[Python-Dev] defaultdict and on_missing() (original) (raw)

Fredrik Lundh fredrik at pythonware.com
Wed Feb 22 12:54:28 CET 2006

Raymond Hettinger wrote:

Aside: Why onmissing() is an oddball among dict methods. When teaching dicts to beginner, all the methods are easily explainable ex- cept this one. You don't call this method directly, you only use it when subclassing, you have to override it to do anything useful, it hooks KeyError but only when raised by getitem and not other methods, etc.

agreed.

My recommendation: Dump the onmissing() hook. That leaves the dict API unmolested and allows a more straight-forward im- plementation/explanation of collections.defaultdict or whatever it ends-up being named. The result is delightfully simple and easy to understand/explain.

agreed.

a separate type in collections, a template object (or factory) passed to the constructor, and implementation inheritance, is more than good en- ough. and if I recall correctly, pretty much what Guido first proposed. I trust his intuition a lot more than I trust the design-by-committee-with- out-use-cases process.

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