Guido van Rossum wrote: >On 2/21/06, Fuzzyman <[EMAIL PROTECTED]> wrote: > > >>I've had problems in code that needs to treat strings, lists and >>dictionaries differently (assigning values to a container where all >>three need different handling) and telling the difference but allowing >>duck typing is *problematic*. >> >> > >Consider designing APIs that don't require you to mae that kind of >distinction, if you're worried about edge cases and classifying >arbitrary other objects correctly. It's totally possible to create an >object that behaves like a hybrid of a string and a dict. > > > Understood.
>If you're only interested in classifying the three specific built-ins >you mention, I'd check for the presense of certain attributes: >hasattr(x, "lower") -> x is a string of some kind; hasattr(x, "sort") >-> x is a list; hasattr(x, "update") -> x is a dict. Also, hasattr(x, >"union") -> x is a set; hasattr(x, "readline") -> x is a file. > >That's duck typing! > > Sure, but that requires a "dictionary like object" to define an update method, and a "list like object" to define a sort method. The mapping and sequence protocols are so loosely defined that some arbitrary decision like this has to be made. (Any object that defines "__getitem__" could follow either or both and duck typing doesn't help you unless you're prepared to make an additional requirement that is outside the loose requirements of the protocol.) I can't remember how we solved it, but I think we decided that an object would be treated as a string if it passed isinstance, and a dictionary or sequence if it has _getitem__ (but isn't a string instance or subclass). If it has update as well as __getitem__ it is a "dictionary-alike". All the best, Michael Foord >-- >--Guido van Rossum (home page: http://www.python.org/~guido/) > > > _______________________________________________ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
