persisting data within a module

[davisn90210 at gmail.com]

On Nov 12, 5:30 pm, "Peter J. Bismuti" <peter.j.bism... at boeing.com>
wrote:
> I'm having trouble understanding how namespaces work in modules.  I want to
> execute a module within the interpreter and then have values that are
> calculated persist so that other modules that get executed can retrieve them.
>

Modules retain state their state across all imports in the same
interpreter instance.  Module state is not shared among different
instances of the interpreter.

> For example, consider the two simple modules below.  The first method fails
> and I'm not sure exactly why.  (Note:  assume one instance of an interpreter.
> In my case a 3rd party software tool that starts an interpreter when it
> launches).
>
> Two alternate ways of running it:
>
> 1. (FAILS: RESULTS A = 0)  Use the module "test" itself as the driver using
> the conditional statement if (__name__=="__main__"):
>
> test.py
> run2.py
>

Ok, what do you mean by this?  Do you mean run test.py and then run
run2.py?  In so, then you will have *two* instances -- one for each
file being executed.  You can only have one main module per
interpreter instance.  I suspect this is the source of your confusion.

> or,
>
> 2.  (SUCCES: RESULTS A = 10)   Use "run.py" as the driver.
>
> run.py
>
> _________test.py__________________
>
> import sys,os
>
> A = 0
>
> def getA():
>   global A
>   return A
>
> def run():
>   global A
>   A = 10
>
> if (__name__=="__main__"):
>   run()
>

Here, A is only initialized when the module is loaded iff it is the
main module.  If it's not the main module, then it will have A set to
0 until some other code calls run().

> _________run.py__________________
>
> import test
>
> test.run()
> print "A = " + str(test.getA())
>

This code calls test.run(), which is necessary for A to be 10.

> _________run2.py__________________
>
> import test
>
> print "A = " + str(test.getA())
>
> --
>

This code gets the value of test.A without calling test.run().  Since
test.run() was not called, A is the value it was initialized when the
test module was loaded -- namely, 0.

Hope this helps,

--Nathan Davis