Operators

Overview

Teaching: 10 min
Exercises: 10 min

Questions

• How do I manipulate objects and data in Python?

Objectives

• Show how to perform operations on python objects

• Introduce the basic operators

We now know how to create variables to store our data but we don’t know how to manipulate them at the moment. This is done using ‘Operators’.

Basic Operators

Operators are symbols that perform specific operations on one or more objects. A subset of these are the arithmetic operators you’re already familiar with:

• Multiplication: a * b
• Addition: a + b
• Subtraction: a - b
• Division: a / b

For example, a+b is the addition operator being applied to the objects pointed at by the variables a and b. What operators you can use depends on what the objects you’re applying them to are so some things won’t work - you can’t divide two strings or add a string and a number for example.

Under the Hood

It’s worth noting that operators in Python are nothing ‘special’ - they can be thought of as basically shorthand for other bits of code. These can range from simple things like numerical addition/subtraction/etc. to concatenation (combining) of two strings or performing the scalar product on matrices.

Going further

As well as these basic operators, there are also more language specific ones:

• Assignment: a = b
• Addition/Subtraction and assignment: a += b, a -= b
• Exponent: a ** b ( e.g. 10**3 = 1000 )
• Modulus: % (e.g. 9 % 4 = 1)
• Floor Division: a // b (e.g. 9 // 4 = 2)
• Subscript/Array: []

You can see that assignment (=) which you’ve already met is just another operator - it will attempt to assign the object or value on the right to the variable on the left. Another of these you will use a lot is the Subscript/Array operator ([]). This is generally used to access elements of collections like getting a single character from a string, e.g.

my_str = "TESTING"
first = my_str[0]
third = my_str[2]

Check Your Understanding

What values do the variables mass and age have after each statement in the following program? Test your answers by executing the commands.

mass = 47.5
age = 122
mass = mass * 2.0
age = age - 20

Challenge

If you assign a = 123, what happens if you try to get the second digit of a via a[1]?

Solution

Numbers are not stored in the written representation, so they can’t be treated like strings.

a = 123
print(a[1])

TypeError: 'int' object is not subscriptable

If you had a string then this would have worked as expected:

a = "123"
print(a[1])

2

In-place Operators

What is the value of a and b after the following code is executed? Try it by running this code yourself

a = 10
b = 20
b += a
print(a, b)

Solution

10 30

The addition and assignment operator (b += a) is shorthand for b = b + a so in this case, the value of a is added to b but a is left unchanged.

Getting Experience with Operators

If you have time, see what happens when you:

1. Multiply a float and an integer
2. Divide a float and an integer
3. Divide two integer

Key Points

• Operators are used to perform actions on objects

• They have different behaviour depending on the objects that are being acted on

• There is nothing special about operators - they are just shorthand for running other bits of code