Indentation

How does Python know which statements belong to the if block and which come after it? Most programming languages answer this by wrapping the statements of a block inside curly brackets. Python takes a different route: it uses indentation — for ordinary blocks and for nested block structures alike.

What indentation means

Indentation is the leading whitespace (spaces and tabs) at the beginning of a statement. In Python, statements written at the same level of indentation are grouped together into a single block of code. Indentation is not decoration — it is the block structure.

Key points to remember:

• The Python interpreter checks indentation levels very strictly and raises a syntax error whenever indentation is incorrect or inconsistent.
• It is common practice to use one tab per level of indentation (deeper nesting simply adds another level).
• In an if..else statement there are two blocks of statements, and the statements inside each block must be indented by the same amount of spaces or tabs.

Blocks in action

# Find the larger of two pre-specified numbers
p = 5
q = 6
if p > q:
    # Block 1 — runs only when the condition is true
    print("first number is larger")
    print("Bye")
else:
    # Block 2 — runs only when the condition is false
    print("second number is larger")
    print("Bye Bye")

second number is larger
Bye Bye
``` …