Raise a ValueError when calling bool() on an ArraySymbol

dwave/optimization/model.pyx

@@ -1625,6 +1625,11 @@ cdef class ArraySymbol(Symbol):
 
         return NotImplemented
 
+    def __bool__(self):
+        # In the future we might want to return a Bool symbol, but __bool__ is so
+        # fundamental that I am hesitant to do even that.
+        raise ValueError("the truth value of an array symbol is ambiguous")
+
     def __eq__(self, rhs):
         if isinstance(rhs, ArraySymbol):
             # We could consider returning a Constant(True) is the case that self is rhs

releasenotes/notes/improve-operators-54e60d66200526cc.yaml

@@ -12,3 +12,5 @@ features:
     In-place multiplication with a ``NaryMultiply`` symbol will no longer create a new symbol. 
 fixes:
   - Return ``NotImplemented`` from ``ArraySymbol`` operator methods for unknown types.
+upgrade:
+  - Raise a ``ValueError`` when calling ``bool()`` on an ``ArraySymbol``.

tests/test_examples.py

@@ -86,7 +86,9 @@ def smart_reduce(f, items):
                 [shifts[day][shift] for day in range(num_days)]
             )
             minutes_worked = total_shifts_worked * minutes_per_shift[shift]
-            total_minutes_worked = total_minutes_worked + minutes_worked if total_minutes_worked else minutes_worked
+            total_minutes_worked = (total_minutes_worked + minutes_worked
+                                    if total_minutes_worked is not None
+                                    else minutes_worked)
 
         min_minutes = model.constant(min_minutes_per_week)
         max_minutes = model.constant(max_minutes_per_week)

tests/test_model.py

@@ -32,6 +32,16 @@ def test_abstract(self):
         with self.assertRaisesRegex(ValueError, "ArraySymbols cannot be constructed directly"):
             ArraySymbol()
 
+    def test_bool(self):
+        from dwave.optimization.model import ArraySymbol
+
+        # bypass the init, this should be done very carefully as it can lead to
+        # segfaults dependig on what methods are accessed!
+        symbol = ArraySymbol.__new__(ArraySymbol)
+
+        with self.assertRaises(ValueError):
+            bool(symbol)
+
     def test_operator_types(self):
         # For each, test that we get the right class from the operator and that
         # incorrect types returns NotImplemented