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

Bernhard Herzog bh at intevation.de
Sat Feb 18 22:41:07 CET 2006

"Guido van Rossum" <guido at python.org> writes:

If the getattr()-like operation that supplies and inserts a dynamic default was a separate method, we wouldn't have this problem.

Why implement it in the dictionary type at all? If, for intance, the default value functionality were provided as a decorator, it could be used with all kinds of mappings. I.e. you could have something along these lines:

class defaultwrapper(object):

def __init__(self, base, factory):
    self.__base = base
    self.__factory = factory

def __getitem__(self, key):
    try:
        return self.__base[key]
    except KeyError:
        value = self.__factory()
        self.__base[key] = value
        return value

def __getattr__(self, attr):
    return getattr(self.__base, attr)

def test(): dd = defaultwrapper({}, list) dd["abc"].append(1) dd["abc"].append(2) dd["def"].append(1) assert sorted(dd.keys()) == ["abc", "def"] assert sorted(dd.values()) == [[1], [1, 2]] assert sorted(dd.items()) == [("abc", [1, 2]), ("def", [1])] assert dd.has_key("abc") assert not dd.has_key("xyz")

The precise semantics would have to be determined yet, of course.

OTOH most reviewers here seem to appreciate onmissing() as a way to do various other ways of alterning a dict's getitem() behavior behind a caller's back -- perhaps it could even be (ab)used to implement case-insensitive lookup.

case-insensitive lookup could be implemented with another wrapper/decorator. If you need both case-insitivity and a default value, you can easily stack the decorators.

Bernhard

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