new_contents: class Foo():
      pass
  
  foo = Foo()
  print(foo)
  print(type(foo))
  
  class Circle():
      PI = 3.14159
      def area(self):
          return self.PI * self.r ** 2
  
  print(Circle.PI)
  
  Circle.r = 0
  print(Circle.r)
  
  c1 = Circle()
  print("---", c1.r)
  c1.r = 10
  
  c2 = Circle()
  c2.r = 20
  
  print(c1.r)
  print(c2.r)
  
  Circle.r = 0
  
  print(c1.r, c2.r)
  
  del c2.r
  print(c2.r)
  
  Circle.r = 2000
  print(c2.r)
  print(Circle.area(c2))
  
  del Circle.r
  print(c1.area())
subject: Add oop demo.