Do not generate default repetition ids if use_repetition_ids=False

Fixes #5418

cirq-core/cirq/circuits/circuit_operation.py

@@ -148,14 +148,15 @@ def __post_init__(self):
                     raise ValueError('repetitions are negative but the circuit is not invertible')
 
             # Initialize repetition_ids to default, if unspecified. Else, validate their length.
-            loop_size = abs(self.repetitions)
-            if not self.repetition_ids:
-                object.__setattr__(self, 'repetition_ids', self._default_repetition_ids())
-            elif len(self.repetition_ids) != loop_size:
-                raise ValueError(
-                    f'Expected repetition_ids to be a list of length {loop_size}, '
-                    f'got: {self.repetition_ids}'
-                )
+            if self.use_repetition_ids:
+                loop_size = abs(self.repetitions)
+                if not self.repetition_ids:
+                    object.__setattr__(self, 'repetition_ids', self._default_repetition_ids())
+                elif len(self.repetition_ids) != loop_size:
+                    raise ValueError(
+                        f'Expected repetition_ids to be a list of length {loop_size}, '
+                        f'got: {self.repetition_ids}'
+                    )
         elif isinstance(self.repetitions, sympy.Expr):
             if self.repetition_ids is not None:
                 raise ValueError('Cannot use repetition ids with parameterized repetitions')
@@ -377,7 +378,7 @@ def __repr__(self):
             args += f'param_resolver={proper_repr(self.param_resolver)},\n'
         if self.parent_path:
             args += f'parent_path={proper_repr(self.parent_path)},\n'
-        if self.repetition_ids != self._default_repetition_ids():
+        if self.use_repetition_ids and (self.repetition_ids != self._default_repetition_ids()):
             # Default repetition_ids need not be specified.
             args += f'repetition_ids={proper_repr(self.repetition_ids)},\n'
         if not self.use_repetition_ids:
@@ -408,7 +409,7 @@ def dict_str(d: Dict) -> str:
             args.append(f'params={self.param_resolver.param_dict}')
         if self.parent_path:
             args.append(f'parent_path={self.parent_path}')
-        if self.repetition_ids != self._default_repetition_ids():
+        if self.use_repetition_ids and (self.repetition_ids != self._default_repetition_ids()):
             # Default repetition_ids need not be specified.
             args.append(f'repetition_ids={self.repetition_ids}')
         elif self.repetitions != 1:

cirq-core/cirq/circuits/circuit_operation_test.py

@@ -11,13 +11,15 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+import unittest.mock as mock
 from typing import Optional
 
 import numpy as np
 import pytest
 import sympy
 
 import cirq
+import cirq.circuits.circuit_operation as circuit_operation
 from cirq.circuits.circuit_operation import _full_join_string_lists
 
 ALL_SIMULATORS = (cirq.Simulator(), cirq.DensityMatrixSimulator(), cirq.CliffordSimulator())
@@ -346,6 +348,21 @@ def test_repeat_zero_times(add_measurements, use_repetition_ids, initial_reps):
     assert np.allclose(result.state_vector(), [1, 0])
 
 
+def test_no_repetition_ids():
+    def default_repetition_ids(self):
+        assert False, "Should not call default_repetition_ids"
+
+    with mock.patch.object(circuit_operation, 'default_repetition_ids', new=default_repetition_ids):
+        q = cirq.LineQubit(0)
+        op = cirq.CircuitOperation(
+            cirq.Circuit(cirq.X(q), cirq.measure(q)).freeze(),
+            repetitions=1_000_000,
+            use_repetition_ids=False,
+        )
+        _ = repr(op)
+        _ = str(op)
+
+
 def test_parameterized_repeat():
     q = cirq.LineQubit(0)
     op = cirq.CircuitOperation(cirq.FrozenCircuit(cirq.X(q))) ** sympy.Symbol('a')