More examples with classes

The following example consists of two classes. The class Point is a model for a point in two-dimensional space. The class Line is a model for a line segment between two points. The code below is commented; please read the comments in order to understand how the classes work.

import math

class Point:
    """ The class represents a point in two-dimensional space """

    def __init__(self, x: float, y: float):
        # These attributes are public because any value is acceptable for x and y
        self.x = x
        self.y = y

    # This class method returns a new Point at origo (0, 0)
    # It is possible to return a new instance of the class from within the class
    @classmethod
    def origo(cls):
        return Point(0, 0)

    # This class method creates a new Point based on an existing Point
    # The original Point can be mirrored on either or both of the x and y axes
    # For example, the Point (1, 3) mirrored on the x-axis is (1, -3)
    @classmethod
    def mirrored(cls, point: "Point", mirror_x: bool, mirror_y: bool):
        x = point.x
        y = point.y
        if mirror_x:
            y = -y
        if mirror_y:
            x = -x

        return Point(x, y)

    def __str__(self):
        return f"({self.x}, {self.y})"


class Line:
    """ The class represents a line segment in two-dimensional space """

    def __init__(self, beginning: Point, end: Point):
        # These attributes are public because any two Points are acceptable
        self.beginning = beginning
        self.end = end

    # This method uses the Pythagorean theorem to calculate the length of the line segment
    def length(self):
        sum_of_squares = (self.end.x - self.beginning.x) ** 2 + (self.end.y - self.beginning.y) ** 2
        return math.sqrt(sum_of_squares)

    # This method returns the Point in the middle of the line segment
    def centre_point(self):
        centre_x = (self.beginning.x + self.end.x) / 2
        centre_y = (self.beginning.y + self.end.y) / 2
        return Point(centre_x, centre_y)

    def __str__(self):
        return f"{self.beginning} ... {self.end}"

point = Point(1,3)
print(point)

origo = Point.origo()
print(origo)

point2 = Point.mirrored(point, True, True)
print(point2)

line = Line(point, point2)
print(line.length())
print(line.centre_point())
print(line)

Sample output

(1, 3) (0, 0) (-1, -3) 6.324555320336759 (0.0, 0.0) (1, 3) ... (-1, -3)

Default values of parameters

In Python programming you can generally set a default value for any parameter. Default values can be used in both functions and methods.

If a parameter has a default value, you do not have to include a value as an argument when calling the function. If an argument is given, the default value is ignored. If not, the default value is used.

Default values are often used in constructors. If it can be expected that not all information is available when an object is created, it is better to include a default value in the definition of the constructor method than to force the client to take care of the issue. This makes using the class easier from the client's point of view, but it also ensures the integrity of the object. For instance, with a set default value we can be sure that an "empty" value is always the same, unless the client specifically wants to supply something different. If a default value is not set, it is up to the client to provide an "empty" value. This could be, for example, an empty string "", the special empty object None, or the string "not set".

Let's have a look at yet another class representing a student. When creating a new Student object the client must provide a name and a student number. The student number is private and should not be changed later. Additionally, a Student object has attributes for study credits and notes, which have default values set in the constructor. New values can be passed as arguments to the constructor, but they can also be left out so that the default values are used instead. Please have a look at the comments in the code to better understand what each method does.

class Student:
    """ This class models a student """

    def __init__(self, name: str, student_number: str, credits: int = 0, notes: str = ""):
        # calling the setter method for the name attribute
        self.name = name

        if len(student_number) < 5:
            raise ValueError("A student number should have at least five characters")

        self.__student_number = student_number

        # calling the setter method for the credits attribute
        self.credits = credits

        self.__notes = notes

    @property
    def name(self):
        return self.__name

    @name.setter
    def name(self, name):
        if name != "":
            self.__name = name
        else:
            raise ValueError("The name cannot be an empty string")

    @property
    def student_number(self):
        return self.__student_number

    @property
    def credits(self):
        return self.__credits

    @credits.setter
    def credits(self, op):
        if op >= 0:
            self.__credits = op
        else:
            raise ValueError("The number of study credits cannot be below zero")

    @property
    def notes(self):
        return self.__notes

    @notes.setter
    def notes(self, notes):
        self.__notes = notes

    def summary(self):
        print(f"Student {self.__name} ({self.student_number}):")
        print(f"- credits: {self.__credits}")
        print(f"- notes: {self.notes}")

# Passing only the name and the student number as arguments to the constructor
student1 = Student("Sally Student", "12345")
student1.summary()

# Passing the name, the student number and the number of study credits
student2 = Student("Sassy Student", "54321", 25)
student2.summary()

# Passing values for all the parameters
student3 = Student("Saul Student", "99999", 140, "extra time in exam")
student3.summary()

# Passing a value for notes, but not for study credits
# NB: the parameter must be named now that the arguments are not in order
student4 = Student("Sandy Student", "98765", notes="absent in academic year 20-21")
student4.summary()

Sample output

Student Sally Student (12345):

credits: 0
notes:

Student Sassy Student (54321):

credits: 25
notes:

Student Saul Student (99999):

credits: 140
notes: extra time in exam

Student Sandy Student (98765):

credits: 0
notes: absent in academic year 20-21

NB: there is no setter method for the attribute student_number as the student number is not supposed to change.

There is one rather significant snag when using default values for parameters. The following example modelling yet another kind of student will shed more light on this:

class Student:
    def __init__(self, name, completed_courses=[]):
        self.name = name
        self.completed_courses = completed_courses

    def add_course(self, course):
        self.completed_courses.append(course)

student1 = Student("Sally Student")
student2 = Student("Sassy Student")

student1.add_course("ItP")
student1.add_course("ACiP")

print(student1.completed_courses)
print(student2.completed_courses)

Sample output

['ItP', 'ACiP'] ['ItP', 'ACiP']

Adding completed courses to Sally's list also adds those courses to Sassy's list. In fact, these two are the exact same list, as Python reuses the reference stored in the default value. Creating the two new Student objects in the above example is equivalent to the following:

courses = []
student1 = Student("Sally Student", courses)
student2 = Student("Sassy Student", courses)

The default values of parameters should never be instances of more complicated, mutable data structures, such as lists. The problem can be circumvented by making the following changes to the constructor of the Student class:

class Student:
    def __init__(self, name, completed_courses=None):
        self.name = name
        if completed_courses is None:
            self.completed_courses = []
        else:
            self.completed_courses = completed_courses

    def add_course(self, course):
        self.completed_courses.append(course)

student1 = Student("Sally Student")
student2 = Student("Sassy Student")

student1.add_course("ItP")
student1.add_course("ACiP")

print(student1.completed_courses)
print(student2.completed_courses)

Sample output

['ItP', 'ACiP'] []

The Grand Finale

Even though the following exercise finishes off this part of the material, the techniques required to solve it were all covered already in the section named objects as attributes. Specifically, you are not required to use the @property decorator or default values for parameters in this exercise. This exercise is very similar to the exercises a box of presents and the shortest person in the room.

Important information regarding the next exercise

Please note that there is an issue resulting from an update in Python, which conflicts with the inbuilt library and the original file name for this exercise. If you experience any issues, we recommend redownloading the exercise folder. Once you have obtained the new local test files, you can use either "code.py" or "code_1.py" as the file name. While using Visual Studio Code, you may receive notifications about problems in the test file. However, these notifications can be safely ignored, as they are caused by the test's inability to import from either the "code.py" or "code_1.py" files.

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