"specialdict" module

[Georg Brandl]

Jeff Epler wrote:
> The software you used to post this message wrapped some of the lines of
> code.  For example:
>>     def __delitem__(self, key):
>>         super(keytransformdict, self).__delitem__(self,
>> self._transformer(key))

Somehow I feared that this would happen.

> In defaultdict, I wonder whether everything should be viewed as a
> factory:
>     def setdefaultvalue(self, value):
>         def factory(): return value
>         self.setdefaultfactory(factory)

That's a reasonable approach. __init__ must be changed too, but this
shouldn't hurt too badly.

> and the "no-default" mode would either cease to exist, or 
>     def cleardefault(self):
>         def factory(): 
>             raise KeyError, "key does not exist and no default defined"
>         self.setdefaultfactory(factory)
> (too bad that the key isn't available in the factory, this degrades the
> quality of the error messge)

That can be worked around with a solution in __getitem__, see below.

> if so, __getitem__ becomes simpler:
>     __slots__ = ['_default']
>     def __getitem__(self, key):
>       try:
>           return super(defaultdict, self).__getitem__(key)
>       except KeyError:
>           return self.setdefault(key, apply(*self._default))

You are peculating the kwargs. Also, apply() is on the verge of being
deprecated, so better not use it.

def __getitem__(self, key):
    try:
        return super(defaultdict, self).__getitem__(key)
    except KeyError, err:
        try:
            return self.setdefault(key,
                      self._default[0](*self._default[1],
                                       **self._default[2]))
        except KeyError:
            raise err

Although I'm not sure whether KeyError would be the right one to raise
(perhaps a custom error?).

> I don't ever have an itch for sorted dictionaries, as far as I can
> remember, and I don't immediately understand the use of
> keytransformdict.  Can you give an example of it?

See the thread "Case-insensitive dict, non-destructive, fast, anyone?",
starting at 04/01/05 12:38.

mfg
Georg