[Python-Dev] Switch statement

Fredrik Lundh fredrik at pythonware.com
Thu Jun 22 18:22:17 CEST 2006

Guido van Rossum wrote:

>> which simply means that expr will be evaluated at function definition
>> time, rather than at runtime.  example usage:
>>      var = expression
>>      if var == constant sre.FOO:
>>          ...
>>      elif var == constant sre.BAR:
>>          ...
>>      elif var in constant (sre.FIE, sre.FUM):
>>          ...
> This gets pretty repetitive. One might suggest that 'case' could imply
> 'constant'...?

possibly, but I find that a tad too magic for my taste.

a "constant" (or perhaps better, "const") primary would also be useful 
in several other cases, including:

- as a replacement for default-argument object binding

- local dispatch tables, and other generated-but-static data structures

- explicit (but still anonymous) constant/expression "folding"

an alternative would be to add a const declaration that can only be used 
in local scopes; i.e.

     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.

yet another alternative would be a const declaration that you could use 
on a global level, but I fail to see how you could propagate the "const- 
ness" property to whoever wants to use a const object -- unless, of 
course, you implement

     const bar = fie.fum

     def foo(value):
        if value == bar:


     class constant_wrapper(object):
         def __init__(self, value):
             self.value = value

     bar = constant_wrapper(fie.fum)

     def foo(value, bar=bar.value):
         if value == bar:

(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...)

it might be too much C# exposure, but I think I prefer the "explicit 
when using" approach...


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