Python Indentation

Why Indentation Matters in Python

Python uses indentation to define blocks of code instead of braces or keywords. Proper indentation is mandatory; incorrect indentation causes IndentationError or changes program behavior.

Basic Rules

• Use consistent indentation throughout your code (commonly 4 spaces).
• Indent blocks after statements like if, for, while, def, and class.
• Never mix tabs and spaces in indentation.

Example 1: Correct Indentation

def greet(name):
    if name:
        print(f"Hello, {name}!")
    else:
        print("Hello, stranger!")

greet("Alice")

Example 2: IndentationError

def greet(name):
if name:  # IndentationError: expected an indented block
    print(f"Hello, {name}!")
else:
    print("Hello, stranger!")

greet("Alice")

Example 3: Mixing Tabs and Spaces (Causes Error)

def foo():
↹print("Hello")  # Tab
    print("World")  # Spaces

# This will raise:
# TabError: inconsistent use of tabs and spaces in indentation

Tips to Avoid Indentation Problems

• Configure your code editor to insert spaces (not tabs) when you press the Tab key.
• Use an editor that highlights indentation errors.
• Use python -m tabnanny your_script.py to detect inconsistent indentation.
• Keep your indentation consistent across your project.

How Python Interprets Indentation

Indented blocks define the body of functions, loops, conditionals, and classes. Example:

for i in range(3):
    print(i)
    if i == 1:
        print("One")
print("Done")  # This line is outside the loop

Output:

0
1
One
2
Done