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

Guido van Rossum guido at python.org
Thu Jun 22 17:52:37 CEST 2006

On 6/21/06, Phillip J. Eby <pje at telecommunity.com> wrote:

At 01:16 PM 6/21/2006 -0700, Guido van Rossum wrote: >Yeah, but if you have names for your constants it would be a shame if >you couldn't use them because they happen to be defined in the same >scope.

Maybe the real answer is to have a "const" declaration, not necessarily the way that Fredrik suggested, but a way to pre-declare constants e.g.: const FOO = 27

The problem with this is that I don't see how to export this property from one module to another. If module A has the above statement and module B does "from A import FOO" then how does the parser know that FOO is a constant when it's parsing B? (Remeber the parser only sees one module at a time; I don't want to drop this separate compilation facility.) It seems you would still end up with lots of duplicate const declarations (e.g. "from A import const FOO"). And it should also be possible to say "import A" and then use "A.FOO" as a constant.

I really don't think this ad-hoc solution is going to work.

And then require case expressions to be either literals or constants. The constants need not be computable at compile time, just runtime. If a constant is defined using a foldable expression (e.g. FOO = 27 + 43), then the compiler can always optimize it down to a code level constant. Otherwise, it can just put constants into cells that the functions use as part of their closure. (For that matter, the switch statement jump tables, if any, can be put in a cell too.)

>I don't like "first use" because it seems to invite tricks. Okay, then I think we need a way to declare a global as being constant. It seems like all the big problems with switch/case basically amount to us trying to wiggle around the need to explicitly declare constants.

And I don't believe declaring constants is going to work; not without a much bigger change to the language and the way we think about it. This is because "const-ness" isn't a property that you can encode as a new type of object. It is a compile-time property of names. The only similar thing in Python is globals. But global declarations are intentionally a bit clunky because we believe overuse of the feature would be a mistake. But we wouldn't want to discourage constant declaration if we had them, so having to repeat the 'constant' keyword in every module that uses a particular constant would be a painful wart.

Is your objection purely based on the problems of getting switch to behave the same way inside and outside a function? I'd rather forbid or cripple switch outside functions than either add constant declarations or switch to first-use semantics.

-- --Guido van Rossum (home page: http://www.python.org/~guido/)

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