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

Michael Urman murman at gmail.com
Sun Feb 19 17:23:15 CET 2006

On 2/19/06, Josiah Carlson <jcarlson at uci.edu> wrote:

My post probably hasn't convinced you, but much of the confusion, I believe, is based on Martin's original belief that 'k in dd' should always return true if there is a default. One can argue that way, but then you end up on the circular train of thought that gets you to "you can't do anything useful if that is the case, .popitem() doesn't work, len() is undefined, ...". Keep it simple, keep it sane.

A default factory implementation fundamentally modifies the behavior of the mapping. There is no single answer to the question "what is the right behavior for contains, len, popitem" as that depends on what the code that consumes the mapping is written like, what it is attempting to do, and what you are attempting to override it to do. Or, simply, on why you are providing a default value. Resisting the temptation to guess the why and just leaving the methods as is seems the best choice; overriding contains to return true is much easier than reversing that behavior would be.

An example when it could theoretically be used, if not particularly useful. The gettext.install() function was just updated to take a names parameter which controls which gettext accessor functions it adds to the builtin namespace. Its implementation looks for "method in names" to decide. Passing a default-true dict would allow the future behavior to be bind all checked names, but only if contains returns True.

Even though it would make a poor base implementation, and these effects aren't a good candidate for it, the code style that could best leverage such a contains exists.

Michael

Michael Urman http://www.tortall.net/mu/blog

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