"once" assigment in Python

[Larry Bates]

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?
> 
> Thx in advance,
> Lorenzo
> 
IMHO variables like what you describe are really data not program variables. 
You might consider putting variables like these in a dictionary and then check 
to see if the keys exist before assignment:

var_dict={}

#
# See if 'varname' initialized, if not it needs to be
#
if 'varname' not in var_dict:
      var_dict[varname]=somevalue

-Larry