This blog post briefly describes the project "Print equations" with PyBaMM during the first three weeks of GSoC'21.

Work Done

1. Added a BaseParameters class
2. Modified all parameters classes to inherit from the base class
3. Added SymPy in dependencies
4. Added to_equation in some classes
5. Printed SPM model equations

Base Parameters class

I added a BaseParameters class to overload the __setattr__ method in the parameters class to record what the variable was called (as suggested by Valentin Sulzer in this comment) and modified all parameters classes to inherit from the base parameters class.

SymPy to render SPM equations

• Added SymPy in PyBaMM's dependencies
• Added to_equation method in
  • Binary Operators
  • Scalars
  • Arrays
  • FunctionParameter
  • Symbol
  • UnaryOperator (base class)
  • Gradient
  • Divergence
  • Variable

I added to_equation in all the classes required to print Single Particle Model (SPM) equations and doing this (will add this function later to the model)

model = pybamm.lithium_ion.SPM()
output = []
for var, eqn in model.rhs.items():
    lhs = sympy.Derivative(var.print_name, 't')
    rhs = sympy.nsimplify(eqn.to_equation())
    final = sympy.Eq(lhs, rhs, evaluate=False)
    output.append(final)

resulted in this

Merged Pull Requests

• Add BaseParameters and SymPy (#1495)
• Add to_equation in Binary and Scalar (#1502)
• Add to_equation for SPM (#1517)

Challenges I ran into

The most challenging part of printing the equations is finding the appropriate SymPy object in to_equation to return to. It sometimes took me days to find the equivalent function in SymPy which returns something similar to what the methods in PyBaMM returns and debugging the entire expression trees by putting import pdb; pdb.set_trace() in various places to see what went wrong.

Later I found that jupyter notebook has a %pdb command which came in very handy. Just say %pdb on any cell and subsequently, the pdb debugger will run on all exceptions, no matter how deep in the call stack.

To Do in the coming week

Now in the coming week, I will try to print the voltage expressions of the SPM

That’s all for this post. Thanks!