Support empty invert_mask in measument gate deserialization

cirq-google/cirq_google/serialization/circuit_serializer.py

@@ -14,7 +14,7 @@
 
 """Support for serializing and deserializing cirq_google.api.v2 protos."""
 
-from typing import cast, Any, Dict, List, Optional
+from typing import Any, Dict, List, Optional
 import sympy
 
 import cirq
@@ -546,20 +546,22 @@ def _deserialize_gate_op(
                 arg_function_language=arg_function_language,
                 required_arg_name=None,
             )
-            invert_mask = cast(
-                List[bool],
-                arg_func_langs.arg_from_proto(
-                    operation_proto.measurementgate.invert_mask,
-                    arg_function_language=arg_function_language,
-                    required_arg_name=None,
-                ),
+            parsed_invert_mask = arg_func_langs.arg_from_proto(
+                operation_proto.measurementgate.invert_mask,
+                arg_function_language=arg_function_language,
+                required_arg_name=None,
             )
-            if isinstance(invert_mask, list) and isinstance(key, str):
-                op = cirq.MeasurementGate(
-                    num_qubits=len(qubits), key=key, invert_mask=tuple(invert_mask)
-                )(*qubits)
+            if (isinstance(parsed_invert_mask, list) or parsed_invert_mask is None) and isinstance(
+                key, str
+            ):
+                invert_mask: tuple[bool, ...] = ()
+                if parsed_invert_mask is not None:
+                    invert_mask = tuple(bool(x) for x in parsed_invert_mask)
+                op = cirq.MeasurementGate(num_qubits=len(qubits), key=key, invert_mask=invert_mask)(
+                    *qubits
+                )
             else:
-                raise ValueError(f'Incorrect types for measurement gate {invert_mask} {key}')
+                raise ValueError(f'Incorrect types for measurement gate {parsed_invert_mask} {key}')
 
         elif which_gate_type == 'waitgate':
             total_nanos = arg_func_langs.float_arg_from_proto(

cirq-google/cirq_google/serialization/circuit_serializer_test.py

@@ -660,3 +660,11 @@ def test_no_constants_table():
 
     with pytest.raises(ValueError, match='Proto has references to constants table'):
         serializer._deserialize_gate_op(op)
+
+
+def test_measurement_gate_deserialize() -> None:
+    q = cirq.NamedQubit('q')
+    circuit = cirq.Circuit(cirq.X(q) ** 0.5, cirq.measure(q))
+    msg = cg.CIRCUIT_SERIALIZER.serialize(circuit)
+
+    assert cg.CIRCUIT_SERIALIZER.deserialize(msg) == circuit