feat: Allow POI-less models via Workspace.model

docs/conf.py

@@ -77,6 +77,7 @@ def setup(app):
     'matplotlib': ('https://matplotlib.org/stable/', None),
     'iminuit': ('https://iminuit.readthedocs.io/en/stable/', None),
     'uproot': ('https://uproot.readthedocs.io/en/latest/', None),
+    'jsonpatch': ('https://python-json-patch.readthedocs.io/en/latest/', None),
 }
 
 # GitHub repo

src/pyhf/workspace.py

@@ -319,24 +319,20 @@ def __repr__(self):
         """Representation of the Workspace."""
         return object.__repr__(self)
 
-    def get_measurement(
-        self, poi_name=None, measurement_name=None, measurement_index=None
-    ):
+    def get_measurement(self, measurement_name=None, measurement_index=None):
         """
-        Get (or create) a measurement object.
+        Get a measurement object.
 
         The following logic is used:
 
-          1. if the poi name is given, create a measurement object for that poi
-          2. if the measurement name is given, find the measurement for the given name
-          3. if the measurement index is given, return the measurement at that index
-          4. if there are measurements but none of the above have been specified, return the 0th measurement
+          1. if the measurement name is given, find the measurement for the given name
+          2. if the measurement index is given, return the measurement at that index
+          3. if there are measurements but none of the above have been specified, return the 0th measurement
 
         Raises:
           ~pyhf.exceptions.InvalidMeasurement: If the measurement was not found
 
         Args:
-            poi_name (:obj:`str`): The name of the parameter of interest to create a new measurement from
             measurement_name (:obj:`str`): The name of the measurement to use
             measurement_index (:obj:`int`): The index of the measurement to use
 
@@ -345,12 +341,7 @@ def get_measurement(
 
         """
         measurement = None
-        if poi_name is not None:
-            measurement = {
-                'name': 'NormalMeasurement',
-                'config': {'poi': poi_name, 'parameters': []},
-            }
-        elif self.measurement_names:
+        if self.measurement_names:
             if measurement_name is not None:
                 if measurement_name not in self.measurement_names:
                     log.debug(f"measurements defined: {self.measurement_names}")
@@ -381,41 +372,57 @@ def get_measurement(
         utils.validate(measurement, 'measurement.json', self.version)
         return measurement
 
-    def model(self, **config_kwargs):
+    def model(
+        self,
+        measurement_name=None,
+        measurement_index=None,
+        patches=None,
+        **config_kwargs,
+    ):
         """
         Create a model object with/without patches applied.
 
         See :func:`pyhf.workspace.Workspace.get_measurement` and :class:`pyhf.pdf.Model` for possible keyword arguments.
 
         Args:
-            patches: A list of JSON patches to apply to the model specification
-            config_kwargs: Possible keyword arguments for the measurement and model configuration
+            measurement_name (:obj:`str`): The name of the measurement to use
+             in :func:`~pyhf.workspace.Workspace.get_measurement`.
+            measurement_index (:obj:`int`): The index of the measurement to use
+             in :func:`~pyhf.workspace.Workspace.get_measurement`.
+            patches (:obj:`list` of :class:`jsonpatch.JsonPatch` or :class:`pyhf.patchset.Patch`):
+             A list of patches to apply to the model specification.
+            config_kwargs: Possible keyword arguments for the model
+             configuration.
+             See :class:`~pyhf.pdf.Model` for more details.
+            poi_name (:obj:`str` or :obj:`None`): Specify this keyword argument
+             to override the default parameter of interest specified in the
+             measurement.
+             Set to :obj:`None` for a POI-less model.
 
         Returns:
             ~pyhf.pdf.Model: A model object adhering to the schema model.json
 
         """
-
-        poi_name = config_kwargs.pop('poi_name', None)
-        measurement_name = config_kwargs.pop('measurement_name', None)
-        measurement_index = config_kwargs.pop('measurement_index', None)
         measurement = self.get_measurement(
-            poi_name=poi_name,
             measurement_name=measurement_name,
             measurement_index=measurement_index,
         )
-        log.debug(f"model being created for measurement {measurement['name']:s}")
 
-        patches = config_kwargs.pop('patches', [])
+        # set poi_name if the user does not provide it
+        config_kwargs.setdefault('poi_name', measurement['config']['poi'])
+
+        log.debug(f"model being created for measurement {measurement['name']:s}")
 
         modelspec = {
             'channels': self['channels'],
             'parameters': measurement['config']['parameters'],
         }
+
+        patches = patches or []
         for patch in patches:
             modelspec = jsonpatch.JsonPatch(patch).apply(modelspec)
 
-        return Model(modelspec, poi_name=measurement['config']['poi'], **config_kwargs)
+        return Model(modelspec, **config_kwargs)
 
     def data(self, model, include_auxdata=True):
         """

tests/test_workspace.py

@@ -67,12 +67,6 @@ def test_get_measurement(workspace_factory):
         assert m['name'] == w.measurement_names[measurement_idx]
 
 
-def test_get_measurement_fake(workspace_factory):
-    w = workspace_factory()
-    m = w.get_measurement(poi_name='fake_poi')
-    assert m
-
-
 def test_get_measurement_nonexist(workspace_factory):
     w = workspace_factory()
     with pytest.raises(pyhf.exceptions.InvalidMeasurement) as excinfo:
@@ -98,12 +92,6 @@ def test_get_measurement_no_measurements_defined(workspace_factory):
 def test_get_workspace_measurement_priority(workspace_factory):
     w = workspace_factory()
 
-    # does poi_name override all others?
-    m = w.get_measurement(
-        poi_name='fake_poi', measurement_name='FakeMeasurement', measurement_index=999
-    )
-    assert m['config']['poi'] == 'fake_poi'
-
     # does measurement_name override measurement_index?
     m = w.get_measurement(
         measurement_name=w.measurement_names[0], measurement_index=999
@@ -136,6 +124,27 @@ def test_get_workspace_model_default(workspace_factory):
     assert m
 
 
+def test_get_workspace_model_nopoi(workspace_factory):
+    w = workspace_factory()
+    m = w.model(poi_name=None)
+
+    assert m.config.poi_name is None
+    assert m.config.poi_index is None
+
+
+def test_get_workspace_model_overridepoi(workspace_factory):
+    w = workspace_factory()
+    m = w.model(poi_name='lumi')
+
+    assert m.config.poi_name == 'lumi'
+
+
+def test_get_workspace_model_fakepoi(workspace_factory):
+    w = workspace_factory()
+    with pytest.raises(pyhf.exceptions.InvalidModel):
+        w.model(poi_name='afakepoi')
+
+
 def test_workspace_observations(workspace_factory):
     w = workspace_factory()
     assert w.observations