Comments

date:2006-08-23 16:22:27
category:Building Skills

In a recent code review, I saw a bizarre habit. The author defended it so vehemently, I gave up on trying to explain the needless confusion it caused. The author was one of those “I always do it this way, and I’m always right.” people, so nothing was gained by pursuing it.

Here’s the practice.

def myFunction( someArgs ):
   """Synopsis...

   Formal Definition...

   Some Design Decisions...

   Overview of Processing:
   1.  Step 1, a summary
   2.  Step 2, a summary
   3.  Step 3, a summary
   4.  Step 4, a summary
   """
   # 1. Step 1, a summary
   the actual code
   # 2. Step 2, a summary
   the actual code
   # 3. Step 3, a summary
   the actual code
   # 3. Step 4, a summary
   the actual code

Besides the silly redundancy in the numbers, there was the total redundancy of the comment sequence.

I asked why the comments were duplicated, and got a strange non-answer. The rework eliminated the in-the-code comments, and kept the in-the-comment-block comments, separated from the code.

I was shocked by this, and had to ask – rhetorically – Who is the audience for the comments?

The answer was something like: “Comments help me, the author, know what to expect in the body of the function.”

I was dumb-founded. I thought my question was rhetorical. I thought everyone knew that comments were for the benefit of maintainers. Further, I thought everyone knew that the distance between a comment and the code was inversely proportional to the accuracy of the comment: the further the comment was from the code, the less likely that the two will agree. I’m sure this is Someone’s law of code-to-comment distance, but I can’t find it anywhere.

Boy was I wrong. I was glad to see Celenko’s Comments Fix Bugs post, it gives me a good reference on what comments are all about.

  1. It’s a good policy to write an overview as comments. It seems simplest to then supplement those comments with the actual code. In my case, the author must have copied and pasted the comments from the comment block to begin making the code. Why the extra step?
  2. It’s a good policy to say things only once. If you say something twice, one of the two may be wrong. In the case of a comment – a summary or interpretation – it isn’t completely correct. If you say something three times, now what are you supposed to believe? In my case, the author didn’t seem to realize that someone else would do maintenance and – very likely – make one or more of the comments disagree with the code.
  3. As Celenko points out, if the comment doesn’t match the code, you’d best think about that situation. When they’re cheek by jowls, that thinking is relatively easy. When they’re separated (or in several places) that thinking is now impossible. How, precisely, did the author expect a maintainer to do a three-way diff between two comments and the code?
  4. Comments (indeed all code) is knowledge capture for the benefit of users and maintainers. The author has the knowledge; the point is to present the knowledge to others. First, of course, it must be correct and complete. Succinct has tremendous value, also. Simple redundancy (via cut and paste of all the rotten techniques) is a make-work procedure that doesn’t create any recognizable value.

I guess that proper use of comments isn’t obvious to some programmers. Why not?

Weinberg, in The Psychology of Computer Programming noted that programmers don’t spend enough time reading code. Jeff Raskin, in ACM Queue (“The Woes of IDEs”), noted that one consequence of this is that some IDE’s make comments a painful exercise. It’s as if the source doesn’t really matter very much.

Maybe I’m confused by my own preference for Literate Programming, but I think code (including comments) can be correct, complete, succinct and still meaningful to authors and their audience. In this era of open-source software, it’s disappointing that every programmer hasn’t read enough source to see what constitutes good practice and bad practice.

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