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[Python-Dev] Proposal: defaultdict

"Martin v. Löwis" martin at v.loewis.de
Sun Feb 19 07:05:11 CET 2006 
Raymond Hettinger wrote:
>>>Also, I think has_key/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[some_key]
>    assert some_key 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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