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

  1. Methods vs Functions
  2. Classes and Instances
  3. Objects and Object-Oriented Programming (OOP)

  1. Methods vs Functions
    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 len on the string s" print(s.replace('could', 'may')) # some methods take additional arguments

    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

  2. Classes and Instances
    • 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)
    • 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).