IndentationError — Indentation Fix

An IndentationError is raised when Python encounters inconsistent or incorrect whitespace. Python uses indentation to define code blocks — every if, for, while, def, class, and with block requires indented code.

Description

Python enforces indentation at the syntax level. Unlike most languages that use braces {} for blocks, Python uses leading whitespace. This makes indentation both structural and functional.

Common error messages:

• unexpected indent — code is indented when it shouldn’t be.
• unindent does not match any outer indentation level — mixed tabs and spaces or inconsistent spacing.
• expected an indented block — a block header (if, def, etc.) is followed by no indented code.
• IndentationError: unindent may not match indentation level — tabs and spaces are mixed in confusing ways.

Python 3 does not allow mixing tabs and spaces. The standard convention is 4 spaces per indent level (PEP 8).

Common Causes

# Cause 1: Mixed tabs and spaces
def greet():
    name = "Alice"      # 4 spaces
    message = "Hello"   # 1 tab — ERROR

# Cause 2: Unexpected indent
x = 10
    y = 20  # IndentationError: unexpected indent

# Cause 3: Missing indented block after header
def calculate():
# IndentationError: expected an indented block after 'def'

# Cause 4: Inconsistent indent levels
if True:
    x = 1       # 4 spaces
      y = 2     # 6 spaces — ERROR

# Cause 5: Dedent to wrong level
def outer():
    def inner():
        x = 1
    return x  # ERROR — 'return' is at outer's indent level but references inner's variable

Solutions

Fix 1: Use consistent spaces everywhere (4 spaces per level)

# Wrong — mixed tabs and spaces
def greet():
    name = "Alice"
	message = "Hello"

# Correct — all spaces, 4 per level
def greet():
    name = "Alice"
    message = "Hello"

Fix 2: Configure your editor to use spaces

In most editors, set these options:

• VS Code: "editor.insertSpaces": true, "editor.tabSize": 4
• PyCharm: Settings > Editor > Code Style > Python > Tabs and Indents > set to 4 spaces
• Sublime Text: "tab_size": 4, "translate_tabs_to_spaces": true

Fix 3: Don’t indent code that isn’t inside a block

# Wrong
x = 10
    y = 20  # Nothing above this is a block header

# Correct
x = 10
y = 20

Fix 4: Always provide a body after block headers

# Wrong
def calculate():
print("doing nothing")

# Correct — either add a body or use 'pass'
def calculate():
    print("doing nothing")

# Or if you intentionally want an empty block:
def calculate():
    pass

Fix 5: Fix dedent alignment

# Wrong — return is at wrong level
def outer():
    def inner():
        x = 1
    return x  # Trying to return inner's x from outer

# Correct
def outer():
    def inner():
        x = 1
    return x  # This works because inner() is defined inside outer()

Fix 6: Use autopep8 to fix indentation automatically

# Install autopep8
pip install autopep8

# Auto-fix indentation issues in a file
autopep8 --in-place myfile.py

# Or preview changes without writing
autopep8 --diff myfile.py

Related Errors

• SyntaxError — broader category; IndentationError is a subclass of SyntaxError.
• TabError — specific to tab/space mixing (subclass of SyntaxError in Python 3).
• TypeError — runtime type mismatch, unrelated to whitespace.