Python IndentationError: how to fix it online

In JavaScript, braces decide where a block starts and ends. In Python, indentation is the block. If the spaces are wrong, the file is not a valid program. You get IndentationError or TabError before any print runs.

Treat the error as a layout problem, not a logic problem. Fix alignment first. Guessing about variables will not help if the parser never built a tree.

Code Arena’s Python editor uses a monospace font so columns line up. Prefer spaces. If a paste from chat mixed tabs and spaces, re-indent the whole block instead of nudging one character.

A line after if, for, while, def, or class must be indented. A line that continues the outer program must return to the previous level. else must line up with its if, not with the body.

Empty blocks need pass. A comment alone is not a body. Students who delete a print and leave a bare if often see an unexpected indent on the next line.

Copying from slides is a frequent source of mixed tabs. If Output mentions TabError, select the block and apply a consistent indent rather than hunting for the hidden tab.

• Every colon (:) starts a deeper block
• The next statement after the block lines up with the if/for/def
• Do not mix tabs and spaces
• Empty blocks need pass — a comment is not enough

Python often points at the first surprising indent. That line may be the victim, not the cause. Look at the line above it. Did you forget a colon? Did you indent an else that belongs with an if?

Once Output is clean, add the next statement. Indentation bugs love to return when you paste a second function under the first.

If you are stuck, delete the body, write pass, Run, then rebuild the body one line at a time. The sandbox is cheap. Use it.

Open the starter snippet and Run it to read IndentationError. Indent the print one level and Run again. Then add an else that is accidentally indented too far and compare the new message.

That five-minute drill is enough for most homework indent failures.