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Python IndentationError explained

An IndentationError is one of the first Python errors a new programmer meets, and its message rarely says what to change. This page reads each message the way Python means it, shows how Python actually counts the spaces at the start of a line, and gives a routine that fixes the error in any editor a US school uses, from IDLE and VS Code to a notebook in the browser. Every error message below comes from the Python documentation or from our own run of Python 3.13.6, and each one is labelled.

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In short

A Python IndentationError means the spaces at the start of a line do not fit the block structure Python expects: a block after a colon is missing, a line is indented for no reason, or a line steps back to a level that never existed. TabError, its subclass, means tabs and spaces were mixed. Re-indent the block with 4 spaces per level. Modern Age Coders teaches Python live to students from Grade 3 up, in a batch or one to one.

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The messages

Five messages, one family: what each IndentationError is telling you

Python reports every indentation problem as an IndentationError or its stricter cousin, TabError. The Python documentation describes IndentationError as the "Base class for syntax errors related to incorrect indentation" and notes that it is a subclass of SyntaxError, which is why the program stops before a single line runs.

The messages, as printed by our run of Python 3.13.6 on 17 October 2026, and what each one means
MessageWhat Python foundThe usual fix
expected an indented block after 'for' statement on line 2A line ending in a colon (for, if, while, def, class, else) with no indented line under itIndent the body 4 spaces, or write pass if the body is not ready
unexpected indentA line pushed right when nothing above it opened a blockMove the line back to the level of the line above
unindent does not match any outer indentation levelA line stepped back to a column no earlier line usedLine it up exactly with the block it belongs to
inconsistent use of tabs and spaces in indentationTabs and spaces mixed so that the meaning depends on tab width (this one is a TabError)Convert the whole file to spaces
expected an indented block after function definition on line 1A def with only a comment, or nothing, underneathAdd the body, or pass as a placeholder

Why the line number can point at the wrong line

Python reports the line where it noticed the problem, not the line you need to edit. For "expected an indented block", the line printed is the first line that should have been indented, and the line named at the end of the message is the one with the colon. Read both.

When the block is empty and the file ends, our run pointed at a comment line: a line that holds only a comment is ignored by Python (the documentation says such a line "is ignored"), so a comment can never be the body of a loop or a function.

Versions word it slightly differently

Python 3.10 added the extra context. Its release notes show the message as "IndentationError: expected an indented block after 'if' statement in line 2", with the word "in".

Our run of Python 3.13.6 printed "on line 2" instead. If your school laptop runs an older Python, the message may be shorter, but the meaning and the fix are the same.

Sources, read 17 October 2026: Built-in Exceptions, What is new in Python 3.10, and our own runs of Python 3.13.6 on Windows 11 (file paths shortened to the file name). We are not connected with the Python Software Foundation.

Broken, then fixed

The most common case: a loop with no body

A student counts quiz answers, presses Run, and gets an error before anything prints. The loop line ends in a colon, which promises a block, and the next line breaks the promise.

Broken · the loop body sits at the left marginpython
# quiz_score.py  (broken on purpose, ours)
score = 0
for answer in ["a", "b", "c"]:
score = score + 1
print(score)

# Our run, Python 3.13.6:
#   File "quiz_score.py", line 3
#     score = score + 1
#     ^^^^^
# IndentationError: expected an indented block
#   after 'for' statement on line 2

The caret sits under the first word of line 3, the line that should have moved right. The end of the message names line 2, the line that opened the block.

Fixed · four spaces put the line inside the looppython
# quiz_score.py  (fixed, ours)
score = 0
for answer in ["a", "b", "c"]:
    score = score + 1   # 4 spaces: inside the loop
print(score)            # back at 0: after the loop

# Our run prints: 3

The print line stays at the left margin on purpose: it runs once, after the loop. Indent it too and the program prints 1, 2 and 3 on separate lines, a logic change rather than an error.

Indentation is meaning, not decoration

The Python tutorial puts it plainly: "indentation is Python’s way of grouping statements." In many languages braces mark a block and spacing is only style. In Python, moving a line four spaces changes which block it belongs to.

That is why the fixed program above and a version with print indented are both valid Python but do different things. The error only appears when the spacing makes no sense at all.

What to check first

Find the line that ends in a colon just above the error. Every line that should run as part of that loop, if or function must start further right than the colon line, and all of them by the same amount.

The tutorial states the same rule: "each line within a basic block must be indented by the same amount." Three spaces on one line and four on the next is enough to stop the program.

How Python counts

How Python decides a line is inside a block: the indentation stack

The Python language reference describes the exact rule, and once a student sees it, "unindent does not match any outer indentation level" stops being mysterious. Python keeps a stack of indentation levels, starting with a single zero, and compares each new line against the top of it.

Broken · a return that matches no levelpython
# grade.py  (broken on purpose, ours)
def grade(points):
    if points >= 90:
        return "A"
      return "B"     # 6 spaces: matches no level

# Our run, Python 3.13.6:
#   File "grade.py", line 4
#     return "B"
#               ^
# IndentationError: unindent does not match
#   any outer indentation level

Line 4 starts at column 6. Python has only seen levels 0, 4 and 8, so it cannot tell which block the return belongs to.

Our walk-through of grade.py using the stack rule in the Python language reference (2.1.8 Indentation)
LineLeading spacesStack before the lineWhat Python does
1 def grade(points):0[0]Equal to the top: nothing happens
2 if points >= 90:4[0]Larger: push 4, one INDENT
3 return "A"8[0, 4]Larger: push 8, one INDENT
4 return "B"6[0, 4, 8]Smaller, but 6 is not on the stack: IndentationError

The rule in the reference

The reference says that when a line is indented less than the one before, its level "must be one of the numbers occurring on the stack". Python pops levels until it reaches that number, producing one DEDENT for each.

Moving line 4 to 4 spaces makes it a DEDENT back to the if level, which means "otherwise, return B". Our run of the fixed file prints A B for grade(95) and grade(70).

Where tabs break the count

Tabs are worth a variable number of spaces: the reference says each tab is replaced by one to eight spaces so that the column becomes a multiple of eight. A tab at the start of a line counts as 8; four spaces and then a tab also count as 8.

Two lines can therefore look identical on screen and mean different things. Python refuses to guess: if the meaning depends "on the worth of a tab in spaces", it raises TabError.

The fix routine

Eight steps that fix any indentation error

This is the routine our teachers walk students through, in this order. It works in IDLE, VS Code, PyCharm and browser notebooks, because it relies on Python's rules rather than any one editor.

  1. Read the last line of the error first. It names the error type and, for a missing block, the line with the colon.
  2. Go to the line the caret points at, then look one line up. The problem is almost always between those two lines.
  3. Find the block owner. That is the nearest line above ending in a colon: for, while, if, elif, else, def, class, try, except or with.
  4. Count the spaces. Put the cursor at the start of each line in the block and read the column number most editors show in the status bar. Every line of one block must start in the same column.
  5. Use 4 spaces per level. The Python tutorial says "Use 4-space indentation, and no tabs." PEP 8, the Python style guide, says the same: "Use 4 spaces per indentation level."
  6. Leave nothing empty. If a loop or function body is not written yet, type pass on an indented line. A comment alone does not count.
  7. Hunt for tabs. If the error is a TabError, or the spacing looks right but Python disagrees, run python -m tabnanny on the file: it prints the line number and the line it finds ambiguous.
  8. Run again and read again. Fixing one indentation error often reveals the next one lower down, because Python stops at the first problem it finds.
Fixed · pass holds the place of an unwritten bodypython
# Unfinished function: Python needs a statement
def greet():
    pass    # placeholder until you write the body

print("hi")

# Our run prints: hi
# Without the pass line, Python 3.13.6 says:
# IndentationError: expected an indented block
#   after function definition on line 1

The Python tutorial describes pass as a statement that "does nothing" and can be used "when a statement is required syntactically but the program requires no action."

What tabnanny printed for us

We ran python -m tabnanny on two of our broken files. It printed e4.py 3 followed by the line that began with a tab, and e7.py 4 followed by the line that used 8 spaces under tab-indented lines. That is the file, the line number and the exact line, which is all a student needs.

The Python documentation describes tabnanny as a tool for the "Detection of ambiguous indentation", intended for the time being to be called as a script.

What not to do

Do not press space until the error goes away. The error may vanish while the logic breaks, for example when a line that should run once ends up inside a loop.

Do not paste a whole file into an AI chatbot and accept whatever comes back. We ask students to explain each block out loud first, because putting a block into words makes a misplaced line easier to spot.

Practice set

Seven snippets: predict the message, then check

Our own practice set, written for this page and run on Python 3.13.6. Cover the right-hand column, decide what Python will say, then compare. It is practice for the idea, not anybody's homework.

Practice set (ours): seven short programs and what our run of Python 3.13.6 printed
#The program, line by lineWhat Python said
1for i in range(3): / print(i) at the left marginexpected an indented block after 'for' statement on line 1
2total = 0 / then total = total + 5 with one leading spaceunexpected indent
3if True: / 4 spaces print("yes") / 2 spaces print("done")unindent does not match any outer indentation level
4while False: / 4 spaces # nothing yetexpected an indented block after 'while' statement on line 1
5def hello(): / 4 spaces then a one-line docstringNo error: a docstring is a statement, so the body is not empty
6class Pet: / a blank line / print("ok")expected an indented block after class definition on line 1
7if name == "Ava": / 4 spaces print("Hi Ava") / else: / print("Who are you?") at the marginexpected an indented block after 'else' statement on line 4

Snippet 4 is the trap

The body holds only a comment, and Python ignores comment-only lines, so the loop has no body at all. Replace the comment with pass, or keep the comment and add pass under it.

Snippet 5 is the surprise

A string on its own line is a real statement, so a function whose body is only a docstring is valid. Many students expect an error here; seeing why there is none teaches what a block really needs.

Snippet 7 is the classic

else opens its own block, so it needs its own indented line. It is easy to fix the if half and forget that the else half follows exactly the same rule.

In your editor

Where indentation errors come from in school editors

US students write Python in whatever their class uses: IDLE on a school laptop, VS Code at home, a notebook in Google Colab, or a browser editor the teacher assigned. The rules are Python's, but each setting has its own usual cause.

Common causes by setting (our teaching notes; the rules themselves come from the Python documentation)
Where you write PythonThe usual causeWhat to check
Code copied from a web page or a PDFNon-standard spaces, or tabs mixed with spacesDelete the indentation and retype it with the Tab key in an editor set to spaces
A notebook cell (Colab or Jupyter)A cell pasted half from one source, half from anotherRe-indent the whole cell, then run that one cell
The interactive prompt (the >>> shell)A compound statement not finished with a blank lineThe tutorial says a compound statement typed interactively "must be followed by a blank line"
Two editors on one fileOne saves tabs, the other saves spacesRun python -m tabnanny, then convert the file to spaces
A long expression across linesWorry that continuation lines need block indentationLines inside brackets may be indented freely; the reference says their indentation "is not important"

Make the editor help you

The tutorial notes that "all decent text editors have an auto-indent facility." Let the editor indent after a colon, and use its option to show whitespace, so a tab and four spaces stop looking the same.

Many editors can convert a file's indentation to spaces in one command. Do that once per file and TabError disappears for good.

A US classroom habit worth keeping

A student moving from block coding in elementary school to typed Python in middle or high school loses the visual nesting that blocks gave them. Indentation is that same nesting, written with spaces.

Students who go on to Java, the language of our AP Computer Science A course, keep the same habit: Java marks blocks with braces, but well-indented Java is far easier to read and debug.

Rules quoted from the Python tutorial, the language reference and PEP 8, read 17 October 2026. PEP 8 also says "Python disallows mixing tabs and spaces for indentation."

Learning it properly

What a teacher changes: from fixing errors to reading structure

Fixing one error at midnight is useful. Understanding blocks well enough that the errors stop is better, and that is what our classes aim for.

How we teach blocks across a first term of Python (our plan, adjusted for each learner)
ClassesWhat the student doesWhat it teaches about indentation
1 to 2First programs with if and elseA colon opens a block; the block is everything indented under it
3 to 4Loops that count and totalWhich lines run every time, and which run once after
5 to 6Functions with returnWhere a function ends, and why a return can end it early
7 to 8Nested loops and conditionsReading three levels at once; the stack walk-through above
9 to 10A small project of their choiceKeeping a long file consistent: 4 spaces, no tabs
11 to 12Reading other people's codeExplaining a program block by block, out loud

We teach the concept so a student can solve their own school assignments; we do not write graded work for anyone. For a wider method that covers every kind of bug, see how to debug code, and for errors that happen while a program runs, exception handling in Python.

Where a student is

Four stages of reading indentation

Find the row that sounds like you. The goal is stage 4.

Stages of understanding Python blocks (ours)
StageWhat it looks like
1. CopierSpacing matches an example, but the student cannot say why
2. FixerCan clear an IndentationError by trial, sometimes breaking the logic
3. ReaderCan say which lines belong to which block before running
4. DesignerPlans blocks first, and indentation errors almost never appear

For parents

Indentation errors are a sign your child is writing real Python, not a sign they are bad at it. Every programmer has met them.

Ask your child to explain one block of their code to you. If they can, they are past stage 2.

For students

Read the colon line and the line under it before touching anything. The message is pointing at one of the two.

Pick 4 spaces and never mix in a tab. That one habit removes TabError for good.

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Questions students ask

Python indentation errors: common questions

What does IndentationError mean in Python?

It means the spaces at the start of a line do not fit the block structure Python expects. The Python documentation describes IndentationError as the base class for syntax errors related to incorrect indentation, and it is a subclass of SyntaxError, so nothing runs until it is fixed.

How do I fix "expected an indented block"?

Find the line ending in a colon that the message names, and indent the line under it by 4 spaces. If the body is not written yet, put pass on an indented line. A comment on its own does not count as a body.

What does "unexpected indent" mean?

A line starts further right than it should, even though nothing above it opened a block. Move it back to line up with the line before it.

What does "unindent does not match any outer indentation level" mean?

A line stepped back to a column that no earlier line used, so Python cannot tell which block it belongs to. Line it up exactly with the block it should join, usually 4 spaces per level.

What is a TabError?

It is a subclass of IndentationError, raised when indentation mixes tabs and spaces inconsistently. Convert the whole file to spaces. Running python -m tabnanny on the file shows the line it finds ambiguous.

Should I use tabs or spaces in Python?

Spaces. The Python tutorial says to use 4-space indentation and no tabs, and PEP 8, the Python style guide, calls spaces the preferred indentation method.

Why do I get an indentation error in Google Colab?

Colab runs ordinary Python, so the same rules apply inside each cell. The usual cause is pasted code with mixed spacing. Re-indent the whole cell with 4 spaces per level and run that cell again.

Can you just fix my homework code?

No. We teach the concept so you can solve your own assignments, and we never write graded work. In a lesson, a teacher will walk through your error with you until you can fix it yourself.

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