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A colleague recently asked the question, "Are single letter variable names evil?". He was trolling, and I kind of knew it, but gosh, what an excuse to rant about code clarity! My comments ended up being multiple pages long, so taking one of Mary Gardiner's suggestions to heart (although I'm sure she regrets it now), I've turned it into a blog post.

In a nutshell, I don't think single variable names are evil. Lots of the time they are a bad idea though.

The classic rule of thumb I've heard is that "the length of the variable name should be proportional to the log of the size of the scope". I can't find who said that originally. My [best guess is Mike Haertel](http://www.jetcafe.org/jim/c-style.html). It seems to be a good rule of thumb.
But the corollary is that sometimes when the code is confusing, the problem isn't short variable names, it's that the scope is too large.
Perhaps there's also something to be said for shorter names being more useful for really generic code? I'm OK with an implementation of `map` taking either `f` and `xs` or `function` and `things`. I don't think either is better or worse.
And maybe single letter variable names are acceptable when they are the standard terms for the problem domain. `(x, y)` might be clearer than `(horizontal_position, vertical_position)`. 
Finally, Python's idiomatic filtering strongly encourages short names, because you have to repeat it three times within the scope of one expression:
[x for x in meaningfully_named_list_of_things if x.relevance > THRESHOLD]
vs:
[meaningful_name

 for meaningful_name in meaningfully_named_list_of_things

 if meaningful_name.relevance > THRESHOLD]
It's probably subjective, but I think the second one is less clear, since it obscures what's going on. Bice Dibley points out that in these list comprehensions, `x` functions much like a pronoun in English.
As is the case when writing prose, being clear is hard. Rules can help – taking knowledge and distilling it into formal principles is one of the best ways for us to progress as a species! – but as Orwell said:

Break any of these rules sooner than say anything outright barbarous.

(And [asking](http://mumak.net/stuff/your-code-sucks.html) [someone](http://blog.labix.org/2013/02/06/ethics-for-code-reviewers) to [look over](http://www.codinghorror.com/blog/2006/01/code-reviews-just-do-it.html) it soon will probably spare [troubles later](http://www.osnews.com/story/19266/WTFs_m))