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

Jim Jewett jimjjewett at gmail.com
Mon Feb 20 20:02:02 CET 2006

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?)

It doesn't actually solve the problem because it doesn't compose well. This makes it hard to use for configuration.

(Use case from plucker web reader, where the config is arguably overdesigned, but ... the version here is simplified)

There is a system-wide default config. Users have config files. A config file can be specified for this program run.

In each of these, settings can be either general settings or channel-specific. The end result is that the value should be pulled from the first of about half a dozen dictionaries to have an answer. Because most settings are never used in most channels, and several channels are typically run at once, it feels wrong to pre-build the whole "anything they might ask" settings dictionary for each of them. On the other hand, I certainly don't want to write

userchannelconfig.getorset(key, systemchannelconfig.getorset(key,

                    ...)

even once, let alone every time I get a config value.

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.

-jJ

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