Very nice! I've been a bit leery though of defaultdict since it contributed to two different bugs in http://norvig.com/spell-correct.html -- in less than a page of code, by a first-rate programmer, which was read by many thousands of others for years before anyone noticed the second bug. I'm not saying don't use this, I just have a little warning light blink on when I see 'defaultdict' anywhere.
Both of those errors seem like fundamental category errors. He's using a dictionary construct which effectively makes it so that every key has a value, but then relies on dictionary membership when testing. This happens to work, but it makes little sense. (One could argue that 'foo in bar' should always be True if bar is a defaultdict.) Seems to me like he should have checked the value of the word rather than using 'in'.
You make a good point about this being written by a known intelligent person and seen by many others who didn't see the bugs, but I think it's ultimately a lesson in thinking about semantics rather than behavior. This code relies too much on the behavior of defaultdict rather than just what such a thing means. As long as what it means is kept in mind I think it should be safe.
At some level it was sloppy thinking, yes; most bugs have some obvious-in-retrospect reason they were stupid. You may be right about how to think about this one.
I've seen other defaultdict bugs -- this is just the case with the highest eyeball-count-times-attention product I can point to.
If this bug was simply due to having the wrong concept of defaultdict, and if the others you saw were the same, then you can avoid the bugs forever by just having the right concept. Lots of ifs there, of course.
