SimultaneousEvent.extend_until: Raise err if empty

If 'extend_until' is called on an empty 'SimultaneousEvent' nothing will
happen. This in counter-intuitive and not obvious. It's therefore better
to raise an explicit error that the operation failed.

mutwo/core_events/basic.py

@@ -845,6 +845,15 @@ def extend_until(
             duration_to_white_space
             or core_events.configurations.DEFAULT_DURATION_TO_WHITE_SPACE
         )
+        # We only append simple events to sequential events, because there
+        # are many problems with the SimultaneousEvent[SimpleEvent] construct
+        # ('extend_until' and 'squash_in' will fail on such a container).
+        # Therefore calling 'extend_until' on an empty SimultaneousEvent is
+        # in fact ineffective: The user would get a SimultaneousEvent which
+        # still has duration = 0, which is absolutely unexpected. Therefore
+        # we raise an error, to avoid confusion by the user.
+        if not self:
+            raise core_utilities.IneffectiveExtendUntilError(self)
         for event in self:
             try:
                 event.extend_until(
@@ -881,8 +890,8 @@ def concatenate_by_index(self, other: SimultaneousEvent) -> SimultaneousEvent:
         >>> s.concatenate_by_index(s)
         SimultaneousEvent([SequentialEvent([SimpleEvent(duration = DirectDuration(duration = 1)), SimpleEvent(duration = DirectDuration(duration = 1))])])
         """
-        self_duration = self.duration
-        self.extend_until(self_duration)
+        if (self_duration := self.duration) > 0:
+            self.extend_until(self_duration)
         for index, event in enumerate(other.copy()):
             try:
                 ancestor = self[index]
@@ -919,8 +928,8 @@ def concatenate_by_tag(self, other: SimultaneousEvent) -> SimultaneousEvent:
         >>> s.concatenate_by_tag(s)
         SimultaneousEvent([TaggedSequentialEvent([SimpleEvent(duration = DirectDuration(duration = 1)), SimpleEvent(duration = DirectDuration(duration = 1))])])
         """
-        self_duration = self.duration
-        self.extend_until(self_duration)
+        if (self_duration := self.duration) > 0:
+            self.extend_until(self_duration)
         for tagged_event in other.copy():
             if not hasattr(tagged_event, "tag"):
                 raise core_utilities.NoTagError(tagged_event)

mutwo/core_utilities/exceptions.py

@@ -13,6 +13,7 @@
     "ImpossibleToSquashInError",
     "ImpossibleToSlideInError",
     "ImpossibleToExtendUntilError",
+    "IneffectiveExtendUntilError",
     "InvalidStartAndEndValueError",
     "InvalidCutOutStartAndEndValuesError",
     "SplitUnavailableChildError",
@@ -95,6 +96,16 @@ def __init__(self, event_to_extend_until):
         )
 
 
+class IneffectiveExtendUntilError(ValueError):
+    def __init__(self, event_to_extend_until):
+        super().__init__(
+            f"Can't extend empty event '{event_to_extend_until}' of type"
+            f"'{type(event_to_extend_until)}'. If you want to extend "
+            "a SimultaneousEvent you should first append an empty "
+            "SequentialEvent to your SimultaneousEvent."
+        )
+
+
 class InvalidStartAndEndValueError(Exception):
     def __init__(self, start, end):
         super().__init__(

tests/events/basic_tests.py

@@ -1047,6 +1047,12 @@ def test_extend_until(self):
             prolong_simple_event=False,
         )
 
+        # We can't call 'extend_until' on an empty SimultaneousEvent: this would
+        # raise an error.
+        self.assertRaises(
+            core_utilities.IneffectiveExtendUntilError, si().extend_until, 10
+        )
+
         # Extend sequential events inside simultaneous event..
         ese = se([s(1), s(2), s(3), s(4)])  # extended sequential event
         self.assertEqual(