Added MSGate to top level (#6466)

* add `MSGate` to top level

* mark `MSGate` as `not_yet_serializable`

* Revert "mark `MSGate` as `not_yet_serializable`"

This reverts commit ffdb3de.

* Test json serialization of cirq.MSGate without custom resolver

Not yet passing.

* Use the "cirq" namespace to JSON-serialize `cirq.MSGate`

The namespace-less cirq_type `MSGate` is used by `cirq_ionq.MSGate`.

---------

Co-authored-by: Tanuj Khattar <tanujkhattar@google.com>
Co-authored-by: Pavol Juhas <juhas@google.com>

cirq-core/cirq/__init__.py

@@ -240,6 +240,7 @@
     MatrixGate,
     MixedUnitaryChannel,
     M,
+    MSGate,
     measure,
     measure_each,
     measure_paulistring_terms,

cirq-core/cirq/json_resolver_cache.py

@@ -157,6 +157,7 @@ def _symmetricalqidpair(qids):
         'LineTopology': cirq.LineTopology,
         'Linspace': cirq.Linspace,
         'ListSweep': cirq.ListSweep,
+        'cirq.MSGate': cirq.MSGate,
         'MatrixGate': cirq.MatrixGate,
         'MixedUnitaryChannel': cirq.MixedUnitaryChannel,
         'MeasurementKey': cirq.MeasurementKey,

cirq-core/cirq/ops/parity_gates.py

@@ -399,6 +399,11 @@ def __repr__(self) -> str:
             return 'cirq.ms(np.pi/2)'
         return f'cirq.ms({self._exponent!r}*np.pi/2)'
 
+    # the default namespace is already occupied by cirq_ionq.MSGate
+    @classmethod
+    def _json_namespace_(cls) -> str:
+        return 'cirq'
+
     def _json_dict_(self) -> Dict[str, Any]:
         return protocols.obj_to_dict_helper(self, ["rads"])
 

cirq-core/cirq/ops/parity_gates_test.py

@@ -258,7 +258,7 @@ def test_trace_distance():
 def test_ms_arguments():
     eq_tester = cirq.testing.EqualsTester()
     eq_tester.add_equality_group(
-        cirq.ms(np.pi / 2), cirq.ops.MSGate(rads=np.pi / 2), cirq.XXPowGate(global_shift=-0.5)
+        cirq.ms(np.pi / 2), cirq.MSGate(rads=np.pi / 2), cirq.XXPowGate(global_shift=-0.5)
     )
     eq_tester.add_equality_group(
         cirq.ms(np.pi / 4), cirq.XXPowGate(exponent=0.5, global_shift=-0.5)
@@ -323,15 +323,7 @@ def test_ms_diagrams():
 
 
 def test_json_serialization():
-    def custom_resolver(cirq_type: str):
-        if cirq_type == "MSGate":
-            return cirq.ops.MSGate
-        return None
-
-    assert cirq.read_json(
-        json_text=cirq.to_json(cirq.ms(np.pi / 2)), resolvers=[custom_resolver]
-    ) == cirq.ms(np.pi / 2)
-    assert custom_resolver('X') is None
+    assert cirq.read_json(json_text=cirq.to_json(cirq.ms(np.pi / 2))) == cirq.ms(np.pi / 2)
 
 
 @pytest.mark.parametrize('gate_cls', (cirq.XXPowGate, cirq.YYPowGate, cirq.ZZPowGate))

cirq-core/cirq/protocols/json_test_data/cirq.MSGate.json

@@ -0,0 +1,4 @@
+{
+  "cirq_type": "cirq.MSGate",
+  "rads": 1.5707963267948966
+}

cirq-core/cirq/protocols/json_test_data/cirq.MSGate.repr

@@ -0,0 +1 @@
+cirq.ms(np.pi/2)

cirq-core/cirq/protocols/json_test_data/spec.py

@@ -22,6 +22,7 @@
     name="cirq",
     packages=[cirq, cirq.work],
     test_data_path=pathlib.Path(__file__).parent,
+    custom_class_name_to_cirq_type={"MSGate": "cirq.MSGate"},
     resolver_cache=_class_resolver_dictionary(),
     not_yet_serializable=[
         'Alignment',