SymPy is great! I use it at work in an in-house web application where the user designs a graph (using d3.js). The graph nodes are electric energy meters, and so

SymPy is great! I use it at work in an in-house web application where the user designs a graph (using d3.js). The graph nodes are electric energy meters, and so there are certain flows thru them. A system of equations is generated and then solved using SymPy, so one can find out what the flow is in a certain node in terms of the flows in the others.