fixes to sequence.create_baseline:

- assert isinstance(res, list) was wrong because duplicate_task returns a tuple not a list
- removed overkill assertion anyway as the usecase is already typed.
- setting optional name or reuse planned schedule name
- usecase now returns created baseline work schedule

src/ifcopenshell-python/ifcopenshell/api/sequence/create_baseline.py

@@ -28,7 +28,7 @@ import ifcopenshell.util.sequence
 
 def create_baseline(
     file: ifcopenshell.file, work_schedule: ifcopenshell.entity_instance, name: Optional[str] = None
-) -> None:
+) -> ifcopenshell.entity_instance:
     """Creates a baseline for your Work Schedule
 
     Using a IfcWorkSchdule having PredefinedType=PLANNED,
@@ -42,7 +42,7 @@ def create_baseline(
     * Same Construction Resources
     * Same Resource Relationships
 
-    :param work_schedule: The planned work_schedule to baseline
+    :param work_schedule: The planned work schedule to baseline
     :param name: baseline work schedule name
     :return: The baseline work_schedule
 
@@ -51,7 +51,7 @@ def create_baseline(
     .. code:: python
 
         # We have a Work Schedule
-        planned_work_schedule = WorkSchedule(name="Design new feature",predefinedType="PLANNED", deadline="2023-03-01")
+        planned_work_schedule = ifcopenshell.api.sequence.add_work_schedule(model, name="Planned Construction Schedule")
 
         # And now we have a baseline for our Work Schedule
         baseline_work_schedule = ifcopenshell.api.sequence.create_baseline(file, work_schedule=planned_work_schedule, name="Baseline 1")
@@ -64,24 +64,23 @@ def create_baseline(
 class Usecase:
     file: ifcopenshell.file
 
-    def execute(self, work_schedule: ifcopenshell.entity_instance, name: Union[str, None]) -> None:
+    def execute(
-        # create work schedule
+        self, work_schedule: ifcopenshell.entity_instance, name: Union[str, None]
-        if not work_schedule.PredefinedType == "PLANNED":
+    ) -> ifcopenshell.entity_instance:
-            return
+        if work_schedule.PredefinedType != "PLANNED":
+            raise ValueError("Only a PLANNED work schedule can be baselined.")
         baseline_work_schedule = ifcopenshell.api.sequence.add_work_schedule(
-            self.file, name=work_schedule.Name, predefined_type="BASELINE"
+            self.file, name=name or work_schedule.Name, predefined_type="BASELINE"
         )
-        baseline_work_schedule.Name = name
         self.create_baseline_reference(work_schedule, baseline_work_schedule)
         for summary_task in ifcopenshell.util.sequence.get_root_tasks(work_schedule):
-            res = ifcopenshell.api.sequence.duplicate_task(self.file, task=summary_task)
+            current, duplicate = ifcopenshell.api.sequence.duplicate_task(self.file, task=summary_task)
-            assert isinstance(res, list)
-            current, duplicate = res
             ifcopenshell.api.control.assign_control(
                 self.file, relating_control=baseline_work_schedule, related_objects=[duplicate[0]]
             )
             for i, task in enumerate(current):
                 self.create_baseline_reference(task, duplicate[i])
+        return baseline_work_schedule
 
     def create_baseline_reference(
         self, relating_object: ifcopenshell.entity_instance, related_object: ifcopenshell.entity_instance

src/ifcopenshell-python/test/api/sequence/test_create_baseline.py

@@ -0,0 +1,146 @@
+# IfcOpenShell - IFC toolkit and geometry engine
+# Copyright (C) 2026 IfcOpenShell contributors
+#
+# This file is part of IfcOpenShell.
+#
+# IfcOpenShell is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Lesser General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+
+
+import pytest
+
+import ifcopenshell.api.root
+import ifcopenshell.api.sequence
+import ifcopenshell.util.sequence
+import test.bootstrap
+
+
+class TestCreateBaseline(test.bootstrap.IFC4):
+    def create_planned_schedule(self, name="Design & Build"):
+        ifcopenshell.api.root.create_entity(self.file, ifc_class="IfcProject")
+        return ifcopenshell.api.sequence.add_work_schedule(self.file, name=name, predefined_type="PLANNED")
+
+    def test_returns_the_created_baseline_schedule(self):
+        planned = self.create_planned_schedule()
+        root_task = ifcopenshell.api.sequence.add_task(self.file, work_schedule=planned, name="Design")
+
+        baseline = ifcopenshell.api.sequence.create_baseline(self.file, work_schedule=planned, name="Baseline 1")
+
+        assert baseline.is_a("IfcWorkSchedule")
+        assert baseline.Name == "Baseline 1"
+        assert baseline.PredefinedType == "BASELINE"
+        baseline_roots = ifcopenshell.util.sequence.get_root_tasks(baseline)
+        assert [task.Name for task in baseline_roots] == [root_task.Name]
+        assert baseline_roots != [root_task]
+
+    def test_falls_back_to_the_planned_schedule_name(self):
+        planned = self.create_planned_schedule()
+
+        baseline = ifcopenshell.api.sequence.create_baseline(self.file, work_schedule=planned)
+
+        assert baseline.Name == "Design & Build"
+
+    def test_leaves_the_name_null_when_both_names_are_omitted(self):
+        planned = self.create_planned_schedule()
+        planned.Name = None
+
+        baseline = ifcopenshell.api.sequence.create_baseline(self.file, work_schedule=planned)
+
+        assert baseline.Name is None
+
+    def test_rejects_a_non_planned_schedule(self):
+        ifcopenshell.api.root.create_entity(self.file, ifc_class="IfcProject")
+        actual = ifcopenshell.api.sequence.add_work_schedule(self.file, predefined_type="ACTUAL")
+
+        with pytest.raises(ValueError):
+            ifcopenshell.api.sequence.create_baseline(self.file, work_schedule=actual)
+
+    def test_baselines_a_schedule_without_tasks(self):
+        planned = self.create_planned_schedule()
+
+        baseline = ifcopenshell.api.sequence.create_baseline(self.file, work_schedule=planned, name="Baseline 1")
+
+        assert ifcopenshell.util.sequence.get_root_tasks(baseline) == []
+
+    def test_baselines_every_root_task(self):
+        planned = self.create_planned_schedule()
+        ifcopenshell.api.sequence.add_task(self.file, work_schedule=planned, name="Design")
+        ifcopenshell.api.sequence.add_task(self.file, work_schedule=planned, name="Construction")
+
+        baseline = ifcopenshell.api.sequence.create_baseline(self.file, work_schedule=planned, name="Baseline 1")
+
+        baseline_roots = ifcopenshell.util.sequence.get_root_tasks(baseline)
+        assert sorted(task.Name for task in baseline_roots) == ["Construction", "Design"]
+
+    def test_baselines_nested_tasks(self):
+        planned = self.create_planned_schedule()
+        root_task = ifcopenshell.api.sequence.add_task(self.file, work_schedule=planned, name="Construction")
+        ifcopenshell.api.sequence.add_task(self.file, parent_task=root_task, name="Foundations")
+        ifcopenshell.api.sequence.add_task(self.file, parent_task=root_task, name="Superstructure")
+
+        baseline = ifcopenshell.api.sequence.create_baseline(self.file, work_schedule=planned, name="Baseline 1")
+
+        baseline_root = ifcopenshell.util.sequence.get_root_tasks(baseline)[0]
+        nested = ifcopenshell.util.sequence.get_nested_tasks(baseline_root)
+        assert sorted(task.Name for task in nested) == ["Foundations", "Superstructure"]
+        assert len(self.file.by_type("IfcTask")) == 6
+
+    def test_baselines_task_attributes_and_times(self):
+        planned = self.create_planned_schedule()
+        task = ifcopenshell.api.sequence.add_task(
+            self.file, work_schedule=planned, name="Foundations", identification="A1", description="Pour concrete"
+        )
+        ifcopenshell.api.sequence.add_task_time(self.file, task=task)
+        ifcopenshell.api.sequence.edit_task_time(
+            self.file, task_time=task.TaskTime, attributes={"ScheduleDuration": "P5D"}
+        )
+
+        baseline = ifcopenshell.api.sequence.create_baseline(self.file, work_schedule=planned, name="Baseline 1")
+
+        baseline_task = ifcopenshell.util.sequence.get_root_tasks(baseline)[0]
+        assert baseline_task.Identification == "A1"
+        assert baseline_task.Description == "Pour concrete"
+        assert baseline_task.TaskTime != task.TaskTime
+        assert baseline_task.TaskTime.ScheduleDuration == "P5D"
+
+    def test_baselines_sequence_relationships_between_tasks(self):
+        planned = self.create_planned_schedule()
+        root_task = ifcopenshell.api.sequence.add_task(self.file, work_schedule=planned, name="Construction")
+        predecessor = ifcopenshell.api.sequence.add_task(self.file, parent_task=root_task, name="Foundations")
+        successor = ifcopenshell.api.sequence.add_task(self.file, parent_task=root_task, name="Superstructure")
+        ifcopenshell.api.sequence.assign_sequence(self.file, relating_process=predecessor, related_process=successor)
+
+        baseline = ifcopenshell.api.sequence.create_baseline(self.file, work_schedule=planned, name="Baseline 1")
+
+        baseline_root = ifcopenshell.util.sequence.get_root_tasks(baseline)[0]
+        nested = {task.Name: task for task in ifcopenshell.util.sequence.get_nested_tasks(baseline_root)}
+        rels = nested["Foundations"].IsPredecessorTo
+        assert len(rels) == 1
+        assert rels[0].RelatedProcess == nested["Superstructure"]
+
+    def test_references_the_planned_schedule_and_tasks(self):
+        planned = self.create_planned_schedule()
+        root_task = ifcopenshell.api.sequence.add_task(self.file, work_schedule=planned, name="Construction")
+        subtask = ifcopenshell.api.sequence.add_task(self.file, parent_task=root_task, name="Foundations")
+
+        baseline = ifcopenshell.api.sequence.create_baseline(self.file, work_schedule=planned, name="Baseline 1")
+
+        baseline_root = ifcopenshell.util.sequence.get_root_tasks(baseline)[0]
+        baseline_subtask = ifcopenshell.util.sequence.get_nested_tasks(baseline_root)[0]
+        references = {
+            rel.RelatingObject: list(rel.RelatedObjects) for rel in self.file.by_type("IfcRelDefinesByObject")
+        }
+        assert references[planned] == [baseline]
+        assert references[root_task] == [baseline_root]
+        assert references[subtask] == [baseline_subtask]
+
+    def test_reuses_the_existing_reference_for_further_baselines(self):
+        planned = self.create_planned_schedule()
+
+        first = ifcopenshell.api.sequence.create_baseline(self.file, work_schedule=planned, name="Baseline 1")
+        second = ifcopenshell.api.sequence.create_baseline(self.file, work_schedule=planned, name="Baseline 2")
+
+        assert len(planned.Declares) == 1
+        assert list(planned.Declares[0].RelatedObjects) == [first, second]