Can no longer set several model config attributes

[alexander-held]

Summary

The changes in #1972 cause model.config.poi_index to not be writable anymore. cabinetry sets this value directly to support running pyhf.infer.hypotest with a configurable POI (without the need to re-build the model).

OS / Environment

n/a

Steps to Reproduce

import pyhf

model = pyhf.simplemodels.uncorrelated_background(
    signal=[24.0, 22.0], bkg=[50.0, 52.0], bkg_uncertainty=[3.0, 7.0]
)
model.config.poi_index = 1

File Upload (optional)

No response

Expected Results

assignment succeeds

Actual Results

Traceback (most recent call last):
  File "test.py", line 7, in <module>
    model.config.poi_index = ""
AttributeError: can't set attribute

pyhf Version

pyhf, version 0.7.0rc4.dev2

Code of Conduct

• I agree to follow the Code of Conduct

[alexander-held]

This affects also model.config.poi_name and presumably the rest of the attributes that were decorated with @property: par_order, auxdata, nmaindata, nauxdata, as well as model.config itself. I could imagine people possibly wanting to override auxdata and potentially the full model.config, I have not had the need to do so myself.

Adding the following two setters restores the functionality cabinetry uses at the moment:

diff --git a/src/pyhf/pdf.py b/src/pyhf/pdf.py
index db59fa1c..969b26db 100644
--- a/src/pyhf/pdf.py
+++ b/src/pyhf/pdf.py
@@ -245,6 +245,13 @@ def poi_name(self):
         """
         return self._poi_name

+    @poi_name.setter
+    def poi_name(self, value):
+        """
+        Set the name of the POI parameter in the model.
+        """
+        self._poi_name = value
+
     @property
     def poi_index(self):
         """
@@ -252,6 +259,13 @@ def poi_index(self):
         """
         return self._poi_index

+    @poi_index.setter
+    def poi_index(self, value):
+        """
+        Set the index of the POI parameter in the model.
+        """
+        self._poi_index = value
+
     @property
     def auxdata(self):
         """

[matthewfeickert]

When this is fixed there should additionally be a test that is added to ensure that these can be set by the user.

[kratsg]

This is not a bug. The API for setting the poi_index is done through model.config.set_poi(name: str). https://scikit-hep.org/pyhf/_generated/pyhf.pdf._ModelConfig.html#pyhf.pdf._ModelConfig.set_poi

We never encouraged anyone to override these other properties you mentioned either. The whole point is to try and make Model less dynamically configurable because there was always confusion about how mutable it was meant to be.

[alexander-held]

Making this explicit by using set_poi is a good solution, but it currently does not support setting POIs back to "no POI" mode where poi_name / poi_index are None. This worked when setting the attribute directly so far.

import pyhf

spec = {
    "channels": [
        {
            "name": "SR",
            "samples": [
                {
                    "data": [50],
                    "modifiers": [
                        {"data": None, "name": "mu", "type": "normfactor"},
                    ],
                    "name": "Signal",
                },
            ],
        }
    ],
    "measurements": [
        {
            "config": {
                "parameters": [],
                "poi": "",
            },
            "name": "meas",
        }
    ],
    "observations": [{"data": [50], "name": "SR"}],
    "version": "1.0.0",
}

ws = pyhf.Workspace(spec)
model = ws.model()
data = ws.data(model)

print(model.config.poi_name is None)  # True
print(model.config.poi_index is None)  # True

model.config.set_poi("mu")  # temporarily set for e.g. hypotest
pyhf.infer.hypotest(1.0, data, model)

# neither of the following works, expects string for a parameter that is in the model
# model.config.set_poi(None)
# model.config.set_poi("")