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3.2. Indentation & Code blocks

Before we move on to learning more about conditionals, it is important that you know about how indentation works in Python as indentation will be used a lot in Python.

Indentation and Code block

Python is one of the many programming languages that use indentation for representing code blocks. Other programmings like C, C++, JavaScript etc. use curly brackets to represent code blocks.

Simply speaking, Indentation are spaces in the beginning of line of code to represent that a code is part of a code block.

A block of code comprises lines of code that are intended to execute under a certain condition. A colon : usually indicates the starting of a code block.

If we look at the following code similar to the code shown on previous page,

age = int(input('Enter your age: '))

if age >= 18:
    print('You are over 18.')
    print('You can drive.')
else:
    print('You are under 18.')
    print('You cannot drive yet.')

Enter your age: 23
You are over 18.
You can drive.

Enter your age: 15
You are under 18
You cannot drive yet.
  1. The colon indicates the starting of code block
  2. The code block for the if statement
  3. The code block for the else statement

In this code, both blocks of code comprise two lines of code only but could have any number of lines.

An error will be raised if indentation is skipped:

if age >= 18:
print('You are over 18 so you can drive')

  File "G:\thepyguide\main.py", line 3
    print('You are over 18 so you can drive.')
    ^
IndentationError: expected an indented block after 'if' statement on line 2

Rules for Indentation

There are a following rules for indentation that are discussed below.

Number of spaces

It doesn't matter how many number of spaces are used for indentation but it should be at least one. The convention is to use 4 spaces (equivalent to a Tab).

In most code editors, pressing the Tab key would automatically indent by four spaces. Note that four spaces are conventionally preferred over a raw Tab character and most code editors treat pressing the Tab as equivalent to pressing spacebar four times.

age = int(input('Enter your age: '))

if age >= 18:
    print('You are over 18')  # 4 spaces
else:
        print('You are not over 18')  # 8 spaces

Enter your age: 23
You are over 18

Though no errors are raised by the code above, it is a good practice to keep the indentation consistent throughout the code.

Consistent indentation

The indentation in same code block must be consistent. That is, in a certain code block that has multiple lines, the indentation for each line should be same throughout that code block.

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age = int(input('Enter your age: '))

if age >= 18:
    print('You are over 18')  # 4 spaces
    print('So you can drive')
else:
        print('You are not over 18')  # 8 spaces
    print('So you cannot drive')  # 4 spaces

  File "G:\thepyguide\main.py", line 8
    print('So you cannot drive')  # 4 spaces
                                        ^
IndentationError: unindent does not match any outer indentation level

As you can see, in the else code block, the first line is indented with 8 spaces and the second line is indented with 4 spaces.

To resolve the error, either the second line in code block has to be indented 8 spaces too or the first line has to be indented with 4 spaces.

Tip

Writing pretty code is as important as writing efficient code. The conventional practice is to stick to 4 spaces for indentation throughout your code.

While Python allows different indentation for individual code blocks, the best practice is to stay consistent.

Tabs and spaces

Either Tab can be used for indentation or four spaces. In most code editors, pressing a Tab adds four spaces. However, both tabs and spaces cannot be mixed or a TabError is raised.

Tip

The preferred and good practice is to always use spaces for indentation because tabs can affect readiblity of code on different code editors.

This is why most code editors treat pressing a Tab as pressing Space four times.