There is a guide to using the Python node that is installed with BRE named "Python Node Getting Started.pdf". It should be included in the following directory:
<InstallDir>\docs
For example:
C:\Program Files\Lavastorm\LAE6.1\docs
The Python node is intended for advanced uses and it is recommended that other nodes (e.g. Agg Ex or Filter) are used as they can achive a simple summation of values without the code overhead of using the Python node. However, an example is attached for your reference.
Attached files
Python_Node_Sum_Input_Field_Values.brg