[Python-Dev] Switch statement

Thu Jun 22 20:21:41 CEST 2006

Guido van Rossum wrote:
>> def foo(value):
>> const bar = fie.fum
>> if value == bar:
>> ...
>>>> which would behave like
>>>> def foo(value, bar=fie.fum):
>> if value == bar:
>> ...
>>>> but without the "what if we pass in more than one argument?" issue.
>> So the constant would be evaluated at function definition time? I find
> that rather confusing.

well, I find the proposed magic behaviour of "case" at least as confusing...
>> (except for the default argument thing; see above). the result is a
>> kind of semi-constant objects that would be useful, but perhaps not
>> constant enough...)
>> I fail to see the usefulness of this wrapper. The wrapper isn't
> completely transparent o some code that uses type checks may need to
> be modified. The wrapper doesn't get removed by a simple assignment;
> after
>> const a = 1
> b = a
>> how do we prevent b from being treated as a constant?

we cannot -- this approaches assigns (a small amount of) const-ness to 
objects, not names.
>> it might be too much C# exposure, but I think I prefer the "explicit
>> when using" approach...
>> It may be not enough C# exposure, but I don't know exactly which
> approach you are referring to.

the original one: if you want to treat an expression as a constant, you 
have to be explicit. examples:
>>> a "constant" (or perhaps better, "const") primary would also be useful
>>> in several other cases, including:
>>>>>> - as a replacement for default-argument object binding

this is used when you want to pass an *object* into an inner function, 
rather than a name:
 def foo(value, bar=fie.fum):
 if value == bar:
 ...
can be written
 def foo(value):
 if value == const bar:
 ...
>>> - local dispatch tables, and other generated-but-static data structures

 def foo(value):
 table = const {
 1: "one",
 2: "two",
 3: fie.fum,
 }
(maybe "static" would be a better keyword?)
>>> - explicit (but still anonymous) constant/expression "folding"

 def foo(value):
 if value < const (math.pi / 2):
 ...
and so on. to implement this, the runtime simply evaluates the "const" 
expressions together with the default value expressions, and assigns the 
result to some func_xxx attribute. everything else works as usual.
</F>