Use of code for no value in ResSim script

For some scripts in ResSim, if you are trying to pull in an external variable or a value from a state variable, you may need to tell ResSim what to assign to the variable if no value is available.

As an example, the code below brings in the variable of a "slave" state variable named "storingstor".

varsv = network.getStateVariable("storingstor")
varsv_prev = varsv.getPreviousValue(currentRuntimestep)

If there is no value or the value is undefined, you need to tell ResSim what to do.

Below are a few examples:

Example 1:

if not varsv_prev:
    varsv_prev = 0

This code seems to work for some cases, but it appears that ResSim will sometimes see the value as being undefined.  When this occurs, I recommend using one of the two examples below.

Example 2:

from hec.script.Constants import UNDEFINED
if varsv_prev == UNDEFINED:
    varsv_prev = 0

Example 3:

from hec.lang.Const import is Undefined
if isUndefined(varsv_prev):
    varsv_prev = 0