4.4 Indentation

Importance

What Is It?

In Python, indentation is not just a style preference — it is the syntax that defines block boundaries, replacing the {} braces used by languages like C or Java.

Why Does It Matter?

Getting indentation wrong doesn’t just look bad, it changes (or breaks) what the code actually does.

Indentation Rules

• Use spaces, not tabs (PEP 8 recommends 4 spaces per level) — mixing tabs and spaces raises a TabError
• Every statement inside the same block must use the exact same indentation
• A block is introduced by a line ending in a colon (:) and must be indented relative to that line

Nested Blocks

What Is It?

A block inside another block (like an if inside a for inside a function), indicated by increasing indentation for each level of nesting.

How Is It Used?

Each additional level of nesting adds one more increment of indentation.

def check_status(code):
    if code == 200:
        print("OK")
        if code < 300:
            print("Success range")
    else:
        print("Error")

Common Indentation Errors

The most common beginner error is inconsistent or missing indentation after a colon:

>>> if True:
... print("bad")   # missing indentation after the colon
  File "<stdin>", line 2
    print("bad")
IndentationError: expected an indented block after 'if' statement on line 1

Quick Interview Answer

“Python uses indentation instead of braces to mark blocks — it’s part of the language grammar, not a style preference. Every statement in the same block must share identical indentation, spaces and tabs can’t be mixed (that raises a TabError), and PEP 8’s standard is 4 spaces per level.”

Common Mistakes

• Mixing tabs and spaces in the same file — raises a TabError in Python 3 rather than silently guessing.
• Using inconsistent indentation widths between sibling statements in the same block, which raises an IndentationError.
• Forgetting to indent at all after a line ending in :.