Lesson 17 Notes: More Class Stuff

• Preview
  • Setting default values
  • Overloading
  • Class attributes
  • Driver functions
• Setting Default Values
  • Functions can be set to have default values
  • These values are used only when a function call does not specify a value
    class Money:
def __init__(self, amount = 0):
self.amount = amount
• Operator Overloading
  • Classes can redefine regular Python commands.
  • Changing how regular commands work in a class is called Operator Overloading.
  • To define operator overloading, you must use special names when defining the methods.
  • Overload definitions must return a value.
• Operator Overloading Example
  class Weird:
    def __init__(self, num1 = 2):
        self.num1 = num1
    def __del__(self):
        print "%d Destroyed!" % self.num1
    def __repr__(self):
        return "Your number is %d" % self.num1
    def __add__(self, newnum):
        #Don't ever do this.
        return newnum*self.num1
• Some Operator Overloading Methods
  • __add__ +
  • __sub__ -
  • __mul__ *
  • __div__ /
  • __pow__ **
  • __repr__ print
  • __len__ len(x)
  • __call__ x()
  • __init__ Creation
  • __del__ Destruction
• Driver Function
  • A "driver function" is put at the end of a class to test it.
  • The function takes the form :
    if __name__ == '__main__':
    do stuff
  • When a class or module is called as the main program, it will run this code.
• Class Attributes
  • Data that needs to be accessed by every instance (or object) of a class can be stored in class attributes
  • Values will be created outside of function definitions in main body of the class
  • Data will be changed or accessed within the function definitions (methods) us Classname.variablename notation.
• Sample Class Attributes
  class Accounts:
     employees = []
     def __init__(self, name):
          self.name = name
          Accounts.employees.append(self.name)
     def __del__(self)
          Accounts.employees.remove(self.name)
  • Note that the employees list is always called by its full name (Accounts.employees)
  • To access the list outside of the class, use its full name (Accounts.employees)
• Review
  • What is operator overloading?
  • How do you use class attributes?
  • Why would you set default values?
  • What is the purpose of a driver function?
  • How do you create a driver function?

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