Python classes

If you’ve not used Python classes before, this guide will give you an introduction to what they are and what they are used for.

In general, a class is a set of instructions for how to create an object. An instance of a class is the object that is created from the class. As an example, we can create a class that defines a fraction.

from math import gcd


class Fraction:
    """A fraction."""

    def __init__(self, numerator, denominator):
        self.numerator = numerator
        self.denominator = denominator

    def print_numerator(self):
        print(self.numerator)

    def __str__(self):
        """Get string representation."""
        return str(self.numerator) + " over " + str(self.denominator)

    def __add__(self, other):
        """Add two fractions."""
        assert isinstance(other, Fraction)
        new_numerator = self.numerator * other.denominator + other.numerator * self.denominator
        new_denominator = self.denominator * other.denominator
        common_factor = gcd(new_numerator, new_denominator)
        return Fraction(new_numerator // common_factor, new_denominator // common_factor)

    def __mul__(self, other):
        """Multiply two fractions."""
        assert isinstance(other, Fraction)
        new_numerator = self.numerator * other.numerator
        new_denominator = self.denominator * other.denominator
        common_factor = gcd(new_numerator, new_denominator)
        return Fraction(new_numerator // common_factor, new_denominator // common_factor)

This class contains a number of functions: functions inside a class are called methods. Each method starts with the self input: this refers to the instance of the class itself and can be used to store information that other methods will need. (You could use another name instead of self but it’s very very common practice to use self.) Function that start and end with a double underscore (__) are special methods.

The special method __init__ is run when an instance of the class is created. In this function, we store the numerator and denominator of the fraction as self.numerator and self.denominator.

We can create a fraction and call the print_numerator method like this:

half = Fraction(1, 2)
half.print_numerator()

This first line will run __init__ with half as self, 1 as numerator and 2 as denominator. The second line will run print_numerator with half as self, and will therefore print 1.

The special method __str__ defines what happens when you print an instance of your class. In our example, print(half) will print 1 over 2.

The special method __add__ defines what the + operator does. If you read the implementation above, you can see that __add__ is adding fractions in the way you would expect. For example, you can add two fractions like this:

half = Fraction(1, 2)
third = Fraction(1, 3)
print(half + third)

The special method __mul__ defines what the * operator does. For example, you can multiply two fractions like this:

half = Fraction(1, 2)
third = Fraction(1, 3)
print(half * third)

There are a lot more special methods that you can use, including those to define the behaviour of -, /, //, **, @, [], =, <, <=, >, and >=. You can find (details of these in the Python documentation](https://docs.python.org/3/reference/datamodel.html#specialnames)