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

Adam Olsen rhamph at gmail.com
Tue Feb 21 04:55:22 CET 2006

On 2/20/06, Jim Jewett <jimjjewett at gmail.com> wrote:

Adam Olsen asked: > ... d.getorset(key, func) would work in your use cases?

It is an improvement over setdefault, because it doesn't always evaluate the expensive func. (But why should every call have to pass in the function, when it is a property of the dictionary?)

Because usually it's a property of how you use it, not a property of the dictionary. The dictionary is just a generic storage mechanism.

[snip] In other words, the program would work correctly if I passed in a normal but huge dictionary; I want to avoid that for reasons of efficiency. This isn't the only use for a mapping, but it is the only one I've seen where KeyError is "expected" by the program's normal flow.

Looking at your explanation, I agree, getorset is useless for that use case.

However, I'm beginning to think we shouldn't be comparing them. defaultdict is a powerful but heavyweight option, intended for complicated behavior. getorset and setdefault are intended to be very lightweight, even lighter than the "try/except KeyError" and "if key not in X: X[key] = default" memes we have right now. getorset's factory function is only appropriate for preexisting functions, not user defined ones.

Essentially, I believe getorset should be discussed on its own merits, independent of the addition of a defaultdict class. Perhaps discussion of it (and the deprecation of setdefault) should wait until after defaultdict has been completed?

-- Adam Olsen, aka Rhamphoryncus

More information about the Python-Dev mailing list