Objects

Python is an object oriented language. As such it allows the definition of classes.

For instance lists are also classes, that's why there are methods associated with them (i.e. append()). Here we will see how to create classes and assign them attributes and methods.

Definition and initialization

A class gathers functions (called methods) and variables (called attributes). The main of goal of having this kind of structure is that the methods can share a common set of inputs to operate and get the desired outcome by the programmer.

In Python classes are defined with the word class and are always initialized with the method __init__, which is a function that always must have as input argument the word self. The arguments that come after self are used to initialize the class attributes.

In the following example we create a class called Circle.



In [ ]:

    
class Circle:
    def __init__(self, radius):
        self.radius = radius #all attributes must be preceded by "self."

To create an instance of this class we do it as follows



In [ ]:

    
A = Circle(5.0)

We can check that the initialization worked out fine by printing its attributes



In [ ]:

    
print(A.radius)

We now redefine the class to add new method called area that computes the area of the circle



In [ ]:

    
class Circle:
    def __init__(self, radius):
        self.radius = radius #all attributes must be preceded by "self."
    def area(self):
        import math
        return math.pi * self.radius * self.radius



In [ ]:

    
A = Circle(1.0)



In [ ]:

    
print(A.radius)
print(A.area())

Exercise 3.1

Redefine the class Circle to include a new method called perimeter that returns the value of the circle's perimeter.



In [ ]:

We now want to define a method that returns a new Circle with twice the radius of the input Circle.



In [ ]:

    
class Circle:
    def __init__(self, radius):
        self.radius = radius #all attributes must be preceded by "self."
    def area(self):
        import math
        return math.pi * self.radius * self.radius
    def enlarge(self):
        return Circle(2.0*self.radius)



In [ ]:

    
A = Circle(5.0) # Create a first circle
B = A.enlarge() # Use the method to create a new Circle
print(B.radius) # Check that the radius is twice as the original one.

We now add a new method that takes as an input another element of the class Circle and returns the total area of the two circles



In [ ]:

    
class Circle:
    def __init__(self, radius):
        self.radius = radius #all attributes must be preceded by "self."
    def area(self):
        import math
        return math.pi * self.radius * self.radius
    def enlarge(self):
        return Circle(2.0*self.radius)
    def add_area(self, c):
        return self.area() + c.area()



In [ ]:

    
A = Circle(1.0)
B = Circle(2.0)
print(A.add_area(B))
print(B.add_area(A))

Exercise 3.2

Define the class Vector3D to represent vectors in 3D. The class must have

Three attributes: x, y, and z, to store the coordinates.
A method called dot that computes the dot product

$$\vec{v} \cdot \vec{w} = v_{x}w_{x} + v_{y}w_{y} + v_{z}w_{z}$$

The method could then be used as follows

v = Vector3D(2, 0, 1)
w = Vector3D(1, -1, 3)

v.dot(w)
5



In [ ]: