On Fri, 14 Sep 2007 06:16:56 +0000, 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.
I could see reasons for doing something like this at a module level or
for attributes; such as setting default values when defaults are
expensive to calculate. You could, as others have said, initialize the
variable to a trivial value and then test whether it still held the
trivial value later, but what's the point?
> 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.
For module level variables you can do something like this:
def once(symbol,method):
g = globals()
if symbol not in g:
g[symbol] = method()
You'd have to pass a symbol as a string, but that's no big deal.
For local variables you're stuck with trying to catch UnboundLocalError.
There's a way to do it by examining stack frames, but I don't really
recommend it: it's inefficient, and the once assignment doesn't make as
much sense for local variables.
Carl Banks
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