text: class Circle:
      """
      This is an example of a class called Circle
      """
      import numpy as np # get some math power
      # define some attributes of the Circle class
      pi=np.pi # pi is now an attribute of this class too.  
      # initialize the class   with the attribute r (no parentheses when called)
      def __init__(self,r):
          self.r=r # define a variable, r
      # define some methods (these have parentheses when called)
      def area(self): 
          return (1./2.)*self.pi*self.r**2
      def circumference(self):
          return 2.*self.pi*self.r
  