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Wed Jan 21 11:14:23 EST 2004 · https://mail.python.org/pipermail/python-dev/2004-January/042157.html

Alex Martelli wrote:
> 
> On 2004 Jan 20, at 19:32, Raymond Hettinger wrote:
>    ...
> 
>> d = {}
>> d.setfactory(list)
>> for k, v in myitems:
>>     d[k].append(v)        # dict of lists
>>
>> d = {}
>> d.setfactory(set):
>> for v in mydata:
>>     d[f(v)].add(v)        # partition into equivalence classes
>>
>> d = {}
>> d.setfactory(int):
>> for v in mydata:
>>     d[k] += 1            # bag
> 
> 
> Yes, except that a .factory property seems preferable to me to a 
> .setfactory setter-method (which would have to come with .getfactory or 
> equivalent if introspection, pickling etc are to work...) except perhaps 
> for the usual "we don't have a built-in curry" issues (so .setfactory 
> might carry arguments after the first [callable factory] one to perform 
> the usual "ad hoc currying" hac^H^H^H idiom).  In fact I'd _love_ this 
> approach, were it not for the fact that in some use cases I'd like the 
> factory to receive the key as its argument.  E.g.:
> 
> squares_of_ints = {}
> def swe_factory(k):
>     assert isinstance(k, (int, long))
>     return k*k
> squares_of_ints.setfactory_receiving_key(swe_factory)
> 
> 
> Alex

This could be seen as favouring the protocol approach that Bob 
suggested. In that approach, the 'defaulting' is done by having dict 
access call __getdefaultitem__ if it exists, and the item being looked 
up is not found. The signature of __getdefaultitem__ is the same as that 
for __getitem__.

If __getdefaultitem__ isn't found, then the current behaviour of raising 
KeyError is retained.

I suspect the aim of this approach would be to reduce errors by avoiding 
rewriting the 'try/except KeyError' block everytime we wanted a 
defaulting dictionary. I imagine it could be made faster than the 
current "k in d" or "try...except Keyerror..." idioms, too.

The more I think about it, the more I'm leaning towards the class-based 
approach - the version with 'factory' or 'setfactory' seems to lend 
itself to too many dangerous usages, especially:

d = {}
...do some stuff... [A]
d.factory = factory_func
...do some more stuff... [B]

The meaning of d[k] is significantly different in section A than it was 
in section B.

Regards,
Nick.

-- 
Nick Coghlan               |     Brisbane, Australia
Email: ncoghlan at email.com  | Mobile: +61 409 573 268

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