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Dictionary Enhancement?

Rich Harkins rharkins at thinkronize.com
Thu Oct 31 10:32:20 EST 2002 
On Thu, 2002-10-31 at 10:06, John Hunter wrote:
> >>>>> "Rich" == Rich Harkins <rich at worldsinfinite.com> writes:
> 
>     Rich> I was wondering if anyone else had been wanting to enhance
>     Rich> Python dictionaries such that instead of raising KeyError in
>     Rich> the case of errors that another method on the dictionary
>     Rich> object, say __makeitem__ would be called to try to
>     Rich> auto-generate an appropriate value.  That __makeitem__
>     Rich> method would then either raise KeyError itself or return the
>     Rich> value to insert into the dictionary and return through
>     Rich> __getitem__.
> 
> As of python 2.2, you can subclass built in types, so you are free to
> enhance dict anyway you like.
> 
> Here is an example that uses makeitem to cube the argument, and raises
> a KeyError if it can't
> 
> # requires python2.2
> class mydict(dict):
> 
>     def __makeitem__(self, key):
>         try: val = key**3
>         except: raise KeyError
>         else: return self.setdefault(key, val)
>         
>     def __getitem__(self, key):
>         return self.get(key, self.__makeitem__(key))
> 
> m = mydict()
> 
> print m[2]  # this is ok
> m[3] = 10   # so is this
> 
> m['John'] = 33
> #print m['Bill']  this raises a key error
> 
> 
> print m
> 
> Is this what you are looking for?
> 

Not quite - __makeitem__ will be called (and its value generated)
whether it needs to be or not - this is my basic objection to
setdefault() as an approach.  I did reply to myself with an example
Python class but I guess I would really like to see this become either a
default built-in behavior or a separate, but built-in, C type to manage
this sort of thing.  The pure Python version will have poor performance
characteristics if __getitem__ is called frequently.

Rich
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