On 6/22/06, Ka-Ping Yee <[EMAIL PROTECTED]> wrote: > On Wed, 21 Jun 2006, Guido van Rossum wrote: > > (Note how I've switched to the switch-for-efficiency camp, since it > > seems better to have clear semantics and a clear reason for the syntax > > to be different from if/elif chains.) > > I don't think switch-for-efficiency (at least if efficiency is the > primary design motivator) makes sense without some strong evidence > that the use of if/elif constructs often causes a severe speed problem > in many Python programs. (Premature optimization and all that.) > Long if/elif chains probably don't occur often enough or slow down > programs enough to invent syntax *just* for speed; and even if they > did, i don't think there's any precedent for a Python statement > invented primarily as a speed optimization.
My position is more nuanced that that. I like having a switch because it can make certain types of code more readable. (I keep referring to sre_parse.py and sre_compile.py -- has anyone else looked at these at all?) But I also like switch because it can be implemented using a single dict lookup with a pre-computed dict, if certain conditions on the cases are met. I find that those conditions generally met by code that lends itself for using a switch. I believe that switch would be better understood if it *always* precomputed the dict, and failed immediately with an exception (at run time) if the precompilation wasn't feasible. > I'm hoping we can talk more about the question: How can a new statement > help programmers express their intent more clearly? > > So far i've seen two possible answers to that question: > > 1. The switched-on expression is written and evaluated just once. > > 2. The cases could help you unpack things into local variables. > > (There was some discussion about unpacking earlier: > http://mail.python.org/pipermail/python-dev/2005-April/052780.html > which petered out, though there may still be the possibility of > designing something quite useful and readable.) I'm not convinced that the matching idea (which your URL seems to refer to) works well enough in Python to consider. Anyway, it's a completely different approach and should probably be discussed separately rather than as a variant of switch/case. -- --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
