Guido van Rossum wrote:
> Without const declarations none of this can work and
> the "at-function-definition-time" freezing is the best, because most
> predictable, approach IMO.
I you like this approach best, then how about using the same
approach as we have for function default argument values:
Variables which are to be regarded as constant within the
scope of the function are declared as such by using a "const"
declaration (much like we already have with the
global declaration).
a,b,c,d = range(4)
defvalue = 1
def switch(x=defvalue):
const a,b,c,d
switch x:
case a: return 'foo'
case b: return 'foo'
case c: return 'foo'
case d: return 'foo'
else: raise ValueError(x)
This declaration would cause the compiler to generate
LOAD_NAME opcodes just like for defvalue which then gets
executed at code object execution time, ie. when the
function is created.
This would also work out for the solution 1 case
in the PEP (if-elif-else)... <hint><hint> :-)
def switch(x=defvalue):
const a,b,c,d
if x == a: return 'foo'
elif x == b: return 'bar'
elif x == c: return 'baz'
elif x == d: return 'bazbar'
else: raise ValueError(x)
Furthermore, the compiler could protect the constant
names from assignments (much in the same way it
applies special treatment to variables declared global
in a scope).
A nice side-effect would be that could easily use the
same approach to replace the often used default-argument-hack,
e.g.
def fraction(x, int=int, float=float):
return float(x) - int(x)
This would then read:
def fraction(x):
const int, float
return float(x) - int(x)
--
Marc-Andre Lemburg
eGenix.com
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