Lecture 25: Classes & Objects

August 3rd, 2021

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Today: JSON, matplotlib (finishing up from lecture 24), classes, and objects

Announcements

• Quiz 2 is on Monday, August 9th starting at 1:30pm PT
• Review materials will come out later today

Object Oriented Programming (OOP)

• There are different paradigms in programming
• So far you have learned imperative programming
  -provide a series of direct commands for program execution
  -commands are changing the program state
• Object Oriented Programming (OOP)
  -define objects that contain data and behavior (functions)
  -program is mostly an interaction between objects
  -you are calling functions of these objects (called "methods")
• Python allows for programming in either paradigm!
• Other paradigms exist, but we won't talk about those in 106A

Classes and Objects

• What are classes and objects?
• Classes are like blueprints
  -They provide a template for a kind of object
  -They define a new type
  -Ex: "CS106A student" would be a class
  -Each of these would have a stanford email, pycharm, a section leader etc.
• Objects are instances of classes
  -Can have multiple objects of the same Class type
  -Ex: YOU would be an instance of the "CS106A student" Class
  -So you have the properties of your Class (CS106A student)
  -There are lots of other 106A students out there too
  -You are all type "CS106A student"
  -You are all objects of the same Class

Example of a Class in Python

Counter Example Zip

• Counter Class
• Let's create a class that gives people a number representing their spot in line for ice cream
• Want to be able to get a ticket (ask the class for the next ticket number)
• Need to keep track of this next ticket number
• Class names start with Uppercase character (Counter)
• No main() function (a Class is not a program)
• Let's see an example

    
class Counter:

    # Constructor (two underscores - double underscores - known as 'dunder')
    def __init__(self): 
        self.ticket_num = 0 # "instance" variable 

    # Method (function) that returns the next ticket value
    def next_value(self):
        self.ticket_numm += 1
        return self.ticket_num
    

Classes in Action

Objects are Mutable:
- When you pass an object into as a parameter, changes to the object in that function persist after the function ends

    
from counter import Counter # import the Class

def count_two_times(counter):
    for i in range(2):
        print(counter.next_value())

def main():
    count1 = Counter()
    count2 = Counter()
    print('Count1: ')
    count_two_times(count1)
    print('Count2: ')
    count_two_times(count2)
    print('Count1: ')
    count_two_times(count1)
    

Output:

Count1:
1
2
Count2:
1
2
Count1:
3
4

How to Write a Class in Python

• Classes are written in their own files
• Filename for the class is usually: classname.py
  -Usually the lower case version of the classname in the file (ex: counter.py)
• Let's take a closer look:

    
class Classname:
    # Constructor
    def __init__(self, additional parameters):
        # body of code
        self.variable_name = value # example instance variable
    
    # Method
    def method_name(self, additional parameters):
        # body of code
    

Constructor of a Class

• Constructor Syntax

  def __init__(self, additional parameters):
      # body

• Called when a new object is being created
  -Does not explicity specify a return value
  -A new object is created and returned
  -Can think of a constructor as the factory that creates new objects
  -Responsible for intializing objects (setting initial values)
  -Generally where instance variables are created (with self)
  

  self.variable_name = value # creates an instance variable
  

Instance Variables

• Instance variables are variables associated with objects
  -Each object gets its own set of instance variables
  -Generally, they are initialized in the constructor for the Class
• Instance variables are accessed using self
self.variable_name = value
• self really refers to the specific object that a method is called on

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Methods (functions) in a Class

• Methods (the name used for functions in objects)
• Syntax:
  

  def method_name(self, additional parameters):
      # body

• Works like regular functions in python
  -Can return values (like a regular function)
  -Has access to instance variables (through self)
  self.variable_name = value
  -Called using an object
  object_name.method_name(additional parameters)
  -Recall that the parameter self is automatically set by Python as the object that this method is being called on
  -Ex: you write number = count1.next_value()
  -Pytohn treats it as number = next_value(count1)

Another Example: Students

Student Class Example Zip

• Want a class to keep track of information for students
• Each student has information:
  -Name
  -ID number
  -Units completed
• Want to specify the name and ID number when creating a student object
  -Initially set units completed to 0
  -Student's number of units completed can be updated over time
  -Also want to be able to check if a student can graduate
  Student needs to have at least UNITS_TO_GRADUATE units
• Let's check it out in Pycharm