[Python-Dev] Switch statement (original) (raw)

Guido van Rossum guido at python.org
Thu Jun 22 22:00:15 CEST 2006

On 6/22/06, Ka-Ping Yee <python-dev at zesty.ca> 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/)

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