Disallow passing types as tags (#6947)

* Disallow passing types as tags

- This has caused confusion when passing a tag type
with PhysicalZTag as a tag when you mean the instance
PhysicalZTag().
- Adds comments to TaggedOperation and Operation to explain the limitation.

cirq-core/cirq/ops/raw_types.py

@@ -554,6 +554,9 @@ def with_tags(self, *new_tags: Hashable) -> 'cirq.Operation':
         resulting operation to be eventually serialized into JSON, you should
         also restrict the operation to be JSON serializable.
 
+        Please note that tags should be instantiated if classes are
+        used.  Raw types are not allowed.
+
         Args:
             *new_tags: The tags to wrap this operation in.
         """
@@ -779,14 +782,17 @@ class TaggedOperation(Operation):
     Tags added can be of any type, but they should be Hashable in order
     to allow equality checking.  If you wish to serialize operations into
     JSON, you should restrict yourself to only use objects that have a JSON
-    serialization.
+    serialization.  Tags cannot be raw types and should be instantiated
+    if classes are used.
 
     See `Operation.with_tags()` for more information on intended usage.
     """
 
     def __init__(self, sub_operation: 'cirq.Operation', *tags: Hashable):
         self._sub_operation = sub_operation
         self._tags = tuple(tags)
+        if any(isinstance(tag, type) for tag in tags):
+            raise ValueError('Tags cannot be types.  Did you forget to instantiate the tag type?')
 
     @property
     def sub_operation(self) -> 'cirq.Operation':
@@ -836,6 +842,9 @@ def with_tags(self, *new_tags: Hashable) -> 'cirq.TaggedOperation':
         Overloads Operation.with_tags to create a new TaggedOperation
         that has the tags of this operation combined with the new_tags
         specified as the parameter.
+
+        Please note that tags should be instantiated if classes are
+        used.  Raw types are not allowed.
         """
         if not new_tags:
             return self

cirq-core/cirq/ops/raw_types_test.py

@@ -458,6 +458,12 @@ def test_tagged_operation():
     assert op.with_qubits(q2).qubits == (q2,)
     assert not cirq.is_measurement(op)
 
+    # Tags can't be types
+    # This is to prevent typos of cirq.X(q1).with_tags(TagType)
+    # when you meant cirq.X(q1).with_tags(TagType())
+    with pytest.raises(ValueError, match="cannot be types"):
+        _ = cirq.X(q1).with_tags(cirq.Circuit)
+
 
 def test_with_tags_returns_same_instance_if_possible():
     untagged = cirq.X(cirq.GridQubit(1, 1))

cirq-google/cirq_google/devices/google_noise_properties.py

@@ -310,6 +310,6 @@ class NoiseModelFromGoogleNoiseProperties(devices.NoiseModelFromNoiseProperties)
     """A noise model defined from noise properties of a Google device."""
 
     def is_virtual(self, op: cirq.Operation) -> bool:
-        return isinstance(op.gate, cirq.ZPowGate) and cirq_google.PhysicalZTag not in op.tags
+        return isinstance(op.gate, cirq.ZPowGate) and cirq_google.PhysicalZTag() not in op.tags
 
     # noisy_moments is implemented by the superclass.

cirq-google/cirq_google/devices/google_noise_properties_test.py

@@ -204,7 +204,7 @@ def test_with_params_opid_with_gate():
 @pytest.mark.parametrize(
     'op',
     [
-        (cirq.Z(cirq.LineQubit(0)) ** 0.3).with_tags(cirq_google.PhysicalZTag),
+        (cirq.Z(cirq.LineQubit(0)) ** 0.3).with_tags(cirq_google.PhysicalZTag()),
         cirq.PhasedXZGate(x_exponent=0.8, z_exponent=0.2, axis_phase_exponent=0.1).on(
             cirq.LineQubit(0)
         ),