[Python-Dev] once [was: Simple Switch statementZ]

Thu Jun 29 19:22:25 CEST 2006

Christos Georgiou wrote:
> I haven't followed the complete discussion about once, but I would assume it 
> would be used as such:
>> once <name> = <expression>
>> that is, always an assignment, with the value stored as a cellvar, perhaps, 
> on first execution 0f the code.
>> Typically I would use it as:
>> def function(a):
> once pathjoin = os.path.join
> <etc>

In the "name = (once expr)" form I gave, the property of a constant name 
that can't be rebound or that of a value that persists across function 
call invocations isn't needed. I was trying to separate the different 
behaviors cleanly and clearly.
 # once as constant assignment and skipped line later.
 for n in range(x, 10):
 once startcube x**3 # assigns constant value, skips later
 print startcube
 startcube += 1 # give an exception
So this is the same as "const startcube x**3", except it's ignored if it 
is executed again instead of giving an excepton.
Here the constantness property isn't needed.
 # once as calc once, use result many times expression.
 for n in range(x, 10):
 startcube = (once x**3) # calculated once used many
 print startcube
 startcube += 1 # Ok to do this
I wasn't suggesting which behavior (or combination of) is correct. That 
would depend on what problem is meant to solved.
A fourth property of external has been touched on in these threads where 
 some of the suggestions require doing a calculation on a yet to be 
known value. That's usually handled by linkers in other languages and 
probably isn't something desired in a dynamic language like Python.
Cheers,
 Ron
* I may not be able to reply, do to leaving on a trip. Already should 
be gone. ;-)