[Python-Dev] Proposal: defaultdict

[Thomas Heller]

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?

Thomas