Perturbations API

[ntessore]

We should start thinking about a cosmoAPI for perturbations. Let's open a couple of threads in this discussion with the relevant questions.

[ntessore]

One fundamental question for me is whether the perturbations API will normally be a separate object or assumed to be part of the cosmology object.

The advantage of the former is that it becomes easy to code up something like growth function and an Eisenstein-Hu power spectrum and call that your perturbations, independent of background. Emulators fall into the same category.

The disadvantage is that Boltzmann codes like CAMB or CLASS would provide both background and perturbations API in one, requiring either wrapping one of their instances in two objects, or passing the same object twice.

[nstarman]

IMO the arguments for separate objects outweighs those for a single object. It's easy to create a helper function to which one passes a single CAMB instance and which returns two objects (or a composite object of both the background cosmology + perturbations).

[ntessore]

Yeah, it wouldn't have to be a separate perturbation object, it could just be the same object supporting both interfaces. You could then imagine functions defined as (but this is a lot of boilerplate...)

@overload
def do_something(cosmo: Background, perturb: Perturbations):
    ...
@overload
def do_something(cosmo: BackgroundAndPerturbations, perturb: None):
    ...
def do_something(cosmo: Background | BackgroundAndPerturbations, perturb: Perturbations | None = None):
    ...

On the other hand, it's also easy to create a helper function that takes a background object and an object defining some perturbation methods and binds them together.

[nstarman]

Agreed. Your example is a very Postel's law piece of code. The signature might reasonably just be

def do_something(cosmo: BackgroundAndPerturbations): ...

and the user has to supply a BackgroundAndPerturbation(cosmo, perturb) object.

[nstarman]

Where BackgroundAndPerturbation is the intersection type BackgroundCosmology & Perturbation.