OPS445 Lab 6

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Concepts: Classes and Objects

  • What's a class
  • What's an object
  • Attributes and methods
  • Object instantiation
  • Scopes and Namespaces: local, nonlocal, and global

Creating a Class

Each class that we define in Python can contain variables, functions, and code, but none of the code can be executed until you create at least one object of that class. In a way a class is similar to a function definition: the function doesn't run until it's executed.

We're going to create a class named Student.

  • Create a new python program student.py in a new lab6 directory.
  • Start with the following contents. Read through the program and the comments inside:
    #!/usr/bin/env python3

class Student:
    
    # Define the name and number when a student object is created, 
    # ex. student1 = Student('john', 025969102)
    def __init__(self, name, number):
        self.name = name
        self.number = number
        self.courses = {}
    
    # Display student name and number
    def displayStudent(self):
        print('Student Name: ' + self.name)
        print('Student Number: ' + self.number)
    
    # Add a new course and grade to students record
    def addGrade(self, courseCode, grade):
        self.courses[course] = grade
    
    # Calculate the grade point average of all courses and display it
    def displayGPA(self):
        gpa = 0.0
        for courseCode in self.courses.keys():
            gpa = gpa + self.courses[courseCode]
        print('GPA of student ' + self.name + ' is ' + str(gpa / len(self.courses)))

From this Student class definition: multiple Student objects can be instantiated (created). Most parts of this program that should look familiar, such as functions, for loops, and variables. Functions which are members of a class are called methods. Variables starting with self. are members of the class (therefore members of each object of that class), and are called public attributes.

Let's break down this class and explain each part in detail.

Begin defining a class

You use the class keyword followed by the name of the class (which is not the same as the name of an object of that class): 

class Student:

When you need to create a new Student object: the class Student is available. Everything indented underneath the class Student: will be available in each Student object.

The constructor: __init__() method

Next indented under our class is the __init__() method. It's just a function which is a member of the Student class, but its name makes it special: it will be executed automatically when we create a Student object. 

def __init__(self, name, number):
    self.name = name
    self.number = number
    self.courses = {}

Inside the __init__() we create variables. These variables do not exist in the class, they will exist as members of each object of that class, and they're called attributes. These are created using self.name, where name is the name of the attribute. The self. portion before the variable name is to let the class know that this variable can be accessed from anywhere inside the class and outside of the class, as a public attribute. We're not going to use other types of attributes in this lab.

displayStudent() method

The next method definition prints out the member variables self.name and self.number. Both of these variables were set in the __init__() method. 

# Display student name and number
def displayStudent(self):
    print('Student Name: ' + self.name)
    print('Student Number: ' + self.number)

Normally creating variables inside a function means they can only be accessed inside of that specific function. Classes give us the ability to share data with all other functions in the class. Placing self. before the variable name sets this up.

addGrade() method

This next method accepts some arguments. 

# Add a new course and grade to students record
def addGrade(self, course, grade):
    self.courses[course] = grade

The first argument of the method is self, this is required syntax for making methods inside of a class. This is a rather unusual requirement for object-oriented languages, so don't overthink it.

The other two arguments are the important ones: course and grade. The method will store the values passed in via the arguements inside the dictionary self.courses, with the key being course and the value being grade.

Again, the courses attribute will persist for as long as the object exists, and each object of type Student will have its own self.courses

displayGPA() method

This final method will do a bit of calculations: 

# Calculate the grade point average of all courses and display it
def displayGPA(self):
    gpa = 0.0
    for course in self.courses.keys():
        gpa = gpa + self.courses[course]
    print('GPA of student ' + self.name + ' is ' + str(gpa / len(self.courses)))

It will get the average of all values stored inside the courses dictionary, add them up, and divide them by the number of values. When it finishes, the method will print out a message with the GPA of the student.

Using your class

Before you can use a class, we must create a new object.

A single class definition can be used to create as many objects of that type as you like. Our Student class will be used to create lots of different student objects, each object will be created using the blueprint of the class, but they will all be separate and contain separate data to store every student's data.

With your own class, as with any other python data structure you've used already (list, set, dictionary): you can create multiple instances of that type and store different data inside.

  • In a new file temp.py for your program, and import the Student class:
    from student import Student

The lowercase student is the name of your file (student.py), the capitalized Student is the name of the class you want to use from that file.

  • Create a new Student object and assign values to it:
    # Creates an instance of the Student class, it will be separate from all other objects created with the Student class:
student1 = Student('John', '013454900')

student1 is the object you're creating, of type Student. The student1 object will be constructed with the values 'John', and '013454900'.

  • Have a look at the contents of the object student1:
    print(student1.name)
print(student1.number)
print(student1.courses)
student1.displayStudent()
  • Before going further with student1, lets create a second object, to demonstrate that the data is stored in the object and not in the class:
    student2 = Student('Jessica', '023384103')
  • Take a closer look at some of these different attributes and methods.
    print("") # Empty line
print(student2.name)
print(student2.number)
print(student2.courses)
student2.displayStudent()
  • Now let's call another method whcih will work on the data in each object's instance:
    # Add new courses for student1
student1.addGrade('uli101', 4.0)
student1.addGrade('ops235', 3.5)
student1.addGrade('ops435', 3.0)

# Add new courses for student2
student2.addGrade('ipc144', 4.0)
student2.addGrade('cpp244', 4.0)
  • See what has changed in each object:
    print("") # Empty line
print(student1.name)
print(student1.courses)
print(student2.name)
print(student2.courses)

The method addGrade() changes the self.courses dictionary. But both student1 and student2 have their OWN courses dictionary.

Once our object is created: the attributes inside may be modified from outside the object. But be careful. For example if the class was designed to have a value is a string, and you (the user of the class) change it to another type such as a integer or a list, it's possible that that object's methods may throw a error.

For example, changing the name to a integer would break the displayStudent method, because it would try and concatenate strings and integers. You might argue that the method should be written better, but the opposing argument is that you shouldn't mess with an object's attributes directly.

print("") # Empty line
# student1.name is a string like any other
print(student1.name)
student1.name = 'Jack'
print(student1.name)
len(student1.name)

Practice using your class

The following python program is broken. It has two major problems to fix and one new feature to add:

  1. The first problem is providing the student number as an integer causes an error(TypeError) when displayStudent() is run. For this lab let's assume that providing the student number as an integer should be allowed.
  2. Second problem is that displayGPA() may divide by zero(ZeroDivisionError) if no courses were added to the dictionary or the grades added to the dictionary are 0.0 floats.
  3. Finally, you need to add a new method to this class which will print out a formatted list of all courses the student has taken.
  • Create the ~/ops435/lab6/lab6a.py script.
  • Use the following as a template (be careful: this is NOT the same as student.py):
    #!/usr/bin/env python3

class Student:

    # Define the name and number when a student object is created, ex. student1 = Student('john', 025969102)
    def __init__(self, name, number):
        self.name = name
        self.number = number
        self.courses = {}

    # Return student name and number
    def displayStudent(self):
        return 'Student Name: ' + self.name + '\n' + 'Student Number: ' + self.number

    # Add a new course and grade to students record
    def addGrade(self, course, grade):
        self.courses[course] = grade

    # Calculate the grade point average of all courses and return a string
    def displayGPA(self):
        gpa = 0.0
        for course in self.courses.keys():
            gpa = gpa + self.courses[course]
        return 'GPA of student ' + self.name + ' is ' + str(gpa / len(self.courses))

    # Return a list of course that the student passed (not a 0.0 grade)
    def displayCourses(self):
        return

if __name__ == '__main__':
    # Create the first student object and add grades for each class
    student1 = Student('John', '013454900')
    student1.addGrade('uli101', 1.0)
    student1.addGrade('ops235', 2.0)
    student1.addGrade('ops435', 3.0)

    # Create the second student object and add grades for each class
    student2 = Student('Jessica', '123456')
    student2.addGrade('ipc144', 4.0)
    student2.addGrade('cpp244', 3.5)
    student2.addGrade('cpp344', 0.0)

    # Display information for the student1 object
    print(student1.displayStudent())
    print(student1.displayGPA())
    print(student1.displayCourses())

    # Display information for the student2 object
    print(student2.displayStudent())
    print(student2.displayGPA())
    print(student2.displayCourses())
  • Modify the code so that:
  1. The displayStudent() method will not break if the object was created with an integer, example, student2 = Student('Jessica', 123456)
  2. The displayGPA() will cleanly handle a ZeroDivisionError
  3. The displayCourses() will return a list of courses that the student passed(not 0.0 grade)
  4. The script should show the exact output as the samples(order of courses in list not important)

Sample Run 1:

./lab6a.py
Student Name: John
Student Number: 013454900
GPA of student John is 2.0
['ops435', 'ops235', 'uli101']
Student Name: Jessica
Student Number: 123456
GPA of student Jessica is 2.5
['cpp244', 'ipc144']

Sample Run 2 (with import):

from lab6a import Student

student1 = Student('Jack', 931686102)

student1.addGrade('ops535', 2.0)

student1.addGrade('win310', 0.0)

student1.displayStudent()
# Will print: 'Student Name: Jack\nStudent Number: 931686102'

student1.displayGPA()
# Will print: 'GPA of student Jack is 1.0'

student1.displayCourses()
# Will print: ['ops535']

student2 = Student('Jen', 987654321)

student2.displayGPA()
# Will print: 'GPA of student Jen is 0.0'

student2.displayCourses()
# Will print: []
  • Download the checking script and check your work:
    cd ~/ops435/lab6/
pwd #confirm that you are in the right directory
ls CheckLab6.py || wget http://ops345.ca/ops445/CheckLab6.py
python3 ./CheckLab6.py -f -v lab6a

Submit evidence of your work

  • Run the following command in a terminal:
    cd ~/ops435/lab6
python3 ./CheckLab6.py -f
  • The output of the lab check command must say OK.
  • To show that you completed the lab, submit a screenshot of the terminal with that output.
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