Indentation Error in Python

IndentationError occurs when Python finds incorrect indentation in the code. Since Python uses indentation (spaces or tabs) to define code blocks, even a small alignment mistake can prevent the program from running. Unlike many other programming languages, indentation in Python is not optional it is a required part of the syntax.

def check_number(a):
if a > 2:
    return "Greater than 2"

a = 5
print(check_number(a))

Output

ERROR!
Traceback (most recent call last):
File "<main.py>", line 2
if a > 2:
^^
IndentationError: expected an indented block after function definition on line 1

Explanation: statement if a > 2: should be indented inside the function definition. Since Python cannot determine which statements belong to the function body, it raises an IndentationError.

Common Causes

1. Missing Indentation After a Statement: Python expects an indented block after statements such as if, for, while, def and class.

age = 18

if age >= 18:
print("Eligible to vote")

Output

ERROR!
Traceback (most recent call last):
File "<main.py>", line 4
print("Eligible to vote")
^^^^^
IndentationError: expected an indented block after 'if' statement on line 3

Explanation: print() statement should be indented because it belongs to the if block.

2. Mixing Tabs and Spaces: Using tabs and spaces together often creates inconsistent indentation.

def display():
	print("Hello")
    print("Python")

Output

ERROR!
Traceback (most recent call last):
File "<main.py>", line 3
print("Python")
^
IndentationError: unindent does not match any outer indentation level

Explanation: One line is indented using a tab while another uses spaces. Python treats them differently and cannot determine the correct block structure.

3. Unexpected Extra Indentation: Adding unnecessary indentation where it is not required can also cause an error.

name = "John"

    print(name)

Output

ERROR!
Traceback (most recent call last):
File "<main.py>", line 3
print(name)
IndentationError: unexpected indent

Explanation: print() statement is indented even though it does not belong to any code block.

Fixing IndentationError

1. Use Proper Indentation: Ensure that all statements inside a block are indented consistently.

def check_number(a):
    if a > 2:
        return "Greater than 2"
    return "2 or less"

a = 5
print(check_number(a))

Greater than 2

Explanation: Each statement is correctly indented according to its block level, so Python can interpret the code without errors.

2. Use Consistent Spaces: PEP 8 recommends using four spaces for each indentation level.

for i in range(3):
    print(i)

0
1
2

Explanation: All statements inside the loop use the same indentation level, making the code valid and easy to read.

3. Use an IDE or Code Editor: Modern editors such as VS Code, PyCharm and Jupyter Notebook automatically manage indentation and highlight indentation-related issues.

def greet():
    print("Hello")

Explanation: Using an editor with auto-indentation helps reduce the chances of indentation mistakes and improves code readability.

Best Practices

1. Use 4 spaces for each indentation level.
2. Avoid mixing tabs and spaces in the same file.
3. Enable auto-formatting in your code editor.
4. Keep indentation consistent throughout the program.
5. Use IDEs that highlight indentation problems before execution.