💬 A junior's Python script won't run and the indentation looks fine to me

A junior at my office sent me a 40 line Python script last night. It wouldn't run. The error said IndentationError, unindent does not match any outer indentation level. Line 27. I opened the file and stared at line 27 for 20 minutes. It looked perfect. Same spacing as the line above, same spacing as the line below. I felt stupid. I've written Python for years, and I know the usual suspects. Missing colon, wrong block, bad copy paste. I checked all three. Then I deleted the line and typed it again by hand. Same error. The junior said, sir, I only copied it from a tutorial site and fixed 2 lines myself. That's the part I keep thinking about. He didn't type that whole block himself, and neither did I. At 11pm I gave up and went to sleep, which is never a good sign. This morning I opened the file in a different editor and the error was still sitting there. Everything lined up on screen, but the file clearly disagrees with my eyes, and the file wins. I don't want to just say it's probably tabs and send him away. I want to understand it properly. So here's my question for people who've been through this. How do you find the bad character? Do you turn on visible whitespace, or run some check that points straight at the bad line? And can one single line break a whole file if the rest of it looks completely fine? I'd like to give him a proper method he can reuse, not a guess. He's 22 and keen.

Python ships with a small checker for exactly this, called tabnanny. You run it on the file from the terminal. It reports lines where tabs and spaces are mixed in a way that could confuse the interpreter. A senior showed me that when I had a similar tutorial script that wouldn't run. I still turn on visible whitespace too. Did your junior copy from a page or a PDF? PDFs add odd characters.

On your method question, turn on Render Whitespace in your editor. Tabs show up as arrows and spaces as small dots. One stray arrow in a block of dots jumps out straight away. You'll spot line 27 in seconds. Then run the editor's convert indentation to spaces command on the whole file so it's all consistent. That fixed the first script I ever pasted from a tutorial site.

Tabs mixed with spaces is the usual cause. Python treats them differently, and tutorial code often carries tabs. Another cause is lines in one block indented differently. A missing colon after if, for or def can also do it. For the unindent message you quoted, one line is probably off by a space or a tab. PEP 8 suggests 4 spaces per level, so I stick to that.