How can i add script with math functions to model builder?

I am very new to model builder + python. (Sorry for my english too)

I am trying to make a model. Don’t know if it matters to upload it. If you want i can.

I want to calculate variance from a points-shapefile with elevation data and want to add this script to my model:

### x = (column: GRID_CODE, from a shapefile from model builder)

average = sum(x) / len(x)
variance = sum((average - value) ** 2 for value in x) / (len(x)-1)

### output to a new shapefile

How could i make this script working? Searched a lot here and at arcgis help but couldn’t find something.
Can’t understand how to “grab” data from shapefile, process them through script and then print the output.