CMU 15-112: Fundamentals of Programming and Computer Science
Class Notes: Object-Oriented Programming (OOP), Part 1
Using Objects and Methods
- 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 upper 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 - 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)
- Classes are also called "Types" in Python.
- 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).
- Every value in Python is an Object.