Test round trip of gate to operation to gate

cirq-core/cirq/ops/gate_operation_test.py

@@ -11,11 +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 collections.abc
+import pathlib
+
 import numpy as np
 import pytest
 import sympy
 
 import cirq
+import cirq.testing
 
 
 def test_gate_operation_init():
@@ -465,3 +469,73 @@ def qubit_index_to_equivalence_group_key(self, index: int) -> int:
     assert gate(qubits[0], qubits[1], qubits[2], qubits[3]) == gate(
         qubits[3], qubits[1], qubits[2], qubits[0]
     )
+
+
+def test_gate_to_operation_to_gate_round_trips():
+    def all_subclasses(cls):
+        return set(cls.__subclasses__()).union(
+            [s for c in cls.__subclasses__() for s in all_subclasses(c)]
+        )
+
+    # Only test gate subclasses in cirq-core.
+    gate_subclasses = {
+        g
+        for g in all_subclasses(cirq.Gate)
+        if "cirq." in g.__module__ and "contrib" not in g.__module__ and "test" not in g.__module__
+    }
+
+    test_module_spec = cirq.testing.json.spec_for("cirq.protocols")
+
+    skip_classes = {
+        # Abstract or private parent classes.
+        cirq.BaseDensePauliString,
+        cirq.EigenGate,
+        cirq.Pauli,
+        # Private gates.
+        cirq.transformers.analytical_decompositions.two_qubit_to_fsim._BGate,
+        cirq.ops.raw_types._InverseCompositeGate,
+        cirq.circuits.qasm_output.QasmTwoQubitGate,
+        cirq.circuits.quil_output.QuilTwoQubitGate,
+        cirq.circuits.quil_output.QuilOneQubitGate,
+        cirq.ion.ion_gates.MSGate,
+        # Gate features.
+        cirq.SingleQubitGate,
+        # Interop gates
+        cirq.interop.quirk.QuirkQubitPermutationGate,
+        cirq.interop.quirk.QuirkArithmeticGate,
+        # No reason given for missing json.
+        # TODO(#5353): Serialize these gates.
+        cirq.PauliInteractionGate,
+        cirq.ArithmeticGate,
+    }
+
+    # Gates that do not satisfy the contract.
+    # TODO(#5167): Fix this case.
+    exceptions = {cirq.PauliStringPhasorGate}
+
+    skipped = set()
+    for gate_cls in gate_subclasses - exceptions:
+        filename = test_module_spec.test_data_path.joinpath(f"{gate_cls.__name__}.json")
+        if pathlib.Path(filename).is_file():
+            gates = cirq.read_json(filename)
+        else:
+            if gate_cls in skip_classes:
+                skipped.add(gate_cls)
+                continue
+            # coverage:ignore
+            raise AssertionError(
+                f"{gate_cls} has no json file, please add a json file or add to the list of "
+                "classes to be skipped if there is a reason this gate should not round trip "
+                "to a gate via creating an operation."
+            )
+
+        if not isinstance(gates, collections.abc.Iterable):
+            gates = [gates]
+        for gate in gates:
+            if gate.num_qubits():
+                qudits = [cirq.LineQid(i, d) for i, d in enumerate(cirq.qid_shape(gate))]
+                assert gate.on(*qudits).gate == gate
+
+    assert (
+        skipped == skip_classes
+    ), "A gate that was supposed to be skipped was not, please update the list of skipped gates."