Pre-PEP: Dictionary accumulator methods

Michael Spencer mahs at telcopartners.com
Sun Mar 20 00:46:42 CET 2005


Raymond Hettinger wrote:
> I would like to get everyone's thoughts on two new dictionary methods:
> 
>         def count(self, value, qty=1):
>             try:
>                 self[key] += qty
>             except KeyError:
>                 self[key] = qty
> 
>         def appendlist(self, key, *values):
>             try:
>                 self[key].extend(values)
>             except KeyError:
>                 self[key] = list(values)
> 

These undoubtedly address common cases, which are unsatisfactory when spelled 
using setdefault.  However...

Use of these methods implicitly specializes the dictionary.  The methods are 
more-or-less mutually exclusive i.e., it would be at least strange to use count 
and appendlist on the same dictionary.  Moreover, on many dictionary instances, 
the methods would fail or produce meaningless results.

This seems to be at odds with the other methods of built-in container types 
which can be meaningfully applied, no matter what the types of the contents. 
(There may be exceptions, but I can't think of any at the moment)

Does anyone else think this is a problem?

Michael






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