Custom data types

Looking at objects and classes, you might have noticed a similarity with variables and their types. In fact this is exactly the same. By defining a class, you create a new type and a class instance is simply a variable of this type. Built-in Python types are also classes. Notice that you have already used their methods:

name = "python"
print(name.upper())  # `upper()` is a method in `str` class

You may define any type you want. Consider a class defining a fraction. It should have two attributes, numerator (named shortly numer) and denominator (denom):

class Fraction:
    def __init__(self, numer, denom):
        self.numer = numer
        self.denom = denom

It is possible to perform mathematical operations on fractions, like addition, subtraction, multiplication and division. Let’s define a function mul that takes two fractions, multiplies them, and returns a Fraction instance, which is their factor:

def mul(a, b):
    return Fraction(a.numer * b.numer, a.denom * b.denom)

Actually this function is specific for fractions only. So, in order to keep things together, it is better to make it a method in a fraction:

class Fraction:
    def __init__(self, numer, denom):
        self.numer = numer
        self.denom = denom

    def mul(self, other):
        return Fraction(self.numer * self.numer, self.denom * self.denom)

We may use this method as follows:

x = Fraction(1, 2)
y = Fraction(5, 6)

z = x.mul(y)

The last line does not look good. Later in this course, you will learn what you can do to be able to use normal mathematical operators (z = x * y) for your classes.

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