[Python-Dev] dict.addlist() (original) (raw)

Bob Ippolito bob at redivi.com
Tue Jan 20 13:50:43 EST 2004

On Jan 20, 2004, at 1:32 PM, Raymond Hettinger wrote:

[GvR]

bookindex = dict()

(What's wrong with {}?) Nothing at all. {} is shorter, faster, and everyone understands it. When teasing out ideas at the interpreter prompt, I tend to use dict() because I find it easier to edit the line and add some initial values using dict(one=1, two=2, s='abc'). works for me. (I think setdefault() was already a minor mistake -- by the time I've realized it applies I have already written working code without it. And when using setdefault() I always worry about the waste of the new empty list passed in each time that's ignored most times.) If you really want library support for this idiom ... Not really. It was more of a time machine question -- if we had setdefault() to do over again, what would be done differently: * Keep setdefault(). * Drop it and make do with get(), try/except, or if k in d. * Martin's idea for dicts to have an optional factory function for defaults. * Have a specialized method that just supports dicts of lists.

Here's another idea: how about adding a special method, similar in implementation to getattr, but for getitem -- let's say it's called getdefaultitem? You could then subclass dict, implement this method, and you'd probably be able to do what you want rather efficiently. You may or may not decide to "cache" the value inside your custom getdefaultitem, and if you do, you may or may not want to change the behavior of contains as well.

warning: untested code

class newdict(dict): """Implements the proposed protocol""" def getitem(self, item): try: return super(newdict, self).getitem(item) except KeyError: return self.getdefaultitem(item)

def __getdefaultitem__(self, item):
    raise KeyError(repr(item))

class listdict(newdict): def getdefaultitem(self, item): self[item] = rval = [] return rval

-bob

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