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I have seen this a lot in code, even vim marks it as a special case. #TODO and #FIXME are two other fix markers vim highlights but what does #XXX mean?

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    The question is asking about codetag or comment best practices in Python programming. There is Python Enhancement Proposal discussion on the subject. The question deserves to be retained on SO. It could have been worded a bit better though. Commented Sep 21, 2009 at 5:28

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XXX in a comment is usually a heads-up. It could be:

  • Something that's not implemented completely correctly.
  • Something that should be fixed later on.
  • Highlighting a possible problem spot.
  • Something you're not sure about, a question.

I've often preferred a more descriptive tag like FIXME or TODO or HACK. XXX is often used as a catch all for the above.

Searching for 'XXX' on the FreeBSD code cross reference is a good example of many of the uses. There are thousands...

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What's funny is that I use this on my own. XXX or #XXX basically trips the compiler and reminds me to go back on something. Generally pointer references or a previously unknown value or variable name.
This confirmed my original guess, It's simply a catch all tag to indicate other programmers to highlight that comment as something to look at.
No trying to troll, but "HACK" is not descriptive. In fact, it is very ambiguous. To me, it could mean at least 3 things.
@ЯрославРахматуллин in source code I would always read it in the "this is an ugly hack job, but seems to work well enough" sense - something you'd like to clean up later, but is not urgent. I've not used it (I typically use TODO or XXX instead), but that's how I would interpret it.
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  • NOTE: Description of how the code works (when it isn't self evident).
  • XXX: Warning about possible pitfalls, can be used as NOTE:XXX:.
  • HACK: Not very well written or malformed code to circumvent a problem/bug. Should be used as HACK:FIXME:.
  • FIXME: This works, sort of, but it could be done better. (usually code written in a hurry that needs rewriting).
  • BUG: There is a problem here.
  • TODO: No problem, but additional code needs to be written, usually when you are skipping something.

At least this is how I was taught about these tags. Basically the first two (NOTE and XXX) are used for information and no action is required. While the last three (FIXME, BUG and TODO) do require action. HACK is somewhere in between (and hardly ever used I think?).

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Good list. I also like LAZY (not as critical as FIXME or HACK) and OCD (known overengineering).
Do you really use XXX like this? I've come to see XXX as much higher priority, meaning "this is something that must be fixed before this code is even submitted to revision control". That's the way it's used inside google, automatically enforced, so you might see XXXs during code review but it's impossible to submit the code until the XXXs are gone. Longer-lived to-do notes can be marked with TODO, which are allowed to be submitted as long as someone's name or a bug id is attached.
My C editor also knows ??? and I use it for every piece of code I don't understand, usually if I wrote it myself long ago !!!
SETUP: on this line, I need to manually choose some option like True/False, or a Number, etc for some testing.
DEV: temporary setup in this line during the development, like: I commend (disable) the command, or change it, etc... to help my debug, after done, I remove
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Some notes from a June 2005 Python Enhancement Proposal that was rejected.

Choosing between FIXME and XXX is difficult.
XXX seems to be more common, but much less descriptive.
Furthermore, XXX is a useful placeholder in a piece of code
having a value that is unknown.

Thus FIXME is the preferred spelling.
Sun says that XXX and FIXME are slightly different, giving XXX higher severity.
However, with decades of chaos on this topic, and too many millions of
developers who won't be influenced by Sun, it is easy to rightly call them synonyms.

The PEP Starts with,

This PEP has been rejected. While the community may be interested,
there is no desire to make the standard library conform to this standard.

...

What Are Codetags?

Programmers widely use ad-hoc code comment markup conventions to serve as reminders of sections of code that need closer inspection or review. Examples of markup include FIXME, TODO, XXX, BUG, but there many more in wide use in existing software. Such markup will henceforth be referred to as codetags. These codetags may show up in application code, unit tests, scripts, general documentation, or wherever suitable.

The PEP is an interesting read.

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Have a look at PEP350. It explains all of TODO, XXX etc. I use it everyday when I can't remember exactly what one of the code tags means.

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Do you know if PEP350 is the origin of these labels? Do you know any older documents (from the Unix era) describing how to use these labels?
I stumbled on this thread while reading some docs from Oracle. The Oracle docs say they were last updated April 20, 1999. Not quite Unix era, but it pre-dates the Python docs. oracle.com/java/technologies/javase/…
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From (old) Java code conventions:

Use XXX in a comment to flag something that is bogus but works. Use FIXME to flag something that is bogus and broken.

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THis is the correct answer. I think it originated with Apache in the early days, but I haven't bothered to look that up.
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I use XXX because it's easier to type than TODO.

XXX is for when you're in a hurry and will get back to this yourself.

TODO is for when you have to hand it off to someone else.

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XXX means "I'm in a hurry and will get back to this myself" TODO means "This is an official part of a future backlog request that will be assigned to someone else." Those are the literal meanings.
And which RFC are those "literal meanings" in? Or is there any other citation for that?
@Randall: "citation"? Sorry, it's just my understanding after reading a lot of code.
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Probably it's for cases that you don't know how to handle.
Check out this: List view of TODO/FIXME/XXX/HACK statements

alt text
(source: googlecode.com)

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XXX is short for caveat which is slightly different from NOTE but quite similar to HACK. It may be a bug in a third party library / code which is being used and the code with // XXX: indicates that either it is workaround due to a bug in third party code or it could mean "caution" to someone looking / modifying the code to indicate why something is done in a certain way which otherwise may seem incorrect / inelegant in first look. HACK is generic term meaning a workaround for an issue which could be present in either your own code base or a third party library.

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I believe while FIXME is for the developer, and HACK is for the maintainer, XXX is for the user.

For example, if you ignore the XXX and call this function elsewhere, without understanding how it works, something unexpected can happen, and the person dealing with this issue will be unhappy (at least the one who added the XXX thinks so). You may think the problem will be gone if you just don't use this function.

But for FIXME, you will feel worthy to just fix the code to make it work. And for HACK, you may have no better choice even if you don't use it.

If you wrote XXX on your own code and someone used it, you may feel unhappy for reasons like you completely rewrote that code, and it then behave in completely different ways, and you broke someone else's code. But if you left a FIXME or TODO instead, you won't care so much.

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I use // XXX so I don't have to remember line numbers. Instead I just search for the XXX when I want to return to that piece of code.

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