Python IndentationError

3IndentationError: explained step by step

An IndentationError means Python found a line whose leading whitespace doesn't match the block structure it expected. Python has no curly braces: the spaces at the start of a line are the structure. A line that ends with a colon opens a block, and every line inside that block must be indented by the same amount.

IndentationError is a special kind of SyntaxError. Python checks the layout of the whole file before it runs a single line, so a bad indent on line 40 stops your program even though lines 1 to 39 were fine. If you want a refresher on the layout rules, see Python syntax.

A small helper to trigger the error safely

A real indentation mistake is raised while the file is being compiled, so you can't catch it with a normal try around the broken line. In this lesson we hand the source code to compile() as a string and catch the error there. That lets every demo run without crashing the page.

def show_error(source):
    try:
        compile(source, "demo.py", "exec")
        print("ok: no indentation problem")
    except IndentationError as err:
        print(f"{type(err).__name__}: {err.msg} (line {err.lineno})")

show_error("price = 10\nprint(price)\n")
show_error("price = 10\n    print(price)\n")

How to read an IndentationError traceback

  File "main.py", line 3
    print("pass")
    ^
IndentationError: expected an indented block after 'if' statement on line 2

1. Read the last line first. It names the exact problem and often the statement that caused it.
2. The line number at the top points at the line Python could not accept. For a missing body that is the line after the colon, while the message tells you which header (here line 2) was waiting for it.
3. If the code looks perfectly aligned but still fails, the difference is invisible: a tab where you expected spaces. Turn on "show whitespace" in your editor.

Why "expected an indented block" happens

Every header that ends in a colon (if, elif, else, for, while, def, class, try, except, with) needs at least one indented line under it. If the next line starts at the same column, Python reports the header it was waiting on. Compare it with how if statements, for loops and functions are written.

def show_error(source):
    try:
        compile(source, "demo.py", "exec")
        print("ok")
    except IndentationError as err:
        print(f"{err.msg} (line {err.lineno})")

show_error("score = 80\nif score > 50:\nprint('pass')\n")
show_error("def greet(name):\nprint(name)\n")
show_error("for n in range(3):\nprint(n)\n")
show_error("for n in range(3):\n    print(n)\n")

The fix is always the same: indent the body by four spaces, or remove the header if you didn't mean to start a block.

An empty block needs pass, not just a comment

A comment is not a statement, so a block that contains only a comment is still empty. When you want to leave a body for later, write pass as a placeholder.

def show_error(source):
    try:
        compile(source, "demo.py", "exec")
        print("ok")
    except IndentationError as err:
        print(f"{err.msg} (line {err.lineno})")

show_error("def apply_coupon(cart):\n    # TODO later\n")
show_error("def apply_coupon(cart):\n    pass\n")

Why "unexpected indent" happens

The opposite problem: a line is indented but nothing opened a block above it. This usually comes from a stray space at the start of a line, or from pasting a snippet that carried its original indentation with it. A line only gets deeper after a line ending with a colon.

def show_error(source):
    try:
        compile(source, "demo.py", "exec")
        print("ok")
    except IndentationError as err:
        print(f"{err.msg} (line {err.lineno})")

show_error("total = 5\n    tax = 1\n")
show_error("total = 5\ntax = 1\n")

Why "unindent does not match any outer indentation level" happens

When a block ends, the next line must return to a column that an earlier line already used. If you step back to a column that was never used, Python can't tell which block you mean. Here the body uses 8 spaces, and the next line uses 4 spaces, which does not match the header's column (0) either.

def show_error(source):
    try:
        compile(source, "demo.py", "exec")
        print("ok")
    except IndentationError as err:
        print(f"{err.msg} (line {err.lineno})")

show_error("def total():\n        a = 1\n    b = 2\n")
show_error("def total():\n    a = 1\n    b = 2\n")

This also happens when an else is lined up one or two spaces away from the if it belongs to. Use the same number of spaces for every line at the same level.

Why a tab causes TabError

A tab and four spaces can look identical on screen, but Python treats them as different characters. If one block mixes them, Python raises TabError, a subclass of IndentationError.

mixed = "def total():\n    a = 1\n\tb = 2\n"
try:
    compile(mixed, "demo.py", "exec")
except IndentationError as err:
    print(type(err).__name__ + ":", err.msg)
    print(repr(mixed.split("\n")[2]))

Printing a line with repr() shows \t for a tab, which makes the invisible visible. Pick four spaces everywhere, as recommended by Python's style guide.

Nested blocks and copy-paste

Each level of nesting adds four more spaces. When you paste code into a loop, the whole pasted chunk must be shifted together, not just its first line. Most editors do this with Tab and Shift+Tab on a selection. Deeper structures such as nested loops make the rule easy to see: the inner body is always four spaces deeper than its header.

Editor settings that prevent these errors

• Turn on show whitespace (dots for spaces, arrows for tabs).
• Set the editor to insert spaces when you press Tab, with a width of 4.
• Use Convert indentation to spaces when a file with tabs arrives.
• Select pasted code and re-indent it as one block instead of fixing lines one at a time.