CostSum support for cost functions accepting a parameter array about iminuit HOT 3 OPEN

The cost functions that accept a parametric model like ExtendedBinnedNLL use the function iminuit.util.describe(model, annotations=True) to detect the parameters of the model (the first argument is skipped).

You can override self._parameters in the cost function, but the intention is to add the special attribute _parameters to your model function. This can be done even if that function is a normal Python function or a lambda, because in Python everything is an object.

def my_model(x, *args): ...

# use my model with 3 parameters
my_model._parameters = {x: None, a: None, b: None, c: None}
nll = ExtendedBinnedNLL(counts, bin_edges, my_model)
Minuit(nll, ...).migrad()  # get results for 3 parameters

# use my model with 2 parameters
my_model._parameters = {x: None, a: None, b: None}
nll = ExtendedBinnedNLL(counts, bin_edges, my_model)
Minuit(nll, ...).migrad()  # get results for 2 parameters

Your solution works as well, but I don't think it is a good idea to add a public interface for this.

I don't want normal users to set parameters manually, because this is supposed to be handled behind the scenes by iminuit. For experts like you who need this, there is the option to provide the special _parameters attribute. This is documented in iminuit.util.describe.

from iminuit.

Adding a name keyword to the constructor of the cost functions seems like a possibility to me, but I need to think about the possible consequences.

from iminuit.

Thanks for the detailed explanation and for pointing me towards iminuit.utils.describe!

from iminuit.