Lecture 3

Lecture 3, Tue 01/16

Python Objects and Classes

Plan for today

What are Python objects and classes? Why do we need them?
How do we create a new class?
How to define a method of a class?
How do we instantiate an object of our class?
How to create a container class?

Python Objects and Classes

Objects are a way for programmers to define their own Python types and create abstractions of things with the programming language.
Each object may have a certain state that gets modified throughout program execution.
Object Oriented (OO) programming is the way programs use and manipulate objects to solve problems and model real-world properties
OO Programming is not REQUIRED
- It’s more of a way to organize, read, maintain, test, and debug software in a manageable way
We’ve been using objects already, for example:

>>> help(int)
Help on class int in module builtins:

class int(object)
 |  int([x]) -> integer
 |  int(x, base=10) -> integer
 |
 |  Convert a number or string to an integer, or return 0 if no arguments
 |  are given.  If x is a number, return x.__int__().  For floating point
 |  numbers, this truncates towards zero.
...

Similarly, for strings, we would be able to store characters, ask about the length of the string, and use built-in methods, such as .upper() or .islower().

Methods are like functions but are associated with an object
In this case, Python already defined its own class called str that we can use, but sometimes we want to create our own specific objects for the applications we’re trying to build!

Student Class Example

# Student.py

class Student:
    """Student class keeps track of the student info:
        storing a name and a PERM number."""
    def __init__(self, name = None, number = None):
        self.name = name
        self.perm = number

    def setName(self, name):
        self.name = name

    def setNumber(self, number):
        self.perm = number

    def info(self):
        print("Name", self.name)
        print("PERM #", self.perm) # not .number

s1 = Student("Chris Gaucho", 1234567)
s2 = Student()
s2.setName("Wilma Gaucho")

When defining methods, the self parameter represents the current object we’re calling the method on
In the example above, s1 is the variable storing the created Student object.
When we define the methods, the first parameter is the instance of the object stored in s1
We can then use and manipulate the state of the object with these methods using the self parameter

Default Constructor

We can provide either default values or set values of the object when constructing it through the parameter list.
In the example above, we can set an empty object without any initial attributes, which may cause an error when trying to use them.
The constructor below will set self.name and self.perm to the value None, which doesn’t require the set methods to set these fields in the object.

def __init__(self):
	self.name = None
	self.perm = None

s1 = Student()
s1.info()

We can even go a step further and define the attributes by putting them in the parameters when we create the object

def __init__(self, name, perm):
	self.name = name
	self.perm = perm

s1 = Student("Ricardo", 1234567)
s1.info()

Initializing default values in the constructor

def __init__(self, name=None, perm=None):
	self.name = name
	self.perm = perm

In this case, we can pass in parameters or not. If not, then None will automatically be assigned to the self.name and self.perm
Or we can pick and choose what to set. For example:

s1 = Student(perm = 1234567)
s1.info()

# Student name: None, perm: 1234567

We used the default value of name (None) and then explicitly set the perm (1234567)

Example of using objects in code

s1 = Student("Jane", 1234567)
s2 = Student("Joe", 7654321)
s3 = Student("Jill", 5555555)

studentList = [s1, s2, s3]

for student in studentList:
	student.info()

Container Classes

Let’s continue to expand on our Student class
Classes are also useful to organize / maintain state of a program
Student objects are useful to organize the attributes of a single student
But let’s imagine we are trying to write a database of various students
- The database may be useful to search for students with certain attributes…
- We will need to add / remove things from our database
- We can define a class to represent a collection of courses containing students
- Let’s set up a collection of courses such that a dictionary key is defined by the course number and the value is a list of actual student objects

# Courses.py

# from [filename (without .py)] import [component]
from Student import Student 

class Courses:
	''' Class representing a collection of courses. Courses are
		organized by a dictionary where the key is the
		course number and the corresponding value is
		a list of Students of this course '''

	def __init__(self):
		self.db = {}

	def addStudent(self, courseNum, student):
		# If course doesn't exist... 
		if self.db.get(courseNum) == None:
			self.db[courseNum] = [student] # new entry in the dictionary; its value is a list
		elif not student in self.db.get(courseNum):
			self.db[courseNum].append(student)

	def showStudentsbyCourse(self, courseNum):
	        if self.db.get(courseNum) == None:
	            print(f"{courseNum} is not found!")
	        else:
	            courseList = self.db.get(courseNum)
	            print("Course:", courseNum)
	            for student in courseList:
	                student.info()
	            print("---")


UCSBCourses = Courses()

s1 = Student("Chris Gaucho", 1234567)
s2 = Student("Wilma Gaucho", 1234567)
s3 = Student("Mickey Mouse", 1234567)

UCSBCourses.addStudent("CS9", s1)
UCSBCourses.addStudent("CS9", s2)
UCSBCourses.addStudent("CS9", s3)

Output of running this code:

>>> UCSBCourses.showStudentsbyCourse("CS8")
CS8 is not found!
>>> UCSBCourses.showStudentsbyCourse("CS9")
Course: CS9
Name Chris Gaucho
PERM # 1234567
Name Wilma Gaucho
PERM # 1234567
Name Mickey Mouse
PERM # 1234567
---

TODO: try writing a method to print all courses and their lists.

Exploring different errors

>>> UCSB.db
{}

>>> UCSB.db()
Traceback (most recent call last):
  File "<pyshell#6>", line 1, in <module>
    UCSB.db()
TypeError: 'dict' object is not callable

The error above reminds us that the class attributes are variables, which we cannot call (adding parentheses () tells Python that .db() is supposed to be a method, which it is not, hence the error that the dictionary db is not callable.

Similarly, below is the difference between assigning the variable UCLA to be the class type versus calling the default constructor for that class type (by adding () after the class name):

>>> UCLA = Courses
>>> UCLA
<class '__main__.Courses'>
>>> UCLA = Courses()
>>> UCLA
<__main__.Courses object at 0x105047f50>

Below is an error that we would get if we had a typo in the method name or we haven’t yet defined the method.

>>> UCSB.addStudent("CS9", s1)
Traceback (most recent call last):
  File "<pyshell#5>", line 1, in <module>
    UCSB.addStudent("CS9", s1)
AttributeError: 'Courses' object has no attribute 'addStudent'