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

"Martin v. Löwis" martin at v.loewis.de
Sun Feb 19 07:05:11 CET 2006

Raymond Hettinger wrote:

Also, I think haskey/in should return True if there is a default. * if contains always returns True, then it is a useless feature (since scripts containing a line such as "if k in dd" can always eliminate that line without affecting the algorithm).

If you mean "if contains always returns True for a default dict, then it is a useless feature", I disagree. The code using "if k in dd" cannot be eliminated if you don't know that you have a default dict.

* if defaultdicts are supposed to be drop-in dict substitutes, then having contains always return True will violate basic dict invariants: del d[somekey] assert somekey not in d

If you have a default value, you cannot ultimately del a key. This sequence is not a basic mapping invariant. If it was, then it would be also an invariant that, after del d[some_key], d[some_key] will raise a KeyError. This kind of invariant doesn't take into account that there might be a default value.

Regards, Martin

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