Why can I not change Tidy3D instances after they are created?

Date	Category
2023-10-24 18:09:43	Simulation Troubleshoot

You may notice in Tidy3D versions 1.5 and above that it is no longer possible to modify instances of Tidy3D components after they are created. Making Tidy3D components immutable like this was an intentional design decision intended to make Tidy3D safer and more performant.

For example, Tidy3D contains several “validators” on input data. If models are mutated, we can’t always guarantee that the resulting instance will still satisfy our validations, and the simulation may be invalid.

Furthermore, making the objects immutable allows us to cache the results of many expensive operations. For example, we can now compute and store the simulation grid without worrying about the value becoming stale later, which significantly speeds up plotting and other operations.

If you have a Tidy3D component that you want to recreate with a new set of parameters, instead of obj.param1 = param1_new, you can call obj_new = obj.copy(update=dict(param1=param1_new)). Note that you may also pass more key value pairs to the dictionary in update. Also, note you can use a convenience method obj_new = obj.updated_copy(param1=param1_new), which is just a shortcut to the obj.copy() call above.