On 5/14/07, Talin <[EMAIL PROTECTED]> wrote: > Guido van Rossum wrote: > > Also, can we overload different-length signatures (like in C++ or > > Java)? This is very common in those languages; while Python typically > > uses default argument values, there are use cases that don't easily > > fit in that pattern (e.g. the signature of range()). > > Well, from an algorithmic purity standpoint, I know exactly how it would > work: You put all of the overloads, regardless of number of arguments, > keywords, defaults, and everything else into a single bin. When you call > that function, you search through ever entry in that bin and throw out > all the ones that don't fit, then sort the remaining ones by specificity. > > The problem of course is that I don't know how to build an efficient > dispatch table to do that, and I'm not even sure that it's possible.
Have a look at sandbox/abc/abc.py, class overloadable (if you don't want to set up a svn workspace, see http://svn.python.org/view/sandbox/trunk/abc/abc.py). It doesn't handle keyword args or defaults, but it does handle positional argument lists of different sizes efficiently, by using a cache indexed with a tuple of the argument types. The first time a particular combination of argument types is seen it does an exhaustive search; the result is then cached. Performance is good assuming there are many calls but few distinct call signatures, per overloaded function. (At least, I think it's efficient; I once timed an earlier implementation of the same idea, and it wasn't too bad. That code is still in sandbox/overload/.) Argument default values could be added relatively easily by treating a function with a default argument value as multiple signatures; e.g. @foo.overload def _(a:str, b:int=1, c:Point=None): ... would register these three signatures: (str,) (str, int) (str, int, Point) Phillip suggested a clever idea to deal with keyword arguments, by compieing a synthesized function that has the expected signature and calls the dispatch machinery. I think it would need some adjustment to deal with variable-length signatures too, but I think it could be made to work as long as the problem isn't fundamentally ambiguous (which it may be when you combine different-sides positional signatures with defaults *and* keywords). The synthetic fuction is just a speed hack; the same thing can be done without synthesizing code, at the cost (considerable, and repeated per call) of decoding *args and **kwds explicitly. -- --Guido van Rossum (home page: http://www.python.org/~guido/) _______________________________________________ Python-3000 mailing list Python-3000@python.org http://mail.python.org/mailman/listinfo/python-3000 Unsubscribe: http://mail.python.org/mailman/options/python-3000/archive%40mail-archive.com
