IndentationError and TabError (tested)

IndentationError means a line's leading whitespace doesn't match what Python expected at that point — too much, too little, or ambiguous. There are three distinct IndentationError messages plus a separate TabError subclass for mixed tabs and spaces. We reproduced all four on a live interpreter so the messages below are exact, not paraphrased.

1. Four distinct errors, same root cause

All four come from inconsistent leading whitespace, but Python reports them differently depending on exactly what went wrong:

TabError is a subclass of IndentationError, but it's a separate exception name with its own message — code that catches IndentationError also catches TabError, but the printed error name is different, so searching for the exact string matters.

2. Cause 1: unexpected indent

This fires when a line is indented more than the line before it, but nothing (no : block) called for a new indentation level.

x = 1
    y = 2

  File "case1.py", line 2
    y = 2
IndentationError: unexpected indent

Fix: remove the extra leading whitespace so the line lines up with the statement above it. This is the most common case — usually a stray space or tab left over from pasting code, or an editor auto-indenting after a colon that was later deleted.

3. Cause 2: expected an indented block

Any line ending in : (if, for, while, def, class, try, with...) requires at least one indented statement under it — even pass counts, but nothing is not allowed.

if True:
pass

  File "case2.py", line 2
    pass
    ^^^^
IndentationError: expected an indented block after 'if' statement on line 1

Python 3.10+ names the specific statement and line number in the message (after 'if' statement on line 1); older Python 3 versions just say IndentationError: expected an indented block with no statement name. Fix: indent the line(s) under the colon, or add a placeholder pass if the block is intentionally empty (e.g. a stub you haven't written yet).

4. Cause 3: unindent does not match any outer indentation level

This fires when a line dedents to a depth that isn't any of the currently-open indentation levels — not "less indented", but indented to a width nothing else used. It's commonly worded as searching for "unexpected unindent", but the actual Python 3.13.5 message is the one below.

if True:
  x = 1
  if True:
    y = 2
   z = 3

  File "case3.py", line 5
    z = 3
         ^
IndentationError: unindent does not match any outer indentation level

Here the outer block is at 2 spaces and the inner block is at 4 spaces; z = 3 at 3 spaces matches neither. Fix: pick one consistent indent width (4 spaces is the PEP 8 standard) and make every dedent line up exactly with an enclosing line's indentation — don't eyeball it.

5. Cause 4: TabError (mixed tabs and spaces)

This is a different exception class from the three above. It fires when a file mixes literal tab characters and space characters in a way Python can't unambiguously resolve into one indentation depth.

if True:
	x = 1
        y = 2

Run cat -A to see the raw whitespace (^I is a literal tab, $ marks line end):

if True:$
^Ix = 1$
        y = 2$

  File "case4.py", line 3
    y = 2
TabError: inconsistent use of tabs and spaces in indentation

Fix: never mix tabs and spaces in the same file. Configure your editor to insert spaces when Tab is pressed, and convert existing tabs (most editors have a "convert indentation to spaces" command).

6. Fix checklist

• Pick one indent width for the whole project — 4 spaces (PEP 8) — and use it everywhere, including continuation blocks.
• Set your editor to insert spaces, not a tab character, when you press Tab. In VS Code: "editor.insertSpaces": true. In Vim: :set expandtab.
• Find mixed indentation before running: python3 -m tabnanny yourfile.py prints the file and line of the first ambiguous indent, with no traceback noise.
• See raw whitespace directly with cat -A yourfile.py (tabs show as ^I) or python3 -c "print(open('yourfile.py').readlines()[N-1])" for one specific line.
• Every : block needs at least one indented statement under it — use pass for an intentionally empty block instead of leaving it blank.

Written with AI assistance. Measured on 2026-10-03 with Python 3.13.5 on Linux using plain python3 one-liners. Every traceback below is copy-pasted output, not paraphrased. PlainHeaders is our extension; Python is free.