Lecture 5

Lecture 5, Thu 10/15

Python Classes cont.

Recorded Lecture: 10_15_20

Container Classes

Let’s continue to expand on our Student class from last lecture.
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 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.courses = {}

	def addStudent(self, student, courseNum):
		# If no perm exists…
		if self.courses.get(courseNum) == None:
			self.courses[courseNum] = [student]
		elif not student in self.courses.get(courseNum):
			self.courses[courseNum].append(student)

	def printCourses(self):
		for courseNum in self.courses:
		print("CourseNum: ", courseNum)
		for student in self.courses[courseNum]:
			student.printAttributes()
		print("---")

# lecture.py

from Student import Student
from Courses import Courses

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

UCSB = Courses()
UCSB.addStudent(s1, "CS8")
UCSB.addStudent(s2, "CS9")
UCSB.addStudent(s3, "CS16")
UCSB.printCourses()

# Try adding the same student - should have no effect
UCSB.addStudent(s2, "CS9")

Let’s write pytests for the Courses class!

# courses_test.py
from Student import Student
from Courses import Courses

def test_addSingleStudent():
	s1 = Student("Gaucho", 1234567)
	UCSB = Courses()
	UCSB.addStudent(s1, "CS9")
	courses = UCSB.getCourses()
	assert courses == {"CS9":[s1]}

def test_addMultipleStudentsToCourse():
	s1 = Student("Gaucho", 1234567)
	s2 = Student("Jane", 7654321)
	UCSB = Courses()
	UCSB.addStudent(s1, "CS9")
	UCSB.addStudent(s2, "CS9")
	courses = UCSB.getCourses()
	assert courses == {"CS9":[s1,s2]}

def test_addStudentsToCourses():
	s1 = Student("Gaucho", 1234567)
	s2 = Student("Jane", 7654321)
	s3 = Student("Jill", 5555555)
	UCSB = Courses()
	UCSB.addStudent(s1, "CS9")
	UCSB.addStudent(s2, "CS9")
	UCSB.addStudent(s3, "CS16")
	courses = UCSB.getCourses()

	# Note: Since Python 3.7, Dictionary keys are in inserted order
	assert courses == {"CS9":[s1,s2], "CS16":[s3]}

def test_addSameStudentTwice():
	s1 = Student("Gaucho", 1234567)
	s2 = Student("Jane", 7654321)
	s3 = Student("Jill", 5555555)
	UCSB = Courses()
	UCSB.addStudent(s1, "CS9")
	UCSB.addStudent(s2, "CS9")
	UCSB.addStudent(s1, "CS9")
	UCSB.addStudent(s3, "CS16")
	courses = UCSB.getCourses()    
	assert courses == {"CS9":[s1,s2], "CS16":[s3]}

Shallow vs. Deep Equality

Python allows us to check for equality using the == operator for our objects
But Python doesn’t have any knowledge of what makes a Student equal to another Student in this case
So by default, Python uses the memory address (not the values) to determine if two objects are the same
This is known as a shallow equality
Example

s1 = Student("Jane", 1234567)
s2 = Student("Jane", 1234567)
print(s1 == s2) # False, doesn’t compare values!

In order to provide our meaning of equality for two Student objects, we will have to define the __eq__ method in our Student class
In this case, we can assume two Students are the same if they have the same perm number
- Comparing values (instead of memory addresses) is called deep equality

# Add the __eq__ method in Student.py

# s1 == s2, self is the left operand (s1), rhs is the right operand (s2)
def __eq__(self, rhs):
        return self.perm == rhs.perm

s1 = Student("Jane", 1234567)
s2 = Student("Jane", 1234567)
print(s1 == s2) # True, compares the perm values!