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

Walter Dörwald walter at livinglogic.de
Fri Feb 17 10:31:25 CET 2006

Guido van Rossum 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) and it also suffers from creating an unnecessary list instance. (Timings were inconclusive; the approaches are within 5-10% of each other in speed.) My conclusion is that setdefault() is a failure -- it was a well-intentioned construct, but doesn't actually create more readable code. Google has an internal data type called a DefaultDict which gets passed a default value upon construction. Its getitem method, instead of raising KeyError, inserts a shallow copy (!) of the given default value into the dict when the value is not found. So the above code, after d = DefaultDict([]) can be written as simply d[key].append(value)

Using a shallow copy of the default seems a bit too magical to me. How would this be done? Via copy.copy?

And passing [] to the constructor of dict has a different meaning already.

Fetching the default via a static/class method would solve both problems:

class default_dict(dict): def getitem(self, key): if key in self: return dict.getitem(self, key) else: default = self.getdefault() self[key] = default return default

class multi_map(default_dict): @staticmethod def getdefault(self): return []

class counting_dict(default_dict): @staticmethod def getdefault(self): return 0

[...]

Bye, Walter Dörwald

More information about the Python-Dev mailing list