On 2/16/06, Guido van Rossum <guido@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