It seems to me that the utility of subclassing certain library classes would
be increased if the user could set a factory that the base class would use
when creating new instances. That way, from the user's point of view,
functionality would not "disappear" after operations e.g.
class MyDate(datetime.date):
def foo(self):
...
nextday = MyDate(...) + datetime.timedelta(days=1)
nextday.foo() # exception
Storing the factory information in the base class has the disadvantage of
causing a global behavior change but can probably be made speed efficient.
Storing the factory in the subclass would require that the super class check
for a class variable when creating instances. It would probably provide the
most obvious semantics from the programmer's POV but would be organizationally
odd i.e. each class would have to provide a factory for every overriden type
that it's superclass creates.
Aside from datime.* this feature seems like it would be useful for other types
where there is potential for making useful subclasses (unlike int) and where
many operations result in new instances. sets come to mind.
Any thoughts?
Cheers,
Brian
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