feat: Add POI-less specification support (#1638)

* Support JSON specifications with empty string for POI by treating as POI-less
* Support poi_name as empty string, in addition to None
* Add test for POI-less workspace

src/pyhf/pdf.py

@@ -629,6 +629,7 @@ def __init__(
             modifier_set, self.config, self.spec, self.batch_size
         )
 
+        poi_name = None if poi_name == "" else poi_name
         if poi_name is not None:
             self.config.set_poi(poi_name)
 

tests/test_pdf.py

@@ -278,6 +278,39 @@ def test_poiless_model(backend):
         pyhf.infer.hypotest(1.0, data, model)
 
 
+def test_poiless_model_empty_string(backend):
+    spec = {
+        'channels': [
+            {
+                'name': 'channel',
+                'samples': [
+                    {
+                        'name': 'goodsample',
+                        'data': [10.0],
+                        'modifiers': [
+                            {
+                                'type': 'normsys',
+                                'name': 'shape',
+                                'data': {"hi": 0.5, "lo": 1.5},
+                            }
+                        ],
+                    },
+                ],
+            }
+        ]
+    }
+    model = pyhf.Model(spec, poi_name="")
+
+    data = [12] + model.config.auxdata
+    pyhf.infer.mle.fit(data, model)
+
+    with pytest.raises(pyhf.exceptions.UnspecifiedPOI):
+        pyhf.infer.mle.fixed_poi_fit(1.0, data, model)
+
+    with pytest.raises(pyhf.exceptions.UnspecifiedPOI):
+        pyhf.infer.hypotest(1.0, data, model)
+
+
 def test_pdf_integration_shapesys_zeros(backend):
     spec = {
         "channels": [

tests/test_workspace.py

@@ -875,3 +875,15 @@ def test_wspace_immutable():
     after = model.config.suggested_init()
 
     assert before == after
+
+
+def test_workspace_poiless(datadir):
+    """
+    Test that a workspace with a measurement with empty POI string is treated as POI-less
+    """
+    spec = json.load(open(datadir.join("poiless.json")))
+    ws = pyhf.Workspace(spec)
+    model = ws.model()
+
+    assert model.config.poi_name is None
+    assert model.config.poi_index is None

tests/test_workspace/poiless.json

@@ -0,0 +1,59 @@
+{
+  "channels": [
+    {
+      "name": "singlechannel",
+      "samples": [
+        {
+          "name": "signal",
+          "data": [
+            12,
+            11
+          ],
+          "modifiers": [
+            {
+              "name": "mu",
+              "type": "normfactor",
+              "data": null
+            }
+          ]
+        },
+        {
+          "name": "background",
+          "data": [
+            50,
+            52
+          ],
+          "modifiers": [
+            {
+              "name": "uncorr_bkguncrt",
+              "type": "shapesys",
+              "data": [
+                3,
+                7
+              ]
+            }
+          ]
+        }
+      ]
+    }
+  ],
+  "observations": [
+    {
+      "name": "singlechannel",
+      "data": [
+        51,
+        48
+      ]
+    }
+  ],
+  "measurements": [
+    {
+      "name": "Measurement",
+      "config": {
+        "poi": "",
+        "parameters": []
+      }
+    }
+  ],
+  "version": "1.0.0"
+}