"specialdict" module

[Michael Spencer]

Georg Brandl wrote:

> 
> I think I like Jeff's approach more (defaultvalues are just special
> cases of default factories); there aren't many "hoops" required.
> Apart from that, the names just get longer ;)

Yes Jeff's approach does simplify the implementation and more-or-less eliminates 
my complexity objection

But why do you write:

     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

rather than:

     def __getitem__(self, key):
         return self.setdefault(key,
                           self._default[0](*self._default[1],
                                            **self._default[2]))

(which could catch AttributeError in the case of _default not set)
I'm sure there's a reason, but I can't see it.

> 
> 
>>2. I would really prefer to have the default value specified in the constructor
...
> 
> Too much specialcased for my liking.
> 
It does set up some gotchas I concede ;-)

> 
...
> 
>>Alternatively, you could provide factory functions to construct the defaultdict. 
>>  Someone (Michele?) recently posted an implementation of this
> 
> 
> Yes, I think this could be reasonable.


...though this would more naturally complement a fixed-default dictionary IMO
Your design permits - even encourages (by providing convenient setters) the 
default to change over the lifetime of the dictionary.  I'm not sure whether 
that's good or bad, but it's a feature worth discussing.

>>3. Can you work in the tally and listappend methods that started this whole 
>>thread off?
> 
> 
> They aren't necessary any longer.
> 
> Use defaultdict.setdefaultvalue(0) instead of the tally approach and
> defaultdict.setdefaultfactory(list) instead of listappend.

Oops, I see what you mean... then use += or append as required.  I still prefer 
the clarity of tally for its specific use-case, but it does suffer from lack of 
generality.

Michael