IndentationError: unindent does not match any outer indentation level

What you see

def total(prices):
    result = 0
    for price in prices:
        result += price
  return result

print(total([3, 4]))

  File "/home/me/project/main.py", line 5
    return result
                 ^
IndentationError: unindent does not match any outer indentation level

What it means

Python remembers the depth at which each open block started: 0, 4 and 8 spaces in the example. When a line moves back to the left, it must land exactly on one of those depths. The return sits at 2 spaces, which matches none of them.

The error points at the line that moved back, and that is the line to fix.

Why it happens

• One or two stray spaces, often after editing by hand.
• Tabs and spaces that look aligned but are not. A tab can display as two, four or eight columns, depending on the editor.
• Code pasted from somewhere with a different indentation width.

How to fix it

Move the line to a depth that exists. Here return belongs to the function, so it goes back to 4 spaces:

def total(prices):
    result = 0
    for price in prices:
        result += price
    return result

When the line looks right but still fails, make whitespace visible. Most editors have a "render whitespace" setting. In a terminal, cat -et main.py on macOS or cat -A main.py on Linux shows tabs as ^I.

Python ships a checker for ambiguous indentation:

python3 -m tabnanny main.py

How each Python version words it

Run on CPython 3.9.6, 3.10.22, 3.11.17, 3.12.15, 3.13.17, 3.14.9 and 3.15.1, on October 25, 2026.

The wording has been the same for every version here.