[Python-Dev] defaultdict proposal round three (original) (raw)

Giovanni Bajo rasky at develer.com
Tue Feb 21 08:51:03 CET 2006

Raymond Hettinger wrote:

- It would be unwise to have a default value that would be called if it was callable: what if I wanted the default to be a class instance that happens to have a call method for unrelated reasons? Callability is an elusive propperty; APIs should not attempt to dynamically decide whether an argument is callable or not. That makes sense, though it seems over-the-top to need a zero-factory for a multiset. An alternative is to have two possible attributes: d.defaultfactory = list or d.defaultvalue = 0 with an exception being raised when both are defined (the test is done when the attribute is created, not when the lookup is performed).

What does this buy over just doing:

d.default_factory = lambda: 0

which is also totally unambiguous wrt the semantic of usage of the default value (copy vs deepcopy vs whatever)? Given that the most of the default values I have ever wanted to use do not even require a lambda (list, set, int come to mind).

Giovanni Bajo

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