Paul Boddie <[EMAIL PROTECTED]> wrote: > Alex Martelli wrote: > > Paul Boddie <[EMAIL PROTECTED]> wrote: > > > > > > Well, range is a function in the current implementation, although its > > > usage is similar to that one would get if it were a class, particularly > > > a subclass of list or one providing a list-style interface. With such a > > > class, you could provide a __contains__ method which could answer the > > > question of what the range contains based on the semantics guaranteed > > > by a range (in contrast to a normal list). > > > > You'd also have to override just about every mutating method to switch > > back to a "normal" __contains__ (or change self's type on the fly) -- a > > pretty heavy price to pay. > > A subclass of list is probably a bad idea in hindsight, due to various > probable requirements of it actually needing to be a list with all its > contents, whereas we wanted to avoid having anything like a list around > until the contents of this "lazy list" were required by the program. If > we really wanted to subclass something, we could consider subclassing > the slice class/type, but that isn't subclassable in today's Python for > some reason, and it doesn't really provide anything substantial, > anyway. However, Python being the language it is, an appropriately > behaving class is quite easily written from scratch.
Nevertheless, that class will still need to implement every single method of the list type; making it a subclass of list has some advantage in that every such implementation of a method can basically fill the real list, self.__class__=list, and leave all the rest, forevermore (explicitly here, implicitly in the future), to class list. Performance should be much better than by working off semi-deprecated UserList. A "hook method" __mutator__ (ideally called _before_ in this case), as I was proposing (for 2.6 or later), would make such approaches way easier and handier (and would help with most use cases I can think of for subclassing list, dict or set). Alex -- http://mail.python.org/mailman/listinfo/python-list
