[Python-Dev] Proposal: defaultdict (original) (raw)

Raymond Hettinger raymond.hettinger at verizon.net
Sun Feb 19 07:33:42 CET 2006

[Martin v. Löwis]

If you have a default value, you cannot ultimately del a key. This sequence is not a basic mapping invariant.

You believe that key deletion is not basic to mappings?

This kind of invariant doesn't take into account that there might be a default value.

Precisely. Therefore, a defaultdict subclass violates the Liskov Substitution Principle.

Of course, the del followed contains sequence is not the only invariant that is thrown-off. There are plenty of examples. Here's one that is absolutely basic to the method's contract:

k, v = dd.popitem()
assert k not in dd

Any code that was expecting a dictionary and uses popitem() as a means of looping over and consuming entries will fail.

No one should kid themselves that a default dictionary is a drop-in substitute. Much of the dict's API has an ambiguous meaning when applied to defaultdicts.

If all keys are in-theory predefined, what is the meaning of len(dd)?

Should dd.items() include any entries where the value is equal to the default or should the collection never store those? If the former, then how do you access the entries without looping over the whole contents? If the latter, then do you worry that "dd[v]=k" does not imply "(k,v) in dd.items()"?

Raymond

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