[Python-Dev] Proposal: defaultdict

Thomas Heller theller at python.net
Fri Feb 17 21:18:42 CET 2006

[cc to py-dev again]

Guido van Rossum wrote:
> On 2/17/06, Thomas Heller <theller at python.net> wrote:
>> Guido van Rossum wrote:
>>> So here's a new proposal.
>>> Let's add a generic missing-key handling method to the dict class, as
>>> well as a default_factory slot initialized to None. The implementation
>>> is like this (but in C):
>>> def on_missing(self, key):
>>>   if self.default_factory is not None:
>>>     value = self.default_factory()
>>>     self[key] = value
>>>     return value
>>>   raise KeyError(key)
>>> When __getitem__() (and *only* __getitem__()) finds that the requested
>>> key is not present in the dict, it calls self.on_missing(key) and
>>> returns whatever it returns -- or raises whatever it raises.
>>> __getitem__() doesn't need to raise KeyError any more, that's done by
>>> on_missing().
>> Will this also work when PyDict_GetItem() does not find the key?
> Ouch, tricky. It should, of course, but the code will be a tad tricky
> because it's not supposed to inc the refcount. Thanks for reminding
> me!

Ahem, I'm still looking for ways to 'overtake' the dict to implement
weird and fancy things.  Can on_missing be overridden in subclasses (writing
the subclass in C would not be a problem)?



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