Fix IdentityGate.on_each failing when given a qubit that is iterable (#…

…3036)

cirq/ops/identity.py

@@ -88,10 +88,10 @@ def on_each(self, *targets: Union['cirq.Qid', Iterable[Any]]
                 'gate.')
         operations: List['cirq.Operation'] = []
         for target in targets:
-            if isinstance(target, Iterable) and not isinstance(target, str):
-                operations.extend(self.on_each(*target))
-            elif isinstance(target, raw_types.Qid):
+            if isinstance(target, raw_types.Qid):
                 operations.append(self.on(target))
+            elif isinstance(target, Iterable) and not isinstance(target, str):
+                operations.extend(self.on_each(*target))
             else:
                 raise ValueError(
                     'Gate was called with type different than Qid. Type: {}'.

cirq/ops/identity_test.py

@@ -12,6 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 import itertools
+from typing import Any
 
 import numpy as np
 import pytest
@@ -46,6 +47,25 @@ def test_identity_on_each():
         cirq.I.on_each('abc')
 
 
+def test_identity_on_each_iter_second():
+
+    class Q(cirq.Qid):
+
+        @property
+        def dimension(self) -> int:
+            return 2
+
+        def _comparison_key(self) -> Any:
+            return 1
+
+        def __iter__(self):
+            # Having this method makes `isinstance(x, Iterable)` return True.
+            raise NotImplementedError()
+
+    q = Q()
+    assert cirq.I.on_each(q) == [cirq.I(q)]
+
+
 def test_identity_on_each_only_single_qubit():
     q0, q1 = cirq.LineQubit.range(2)
     q0_3, q1_3 = q0.with_dimension(3), q1.with_dimension(3)