Model parameter index for components of non-scalar parameters

Summary

With the addition of model.config.par_names, there is a well-defined name for components of non-scalar parameters available within pyhf. It would be useful if model.config.par_slice would support parameter indexing to access components of non-scalar parameters. At the moment it will return the slice of all components, and return a KeyError when using it with the name of a component.

This can be worked around (example utility in cabinetry: model_utils._parameter_index), but it would be convenient to support this in the pyhf API.

related:

• #1235
• possible future addition: #994
• changes to support model.config.par_names:
  • #867
  • #1536
  • #1560

Additional Information

Example usage for a model with multiple bins for a channel and staterror modifier

model: pyhf.pdf.Model
model.config.par_names()  # ['mu', 'staterror_SR[0]', 'staterror_SR[1]', 'staterror_SR[2]', 'staterror_SR[3]'] 
model.config.par_slice("staterror_SR")  # returns e.g. slice(1, 5, None)
model.config.par_slice("staterror_SR[0]")  # KeyError: 'staterror_SR[0]'

Code of Conduct

• I agree to follow the Code of Conduct

[kratsg]

I think arguably, if you want to support that, we could have experimental query language support plugged in, so that the par_slice can delegate to the query language functionality…

[alexander-held]

As far as I remember, yes #994 allows for this too. I opened this issue because I think it makes sense to have it in the model.config.par_slice API directly instead of just in the pyhf.experimental API. I would say that

model.config.par_slice("staterror_SR[0]")  # KeyError: 'staterror_SR[0]

not working could almost be viewed as a bug, since par_names is perfectly happy to provide that name.