"once" assigment in Python

[Lorenzo Di Gregorio]

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

[Steve Holden]

Lorenzo Di Gregorio wrote:

On 17 Sep., 16:54, Larry Bates <larry.ba...@we bsafe.comwrote:

>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

This is a good point!

I could write something like:

instantiate('co mponent',functi on)

and have instantiate() to conditionally do:

instance['component']=Module(functio n)

Of course this means using:

instance['component'].method()

instead of just:

component.metho d()

Well, you could do that. Or you could define a class, with a __getattr__
that returns the required default value in the case where no setting had
been made.

Then you can assign components (with their own hierarchies) to the
attributes of your top-level design ...

A more annoying difficulty is with the fact that, while the scope of
ordinary variables is limited (function, package), the scope of
'component' would be global (the whole instance[]): having global
names could be pretty annoying in modeling a hierarchy. Anyway, some
scoping mechanism could be implemented over the global dictionary and
this could be a good price to pay to avoid other tricks.
Further suggestions?

It's not actually that hard to create objects that allow you to access
their attributes using *either* attribute or indexing notation.

regards
Steve
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