How to write tests to make sure this is working correctly? I would need to catch exception

You could write rust unit tests, but in this case it might be better to write a python test snippet which catches the exception and tests if the proper exception is raised. You could use try-except-else for this, like is done here: https://github.com/RustPython/RustPython/blob/master/tests/snippets/builtin_hex.py

Sorry about being slow to respond to this.

I'd prefer to see this named something like apply_operator. I'd probably give d and r names like lhs_method_name and rhs_method_name.