Chapter 3 (Variables and Assignments)

Let's go back to maths. In algebra, x is used as a representation of a number. If I let x = 33241, it follows that 3 x = 99723, x + 1 = 33242, without me needing to write the original number again. Since the value of x can vary, we call it a variable.

Most languages (including Python) have variables at their core. In Python, you can imagine the variable as a labelled box storing a value. You can replace the value in the box at any time. For the example of x = 3, you can visualize it like so:

Variables consist of two important parts: the name and the value.

Variable Name

Unlike in maths: * Variable names must be unique (there are minor exceptions) * Variable names can be more than a letter. "score", "camel_1" and "x" are all valid variable names * Variable names CANNOT start with a number or symbol. "123_o" and "$" are not allowed * Variable names CANNOT be special names used in Python. For example, since print() is a Python function, "print" is not a valid name. However, "print_1" and other names that do not match exactly are allowed * Variables are case sensitive. That is, "variable_x" is different from "Variable_x" which is in turn also different from "variable_X".

Conventionally, * Variables are written with short, meaningful names * Variables are named with lowercase letters, using underscores in place of spaces

Variable Value

Unlike in maths: * Variables need not represent numbers. You can store other things in variables, like letters/words (known as strings) (Chapter 4), functions (Chapter 7), or lists (Chapter 8)

To define a variable with name "var_1" and value of 2.5,

var_1 = 2.5

This is called variable assignment, since we are assigning a value to a variable. The equals sign is know as an assignment operator. It is an operator, just like the mathematical operators in Chapter 2.

Now, we can check the value of the variables var_1:

print(var_1)
2.5

We can reassign the value of var_1 to an integer:

var_1 = 5
print(var_1)
5

We can also define a variable using an existing variable:

var_2 = var_1
print(var_2)
5

var_3 = var_1 * var_2 + 10
print(var_3)
35

We can even change a variable using itself:

var_1 = 12 - var_1
print(var_1)
7

There are other assignment operators which act as shortcuts for these cases:

Statement	Equivalent Statement (using Assignment Operators)
var = var + 1	var += 1
var = var - 1	var -= 1
var = var * 10	var *= 10
var = var / 15	var /= 15
var = var % 2	var %= 2