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

Adam Olsen rhamph at gmail.com
Fri Feb 17 15:13:51 CET 2006

On 2/16/06, Guido van Rossum <guido at python.org> wrote:

A bunch of Googlers were discussing the best way of doing the following (a common idiom when maintaining a dict of lists of values relating to a key, sometimes called a multimap):

if key not in d: d[key] = [] d[key].append(value) An alternative way to spell this uses setdefault(), but it's not very readable: d.setdefault(key, []).append(value)

I'd like to see it done passing a factory function (and with a better name):

d.getorset(key, list).append(value)

The name is slightly odd but it is effective. Plus it avoids creating a new class when a slight tweak to an existing one will do.

Over lunch with Alex Martelli, he proposed that a subclass of dict with this behavior (but implemented in C) would be a good addition to the language. It looks like it wouldn't be hard to implement. It could be a builtin named defaultdict. The first, required, argument to the constructor should be the default value. Remaining arguments (even keyword args) are passed unchanged to the dict constructor.

-1 (atleast until you can explain why that's better than .getorset())

-- Adam Olsen, aka Rhamphoryncus

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