Question

Several times (even several in a row) I’ve been bitten by the defaultdict bug: forgetting that something is actually a defaultdict and treating it like a regular dictionary.

d = defaultdict(list)

...

try:
  v = d["key"]
except KeyError:
  print "Sorry, no dice!"

For those who have been bitten too, the problem is evident: when d has no key ‘key’, the v = d["key"] magically creates an empty list and assigns it to both d["key"] and v instead of raising an exception. Which can be quite a pain to track down if d comes from some module whose details one doesn’t remember very well.

I’m looking for a way to take the sting out of this bug. For me, the best solution would be to somehow disable a defaultdict’s magic before returning it to the client.

Answer 1

You can prevent creation of default values by assigning d.default_factory = None. However, I don’t quite like the idea of object suddenly changing behavior. I’d prefer copying values to the new dict unless it imposes severe performance penalty.