CMU 15-112: Fundamentals of Programming and Computer Science
Class Notes: Object-Oriented Programming (OOP), Part 1
Using Objects and Writing Classes

1. Methods vs Functions
2. Classes and Instances
3. Objects and Object-Oriented Programming (OOP)
4. Writing Classes
5. Writing Constructors
6. Writing Methods
7. Advantages of Classes and Methods
8. Objects and Aliases
9. Example: Animations with OOP

1. Methods vs Functions   video
   We call methods using s.f() rather than f(s):

   s = 'This could be any string!'
   
   print(len(s))     # len is a function
   
   print(s.upper())  # upper is a string method, called using the . notation
                     # we say that we "call the method upper on the string s"
   
   print(s.replace('could', 'may')) # some methods take additional arguments

   Select Visualize Run
   
   See how we get different errors for improperly calling methods vs functions:

   n = 123
   print(len(n))    # TypeError: object of type 'int' has no len()
                    # This means that len() cannot work properly with int's
   
   n = 123
   print(n.upper()) # AttributeError: 'int' object has no attribute 'upper'
                    # This means that there is no method upper() for int's

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2. Classes and Instances   video
   • Classes are also called "Types" in Python.
     • For example, these are classes: int, float, str, bool
   • Instances are values of a given class or type.
     • For example: 'abc' is a str instance (also called a string)


3. Objects and Object-Oriented Programming (OOP)   video
   • Every value in Python is an Object.
     • Every instance is an object, and its type is some class.
     • Every class is an object, too (its type is type!).
   • That is why we call this Object-Oriented Programming
     • We are using objects only a little bit now.
     • Soon we will write our own classes.
     • Then we will add some sophistication to how we write and use classes and objects.
     • Even so, because we are using objects now, we are already using Object-Oriented Programming (OOP).


4. Writing Classes

   # Create our own class:
   class Dog:
       # a class must have a body, even if it does nothing, so we will
       # use 'pass' for now...
       pass
   
   # Create instances of our class:
   d1 = Dog()
   d2 = Dog()
   
   # Verify the type of these instances:
   print(type(d1))             # Dog (actually, class '__main__.Dog')
   print(isinstance(d2, Dog))  # True
   
   # Set and get properties (aka 'fields' or 'attributes') of these instances:
   d1.name = 'Dot'
   d1.age = 4
   d2.name = 'Elf'
   d2.age = 3
   print(d1.name, d1.age) # Dot 4
   print(d2.name, d2.age) # Elf 3

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5. Writing Constructors
   • Constructors let us pre-load our new instances with properties.
   • This lets us write code like so:

     d1 = Dog('fred', 4) # now d1 is a Dog instance with name 'fred' and age 4
     

   • We would like to write our constructor like this:

     def constructor(dog, name, age):
         # pre-load the dog instance with the given name and age:
         dog.name = name
         dog.age = age
     

   • Unfortunately, Python does not use 'constructor' as the constructor name. Instead, it uses '__init__' (sorry about that), like so:

     def __init__(dog, name, age):
         # pre-load the dog instance with the given name and age:
         dog.name = name
         dog.age = age
     

   • Also, unfortunately, while we could name the instance 'dog' like we did, standard convention requires that we name it 'self' (sorry again), like so:

     def __init__(self, name, age):
         # pre-load the dog instance with the given name and age:
         self.name = name
         self.age = age
     

   • Finally, we place this method inside the class and we have a constructor that we can use, like so:

     class Dog:
         def __init__(self, name, age):
             # pre-load the dog instance with the given name and age:
             self.name = name
             self.age = age
      
     # Create instances of our class, using our new constructor
     d1 = Dog('Dot', 4)
     d2 = Dog('Elf', 3)
     
     print(d1.name, d1.age) # Dot 4
     print(d2.name, d2.age) # Elf 3

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6. Writing Methods
   • Start with a function:

     class Dog:
         def __init__(self, name, age):
             self.name = name
             self.age = age
     
     # Here is a function we will turn into a method:
     def sayHi(dog):
         print(f'Hi, my name is {dog.name} and I am {dog.age} years old!')
     
     d1 = Dog('Dot', 4)
     d2 = Dog('Elf', 3)
     
     sayHi(d1) # Hi, my name is Dot and I am 4 years old!
     sayHi(d2) # Hi, my name is Elf and I am 3 years old!

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   • Turn the function into a method, and the function call into a method call, like this:

     class Dog:
         def __init__(self, name, age):
             self.name = name
             self.age = age
     
         # Now it is a method (simply by indenting it inside the class!)
         def sayHi(dog):
             print(f'Hi, my name is {dog.name} and I am {dog.age} years old!')
     
     d1 = Dog('Dot', 4)
     d2 = Dog('Elf', 3)
     
     # Notice how we change the function calls into method calls:
     
     d1.sayHi() # Hi, my name is Dot and I am 4 years old!
     d2.sayHi() # Hi, my name is Elf and I am 3 years old!

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   • Finally, use self, as convention requires:

     class Dog:
         def __init__(self, name, age):
             self.name = name
             self.age = age
     
         # Now we are using self, as convention requires:
         def sayHi(self):
             print(f'Hi, my name is {self.name} and I am {self.age} years old!')
     
     d1 = Dog('Dot', 4)
     d2 = Dog('Elf', 3)
     
     # Notice how we change the function calls into method calls:
     
     d1.sayHi() # Hi, my name is Dot and I am 4 years old!
     d2.sayHi() # Hi, my name is Elf and I am 3 years old!

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   • Methods can take additional parameters, like so:

     class Dog:
         def __init__(self, name, age):
             self.name = name
             self.age = age
     
         # This method takes a second parameter -- times
         def bark(self, times):
             print(f'{self.name} says: {"woof!" * times}')
     
     d = Dog('Dot', 4)
     
     d.bark(1) # Dot says: woof!
     d.bark(4) # Dot says: woof!woof!woof!woof!

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   • Methods can also set properties, like so:

     class Dog:
         def __init__(self, name, age):
             self.name = name
             self.age = age
             self.woofCount = 0   # we initialize the property in the constructor!
     
         def bark(self, times):
             # Then we can set and get the property in this method
             self.woofCount += times
             print(f'{self.name} says: {"woof!" * times} ({self.woofCount} woofs!)')
     
     d = Dog('Dot', 4)
     
     d.bark(1) # Dot says: woof! (1 woofs!)
     d.bark(4) # Dot says: woof!woof!woof!woof! (5 woofs!)

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7. Advantages of Classes and Methods
   • Encapsulation
     • Organizes code
       A class includes the data and methods for that class.
     • Promotes intuitive design
       Well-designed classes should be intuitive, so the data and methods in the class match commonsense expectations.
     • Restricts access
       • len is a function, so we can call len(True) (which crashes)
       • upper is a method on strings but not booleans, so we cannot even call True.upper()
   • Polymorphism
     Polymorphism: the same method name can run different code based on type, like so:

     class Dog:
         def speak(self):
             print('woof!')
     
     class Cat:
         def speak(self):
             print('meow!')
     
     for animal in [ Dog(), Cat() ]:
         animal.speak() # same method name, but one woofs and one meows!

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8. Objects and Aliases
   

   # Objects are mutable so aliases change!
   # Run this with the visualizer to make it clear!
   
   import copy
   
   class Dog:
       def __init__(self, name, age, breed):
           self.name = name
           self.age = age
           self.breed = breed
   
   dog1 = Dog('Dino', 10, 'shepherd')
   dog2 = dog1            # this is an alias
   dog3 = copy.copy(dog1) # this is a copy, not an alias
   
   dog1.name = 'Spot'
   print(dog2.name) # Spot (the alias changed, since it is the same object)
   print(dog3.name) # Dino (the copy did not change, since it is a different object)

   Select Visualize Run


9. Example: Animations with OOP
   Here is an updated version of the adding-and-deleting dots demo from here. This version adds methods to the Dot class so each dot is responsible for its own drawing and mouse handling.

   from cmu_112_graphics import *
   import random
   
   class Dot:
       def __init__(self, cx, cy, r, counter, color):
           self.cx = cx
           self.cy = cy
           self.r = r
           self.counter = counter
           self.color = color
   
       def redraw(self, app, canvas):
           # Only redraw this dot
           canvas.create_oval(self.cx-self.r, self.cy-self.r,
                              self.cx+self.r, self.cy+self.r,
                              fill='white', outline=self.color, width=15)
           canvas.create_text(self.cx, self.cy, text=str(self.counter),
                              fill='black')
   
       def containsPoint(self, x, y):
           return (((self.cx - x)**2 + (self.cy - y)**2)**0.5 <= self.r)
   
       def mousePressed(self, event):
           # We are guaranteed (event.x, event.y) is in this dot
           self.counter += 1
           self.color = getRandomColor()
   
       def timerFired(self, app):
           self.counter += 1
   
   def appStarted(app):
       app.dots = [ ]
       app.timerDelay = 1000 # once per second
   
   def getRandomColor():
       colors = ['red', 'orange', 'yellow', 'green', 'blue', 'pink',
                 'lightGreen', 'gold', 'magenta', 'maroon', 'salmon',
                 'cyan', 'brown', 'orchid', 'purple']
       return random.choice(colors)
   
   def mousePressed(app, event):
       # go through dots in reverse order so that
       # we find the topmost dot that intersects
       for dot in reversed(app.dots):
           if dot.containsPoint(event.x, event.y):
               dot.mousePressed(event)
               return
       # mouse click was not in any dot, so create a new dot
       newDot = Dot(cx=event.x, cy=event.y, r=20, counter=0, color='cyan')
       app.dots.append(newDot)
   
   def keyPressed(app, event):
       if (event.key == 'd'):
           if (len(app.dots) > 0):
               app.dots.pop(0)
           else:
               print('No more dots to delete!')
   
   def timerFired(app):
       for dot in app.dots:
           dot.timerFired(app)
   
   def redrawAll(app, canvas):
       for dot in app.dots:
           dot.redraw(app, canvas)
   
       # draw the text
       canvas.create_text(app.width/2, 20,
                          text='Example: Adding and Deleting Shapes',
                          fill='black')
       canvas.create_text(app.width/2, 40,
                          text='Mouse clicks outside dots create new dots',
                          fill='black')
       canvas.create_text(app.width/2, 60,
                          text='Mouse clicks inside dots increase their counter',
                          fill='black')
       canvas.create_text(app.width/2, 70,
                          text='and randomize their color.', fill='black')
       canvas.create_text(app.width/2, 90,
                          text='Pressing "d" deletes circles', fill='black')
   
   runApp(width=400, height=400)

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