a.py

class MyClass:
    myVar = 1

    def HelloWorld(self):
        MyClass.myVar = 2
        self.myVar = 3

a = MyClass()
print(MyClass.myVar)
print(a.myVar)

a.HelloWorld()

print(MyClass.myVar)
print(a.myVar)

b = MyClass()
print(b.myVar)