Return self in Operation.with_tags if adding no tags (quantumlib#4301)

Review: @viathor

Author: maffoo

cirq-core/cirq/devices/noise_model.py

@@ -88,12 +88,7 @@ def is_virtual_moment(self, moment: 'cirq.Moment') -> bool:
         """
         if not moment.operations:
             return False
-        return all(
-            [
-                isinstance(op, ops.TaggedOperation) and ops.VirtualTag() in op.tags
-                for op in moment.operations
-            ]
-        )
+        return all(ops.VirtualTag() in op.tags for op in moment)
 
     def _noisy_moments_impl_moment(
         self, moments: 'Iterable[cirq.Moment]', system_qubits: Sequence['cirq.Qid']

cirq-core/cirq/ops/raw_types.py

@@ -425,7 +425,7 @@ def untagged(self) -> 'cirq.Operation':
         """Returns the underlying operation without any tags."""
         return self
 
-    def with_tags(self, *new_tags: Hashable) -> 'cirq.TaggedOperation':
+    def with_tags(self, *new_tags: Hashable) -> 'cirq.Operation':
         """Creates a new TaggedOperation, with this op and the specified tags.
 
         This method can be used to attach meta-data to specific operations
@@ -443,6 +443,8 @@ def with_tags(self, *new_tags: Hashable) -> 'cirq.TaggedOperation':
         Args:
             new_tags: The tags to wrap this operation in.
         """
+        if not new_tags:
+            return self
         return TaggedOperation(self, *new_tags)
 
     def transform_qubits(
@@ -608,6 +610,8 @@ def with_tags(self, *new_tags: Hashable) -> 'cirq.TaggedOperation':
         that has the tags of this operation combined with the new_tags
         specified as the parameter.
         """
+        if not new_tags:
+            return self
         return TaggedOperation(self.sub_operation, *self._tags, *new_tags)
 
     def __str__(self) -> str:

cirq-core/cirq/ops/raw_types_test.py

@@ -434,6 +434,14 @@ def test_tagged_operation():
     assert not cirq.is_measurement(op)
 
 
+def test_with_tags_returns_same_instance_if_possible():
+    untagged = cirq.X(cirq.GridQubit(1, 1))
+    assert untagged.with_tags() is untagged
+
+    tagged = untagged.with_tags('foo')
+    assert tagged.with_tags() is tagged
+
+
 def test_tagged_measurement():
     assert not cirq.is_measurement(cirq.GlobalPhaseOperation(coefficient=-1.0).with_tags('tag0'))