Classes and Objects

• As programs get larger, it can become challenging to organize our code and keep track of all the parts/functions.
• Modules do help, but it is an incomplete solution.
• A second key component is being able to bundle data into one single object.
  • For example a single variable called date instead of managing separate month, day, and year variables.
  • Named tuples help with this.
• Object-oriented programming combines these two ideas, and is currently the dominant technique used for organizing code.
• We interact with the word in terms of objects: table, chair, car, person, stove, etc.
• When thinking about an object, we consider two main things:
  • It’s properties
  • How we can interact with it.
• A car may have properties like
  • number of seats,
  • engine size/type
  • number of doors
  • color
• We interact with a car using the steering wheel and pedals (among other things).
• All variables refer to some object.
• Some objects have very simply properties:
  • i = 17 is “just” a number
• Most objects naturally have many properties.
  • A Date has a month, day and year.
• Objects have a set of actions that let us interact with them.
  • ordinal_date
  • days_elapsed etc.

• A class is a blueprint that describes what properties an object has and what actions can be performed on it.

• This says that an object that is a Date has year, month, and day as a property.

  class Date:
    """ A class that represents a date """
    def __init__(self):
        self.year = 0
        self.month = 0
        self.day = 0
  

• This says that an object that is a Time has a hour and a minute.

  class Time:
      """ A class that represents a time of day """
      def __init__(self):
          self.hours = 0
          self.minutes = 0
  

  date1 = Date()
  date1.year = 2022
  date1.month = 11
  date1.day = 4
  
  print(f"date1 is {date1.year}-{date1.month}-{date1.day}")
  
  date2 = Date()
  date2.year = 2024
  date2.month = 7
  date2.day = 4
  

• Notice above that date1 and date2 are different objects. (Just like these are different pens/desks, etc.).
• Each object has different values for it’s properties.
• But, since date1 and date2 are members of the same class, they have the same set of properties (month, day, year)
• In some sense, a class is a “blueprint” that tells how to construct an object.

Methods

• instance methods are functions defined within a class. They provide the means for interacting with an object.

  def make_string(self):
    return f"{self.year}-{self.month}-{self.day}"
    
  print(date1.make_string())
  

  def increment_minute(self):
    self.minute += 1
    if (self.minute >= 60):
      self.hour += 1
  

Next steps

• Constructor
• class variables
• encapsulation / information hiding.
  • alarm clock example
  • separate interface from implementation
  • put underscore in front of “private” methods.
  • Avoid accessing instance variables directly.
  • “Perl doesn’t have an infatuation with enforced privacy. It would prefer that you stayed out of its living room because you weren’t invited, not because it has a shotgun”