fix: added hasattr check to NanoAODEvents __repr__

src/coffea/nanoevents/methods/nanoaod.py

@@ -13,7 +13,9 @@
 
 class _NanoAODEvents(behavior["NanoEvents"]):
     def __repr__(self):
-        return f"<event {self.run}:{self.luminosityBlock}:{self.event}>"
+        return f"<event {getattr(self,'run','??')}:\
+                {getattr(self,'luminosityBlock','??')}:\
+                {getattr(self,'event','??')}>"
 
 
 behavior["NanoEvents"] = _NanoAODEvents

src/coffea/nanoevents/schemas/nanoaod.py

@@ -37,10 +37,18 @@ class NanoAODSchema(BaseSchema):
     There is a class-level variable ``warn_missing_crossrefs`` which will alter the behavior of
     NanoAODSchema. If warn_missing_crossrefs is true then when a missing global index cross-ref
     target is encountered a warning will be issued. Regardless, the cross-reference is dropped.
+
+    The same holds for ``error_missing_events_id``. If error_missing_events_id is true, then when the 'run', 'event',
+    or 'luminosityBlock' fields are missing, an exception will be thrown; if it is false, just a warning will be issued.
     """
 
     __dask_capable__ = True
     warn_missing_crossrefs = True
+    error_missing_event_ids = True
+
+    event_ids = ["run", "luminosityBlock", "event"]
+    """List of NanoAOD event IDs
+    """
 
     mixins = {
         "CaloMET": "MissingET",
@@ -206,6 +214,26 @@ def _build_collections(self, field_names, input_contents):
                     branch_forms["n" + name]
                 )
 
+        # Check the presence of the event_ids
+        missing_event_ids = [
+            event_id for event_id in self.event_ids if event_id not in branch_forms
+        ]
+        if len(missing_event_ids) > 0:
+            if self.error_missing_event_ids:
+                raise RuntimeError(
+                    f"There are missing event ID fields: {missing_event_ids} \n\n\
+                    The event ID fields {self.event_ids} are necessary to perform sub-run identification \
+                    (e.g. for corrections and sub-dividing data during different detector conditions),\
+                    to cross-validate MC and Data (i.e. matching events for comparison), and to generate event displays. \
+                    It's advised to never drop these branches from the dataformat.\n\n\
+                    This error can be demoted to a warning by setting the class level variable error_missing_event_ids to False."
+                )
+            else:
+                warnings.warn(
+                    f"Missing event_ids : {missing_event_ids}",
+                    RuntimeWarning,
+                )
+
         # Create global index virtual arrays for indirection
         for indexer, target in self.cross_references.items():
             if target.startswith("Gen") and isData:

tests/test_nanoevents.py

@@ -156,3 +156,20 @@ def test_read_nanodata(suffix):
 
     crossref(events)
     crossref(events[ak.num(events.Jet) > 2])
+
+
+def test_missing_eventIds_error():
+    path = os.path.abspath("tests/samples/missing_luminosityBlock.root") + ":Events"
+    with pytest.raises(RuntimeError):
+        factory = NanoEventsFactory.from_root(path, schemaclass=NanoAODSchema)
+        factory.events()
+
+
+def test_missing_eventIds_warning():
+    path = os.path.abspath("tests/samples/missing_luminosityBlock.root") + ":Events"
+    with pytest.warns(
+        RuntimeWarning, match=r"Missing event_ids \: \[\'luminosityBlock\'\]"
+    ):
+        NanoAODSchema.error_missing_event_ids = False
+        factory = NanoEventsFactory.from_root(path, schemaclass=NanoAODSchema)
+        factory.events()