# Classes¶

Especially for larger code bases the rather mathematical way of programming may become slightly convoluted. A paradigm that tries to alleviate this is object-oriented programming. Let us just dive right in with the definition of a rectangle class. Each rectangle has a length and a width, from which one can compute its area and perimeter. Here is the definition:

class Rectangle:
"""A rectangle that is described by its length and width."""

def __init__(self, length, width):
self.length = length
self.width = width

def area(self):
"""Return the area of the rectangle."""
return self.length*self.width

def perimeter(self):
"""Return the perimeter of the rectangle."""
return 2*(self.length + self.width)


Let us go over this step by step. The definition of a class starts with the keyword class followed by the name of the class. Within the class you can define methods. They are like functions that are attached to the class. What is slightly peculiar is, that all take self as their first argument— we will come to that in a second. The first method is __init__.

def __init__(self, length, width):
self.length = length
self.width = width


As you use classes you instantiate objects of that class. During the instantiation you can provide some initial arguments to the class to customize the resulting object using the special __init__ method. In this example a rectangle object can be initialized with values for its length $$l$$ and for its width $$w$$. These values are then stored in the instance as attributes. The reference to the instance is the self argument, that each method has as its first argument. So by attaching information to self, each distinct object has its own state.

Now we can exploit this in other methods. The area $$A$$ of a rectangle is defined as

$A = l \cdot b$

In the code this is described by the following definition:

def area(self):
"""Return the area of the rectangle."""
return self.length*self.width


Where a method is defined, that takes the length of the rectangle self.length, multiplies it with self.width and then returns it. If we did not use self.length but just length or used width instead of self.width we would not have accessed the values we stored in this rectangle during its initialization, but some global values instead. So to make sure that we only use the attributes of our specific rectangle we access them from self.

The method used to get the perimeter $$P$$, which, for a rectangle, is defined as

$P = 2 (l + b)$

follows the same theme:

def perimeter(self):
"""Return the perimeter of the rectangle."""
return 2*(self.length + self.width)


In the following the usage of this class is shown.

first_rectangle = Rectangle(length=2, width=3)
print('Information about the first rectangle')
print('Length:', first_rectangle.length)
print('Width:', first_rectangle.width)
print('Area:', first_rectangle.area())
print('Perimeter:', first_rectangle.perimeter())


First an object of the class Rectangle is instantiated with the length argument set to 2 and the width argument set to 3. The name of our first Rectangle object is first_rectangle. Subsequently the attributes length and width of the object first_rectangle can be accessed via first_rectangle.length and first_rectangle.width, respectively. One could say that what ever has been self in the class definition now is replaced by the name of the object. In the case of the methods the self argument is implicitly supplied by calling the method from the object. So it is sufficient to use first_rectangle.area(), and not first_rectangle.area(self) or first_rectangle.area(first_rectangle)— both of which would be wrong. The output of the above code is

Information about the first rectangle
Length: 2
Width: 3
Area: 6
Perimeter: 10


If another rectangle is instantiated with different values the information changes accordingly. So in the case of a second_rectangle

second_rectangle = Rectangle(length=5, width=7)
print('Information about the second rectangle')
print('Length:', second_rectangle.length)
print('Width:', second_rectangle.width)
print('Area:', second_rectangle.area())
print('Perimeter:', second_rectangle.perimeter())


the output would be

Information about the second rectangle
Length: 5
Width: 7
Area: 35
Perimeter: 24


## Inheritance¶

A square is a special case of a rectangle, i.e., a rectangle with equal sides. As the computation of the geometrical properties remains the same, one option of initializing a square could be

square = Rectangle(length=2, width=2)


But maybe you want to be more explicit when initializing squares. This is where inheritance kicks in. Let us take a look at the definition of a Square class that inherits from the previously defined Rectangle class:

class Square(Rectangle):
"""A square that is described by its side length."""

def __init__(self, side_length):
super().__init__(length=side_length, width=side_length)


As opposed to the Rectangle class the name of the Square class is followed by parenthesis containing Rectangle. This tells Python that the Rectangle class is the superclass of Square, i.e., Square inherits from Rectangle. To inherit means that, if not otherwise defined, Square has the exact same method definitions as its superclass Rectangle.

But as the __init__ method of Rectangle takes the arguments length and width, which is not required for the definition of a square, we can simplify it. Now it takes only one argument side_length. If we stored it as we did in the Rectangle class, i.e., as

def __init__(self, side_length):
self.side_length = side_length


The methods that Square inherits from Rectangle would fail to be callable, as they access the attributes length and width, which would not be defined if we took this definition of the __init__ method. Instead we could do this:

def __init__(self, side_length):
self.length = side_length
self.width = side_length


So now the definition looks awfully similar to the one of the Rectangle class. A bit too similar maybe, and we do not want to repeat ourselves. Another side-effect is that, should the __init__ method of the Rectangle implement some more code, it would have to be copied to Square as well. As this is error-prone there is a way to leverage the method of a superclass within the child class, and this is done using the super() function. If used within a method definition followed by calling a method it will resolve to the first parent class that implements a method with this name and call it for the current object. So by implementing it via

def __init__(self, side_length):
super().__init__(length=side_length, width=side_length)


we tell Python to call the __init__ method of the superclass of Square and pass the side_length for the length and width.

Using the class can now be done like this:

first_square = Square(side_length=2)
print('Information about the first square')
print('Length:', first_square.length)
print('Width:', first_square.width)
print('Area:', first_square.area())
print('Perimeter:', first_square.perimeter())


The respective output:

Information about the first square
Length: 2
Width: 2
Area: 4
Perimeter: 8


## Type checking¶

Checking whether an object is of a certain type, or is a child of a certain type, is done by using the isinstance() function. The first argument is the object whose type should be checked, the second argument is the class for which to check.

def what_is_it(object):
if isinstance(object, Rectangle):
print('It is a rectangle.')
if isinstance(object, Square):
print('It is a square.')


If we use this simple function on our geometrical objects you can see how it works.

what_is_it(first_rectangle)


As first_rectangle is a Rectangle, but not a Square, the output is:

It is a rectangle.


Now for the first_square:

what_is_it(first_square)


As Square is a specialization of Rectangle you can also see that it identifies as such in addition to being a Square.

It is a rectangle.
It is a square.


## Special methods¶

Some behavior of Python classes is implemented in terms of so-called Special method names. An example of how they may be used can be seen in the following:

import math

class FreeVector:
"""A vector that is not bound by an initial or terminal point."""

def __init__(self, vector):
self.vector = tuple(vector)

@property
def magnitude(self):
return math.sqrt(math.fsum(v**2 for v in self.vector))

@property
def direction(self):
magnitude = self.magnitude
return tuple(v/magnitude for v in self.vector)

def __repr__(self):
return '{self.__class__.__name__}(vector={self.vector!r})'.format(
self=self)

def __str__(self):
return str(self.vector)

def __eq__(self, other):
if (isinstance(other, FreeVector) and
all(math.isclose(a, b) for a, b in zip(
other.vector, self.vector))):
return True
else:
return False

def __neq__(self, other):
return not self.__eq__(self, other)

def __add__(self, other):
if not isinstance(other, FreeVector):
return NotImplemented
return tuple(a + b for a, b in zip(self.vector, other.vector))

def __sub__(self, other):
if not isinstance(other, FreeVector):
return NotImplemented
return tuple(a - b for a, b in zip(self.vector, other.vector))


The usage may be as follows:

>>> a = FreeVector((1, 2, 3))
>>> a
FreeVector(vector=(1, 2, 3))
>>> str(a)
'(1, 2, 3)'
>>> b = FreeVector((1, 2, 3))
>>> c = FreeVector((4, 5, 6))
>>> a == b
True
>>> a == c
False
>>> a + c
(5, 7, 9)
>>> c - a
(3, 3, 3)


## Exercises¶

• Copy the definition of the Rectangle class and extend it by adding a method aspect_ratio which returns the ratio of its length to its width.
• Define a Circle class with the radius $$r$$ as defining attribute. Implement the area and perimeter class accordingly.
• Read and work on the book “Building Skills in Object-Oriented Design” to understand the process of object-oriented design.