Ian Bicking wrote:
> Guido van Rossum wrote:
>
>[...]
>> What does when(object=list) mean? Does it do an isinstance() check?
>
> Yes; I think RuleDispatch has a form (though I can't remember what the
> form is -- it isn't .when()).
What happens, if I do the following
@PrettyPrinter.pformat.when(object=list)
def foo(...):
...
@PrettyPrinter.pformat.when(object=object)
def foo(...):
...
How does it know which isinstance() check to do first?
And what happens with performance if I have registered many handler
functions?
> [...]
>
> The implementation of my simplistic form of generic function isn't too
> hard. Ignoring keyword arguments, it might work like:
>
> class generic(object):
> def __init__(self, func):
> self.func = func
> self.registry = {}
> def __call__(self, *args):
> for pattern, implementation in self.registry.items():
> for passed, expected in zip(args, pattern):
> # None is a wildcard here:
> if (expected is not None and
> not isinstance(passed, expected)):
> break
> else:
> return implementation(*args)
> return self.func(*args)
> def when(self, *args):
> def decorator(func):
> self.registry[args] = func
> return func
> return decorator
> def __get__(self, obj, type=None):
> if obj is None:
> return self
> return types.MethodType(self, obj, type)
>
> There's lots of details, and handling keyword arguments, dealing
> intelligently with subclasses, and other things I probably haven't
> thought of. But anyway, this allows:
>
> class PrettyPrinter:
> def pformat(self, object): ...
>
> # Without keyword arguments I have to give a wildcard for the self
> # argument...
> @PrettyPrinter.pformat(None, list)
> def pformat_list(self, object):
> ...
I don't understand! There's no generic in sight here!
Bye,
Walter Dörwald
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