Test gate being added multiple times.

cirq/google/devices/known_devices_test.py

@@ -458,3 +458,54 @@ def test_proto_with_waitgate():
   }
 }
 """
+
+
+def test_adding_gates_multiple_times():
+    waiting_for_godot = cg.serializable_gate_set.SerializableGateSet(
+        gate_set_name='wait_gateset',
+        serializers=[
+            cgc.WAIT_GATE_SERIALIZER, cgc.WAIT_GATE_SERIALIZER,
+            cgc.WAIT_GATE_SERIALIZER
+        ],
+        deserializers=[
+            cgc.WAIT_GATE_DESERIALIZER, cgc.WAIT_GATE_DESERIALIZER,
+            cgc.WAIT_GATE_DESERIALIZER
+        ],
+    )
+    wait_proto = cg.devices.known_devices.create_device_proto_from_diagram(
+        "aa",
+        [waiting_for_godot],
+    )
+    wait_device = cg.SerializableDevice.from_proto(
+        proto=wait_proto, gate_sets=[waiting_for_godot])
+    q0 = cirq.GridQubit(0, 0)
+    wait_op = cirq.WaitGate(duration=cirq.Duration(nanos=25))(q0)
+    wait_device.validate_operation(wait_op)
+
+    assert str(wait_proto) == """\
+valid_gate_sets {
+  name: "wait_gateset"
+  valid_gates {
+    id: "wait"
+    number_of_qubits: 1
+    valid_args {
+      name: "nanos"
+      type: FLOAT
+    }
+  }
+}
+valid_qubits: "0_0"
+valid_qubits: "0_1"
+valid_targets {
+  name: "meas_targets"
+  target_ordering: SUBSET_PERMUTATION
+}
+valid_targets {
+  name: "2_qubit_targets"
+  target_ordering: SYMMETRIC
+  targets {
+    ids: "0_0"
+    ids: "0_1"
+  }
+}
+"""

cirq/google/serializable_gate_set_test.py

@@ -54,7 +54,7 @@
 
 Y_DESERIALIZER = cg.GateOpDeserializer(
     serialized_gate_id='y_pow',
-    gate_constructor=cirq.XPowGate,
+    gate_constructor=cirq.YPowGate,
     args=[
         cg.DeserializingArg(
             serialized_name='half_turns',
@@ -388,6 +388,66 @@ def test_gateset_with_added_gates():
     assert xy_gateset.is_supported_gate(cirq.X)
     assert xy_gateset.is_supported_gate(cirq.Y)
 
+    # test serialization and deserialization
+    proto = {
+        'gate': {
+            'id': 'y_pow'
+        },
+        'args': {
+            'half_turns': {
+                'arg_value': {
+                    'float_value': 0.125
+                }
+            },
+        },
+        'qubits': [{
+            'id': '1_1'
+        }]
+    }
+
+    expected_gate = cirq.YPowGate(exponent=0.125)(cirq.GridQubit(1, 1))
+    assert xy_gateset.serialize_op_dict(expected_gate) == proto
+    assert xy_gateset.deserialize_op_dict(proto) == expected_gate
+
+
+def test_gateset_with_added_gates_again():
+    """Verify that adding a serializer twice doesn't mess anything up."""
+    x_gateset = cg.SerializableGateSet(
+        gate_set_name='x',
+        serializers=[X_SERIALIZER],
+        deserializers=[X_DESERIALIZER],
+    )
+    xx_gateset = x_gateset.with_added_gates(
+        gate_set_name='xx',
+        serializers=[X_SERIALIZER],
+        deserializers=[X_DESERIALIZER],
+    )
+
+    assert xx_gateset.gate_set_name == 'xx'
+    assert xx_gateset.is_supported_gate(cirq.X)
+    assert not xx_gateset.is_supported_gate(cirq.Y)
+
+    # test serialization and deserialization
+    proto = {
+        'gate': {
+            'id': 'x_pow'
+        },
+        'args': {
+            'half_turns': {
+                'arg_value': {
+                    'float_value': 0.125
+                }
+            },
+        },
+        'qubits': [{
+            'id': '1_1'
+        }]
+    }
+
+    expected_gate = cirq.XPowGate(exponent=0.125)(cirq.GridQubit(1, 1))
+    assert xx_gateset.serialize_op_dict(expected_gate) == proto
+    assert xx_gateset.deserialize_op_dict(proto) == expected_gate
+
 
 def test_deserialize_op_invalid_gate():
     proto = {