameter to generate the circle with given diameter.

class Circle:

    @classmethod
    def from_diameter(cls, diameter):
        cls.diameter = diameter
        return cls

diameter is a class (static) variable. However - it looks like it exists as an instance variable and class variable as well.

myCircle = Circle.from_diameter(10)
print Circle.diameter
print myCircle.diameter

Output:

10
10

Why does it work? There is no instance variable diameter. I suppose print myCircle.diameter should throw an error.

Answer 1

when you try to access the variable with a class it look into only

cls.__dict__

but when you try to access the variable with instance it looks first

self.__dict__ 

if find then return or if can't find then it also looks in

cls.__dict__

here cls is the class

class Test:
    temp_1=10
    temp_2=20

    def __init__(self):
        self.test_1=10
        self.test_2=20

    @classmethod
    def c_test(cls):
        pass

    def t_method(self):
        pass


print Test.__dict__
print Test().__dict__

Output:

{'c_test': <classmethod object at 0x7fede8f35a60>, '__module__': '__main__', 't_method': <function t_method at 0x7fede8f336e0>, 'temp_1': 10, '__doc__': None, '__init__': <function __init__ at 0x7fede8f335f0>, 'temp_2': 20}

{'test_2': 20, 'test_1': 10}

For detail class special attribute