"once" assigment in Python

[Steve Holden]

Lorenzo Di Gregorio wrote:
> Hello,
> 
> I've been using Python for some DES simulations because we don't need
> full C speed and it's so much faster for writing models.  During
> coding I find it handy to assign a variable *unless it has been
> already assigned*: I've found that this is often referred to as "once"
> assigment.
> 
> The best I could come up with in Python is:
> 
> try:
>   variable
> except NameError:
>   variable = method()
> 
> I wonder if sombody has a solution (trick, whatever ...) which looks
> more compact in coding.  Something like:
> 
> once(variable, method)
> 
> doesn't work, but it would be perfect.  Of course I can preprocess the
> Python code but an all-Python solution would be more handy.
> 
> Any suggestions?
> 
Without being fatuous, I would suggest you rethink your approach to the 
problem.

Do your variables have default values? If not, what happens if the user 
does not set them and your code tries to refer to them? You seem to be 
trying to apply defaults if the user hasn't set a value, when the 
correct things to do is to apply the defaults *before the user gets a 
chance to do anything at all*. Then any changes made by the user will 
overwrite the defaults.

Even better, if its practical, is to provide your functionality as one 
or more functions, whose definitions can set default values for keyword 
arguments.

regards
  Steve
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