defaultdict is danger

Simply looking at a defaultdict dictionary can change its value and break things.

Suppose you pass your defaultdict to a function that says:

try:
    return d[name]
except KeyError:
  pass

It will add an empty value to the dictionary unexpectedly.

I'm against defaultdict nearly always, and that's because you get a thing that looks like a dict and can get passed around like a dict but isn't a dict in a very important way.

I see code snippets like collection[key] hundreds of times every day. Nearly always, this snippet does not change collection and raises an exception if key is not in collection.

If this is not true, I have to stop and think about it, which wastes my time, but sometimes I'll miss that, and perhaps make a mistake.

Using defaultdict is a little less work for the writer, but considerable more effort for the reader. But code is written once and read a hundred times.

Down with defaultdict!