Time query in python expression

[dpugmire]

I'm working with a user at OLCF who is developing a python expression. He needs to get access to the time either inside the expression. He tried to issue a Query("Time") inside the expression, but it isn't working. Is there a better way to get the Time into an expression?

[biagas]

Depending on the nature of the script that uses the python expression, it should be possible to do the query beforehand and then pass the result as an argument to the python expression.

Otherwise, I would suggest the time expression. It creates a constant-valued scalar variable. Not quite as elegant as just retrieving the single-valued output of the Query, but should work.

Time Function: time()time(expr0)

    Return a constant scalar variable which is everywhere the time of the associated input argument within its time-series.

[chrspy1]

Hello, I am the user that @dpugmire is speaking of. I thought that it might be good to outline my problem. I have a code that I want to verify by computing L2 norms.

I plan to accomplish this task in two steps, (1) compute the differences between each physical quantity in my code to its known solution, and then (2) sum up all the differences to obtain the L2-norm.

To carry out task 1, I use visit in cli mode, and I then call a custom python expression of the form:
DefinePythonExpression("differences",("coordx","coordy","coordz","quantity_1","quantity_2","quantity_3"),file='/home/ui0/my_filter.py')

The purpose of my_filter.py to evaluate known_quant1,known_quant2,known_quant3 = Solution(x,y,z,t) at each node point, and then compare the known field quantities to those from the code ("quantity_1","quantity_2", and "quantity_3" above). The python function Solution will be imported into filter.py, since it can be very complicated for some problems. Ultimately, my hope is to have a variable called "differences" that is something like (known_quant1-quantity_1)^2+(known_quant2-quantity_2)^2 + ..., where I could plot it. Afterward, I will write something to sum it all.

My current problem is that in order to run Solution(x,y,z,t) I need to know the time t. As was suggested, I tried to include it as an input into the DefinePythonExpression call, but I am not sure how to do this. Nothing I tried worked. The form of my_filter.py follows the general form in the documentation:
class MyExpression(SimplePythonExpression): def __init__(self): SimplePythonExpression.__init__(self) self.name = "PythonExpression" self.description = "Add two scalar variables together" self.output_is_point_var = True self.output_dimension = 1 def modify_contract(self,contract): pass def derive_variable(self,ds_in,domain_id): #stuff
If there is an easier way to do this, that is easier to parallelize, I am open to suggestions.
-Chris

[biagas]

So, I think you will need to define a new expression:

DefineScalarExpression("my_time", "time(mymeshname)")

Use whatever name you want instead of my_time.
substitute the correct mesh name for mymeshname.

Pass in this variable to your DefinePythonExpression, and then you can access it the same as the other variables.