Fix __radd__ losing device (#2451)

quantumlib · Nov 20, 2019 · 52b14db · 52b14db
1 parent f14ed20
commit 52b14db
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Showing 2 changed files with 15 additions and 2 deletions.
diff --git a/cirq/circuits/circuit.py b/cirq/circuits/circuit.py
@@ -276,8 +276,10 @@ def __radd__(self, other):
         if not isinstance(other, (ops.Operation, Iterable)):
             return NotImplemented
         # Auto wrap OP_TREE inputs into a circuit.
-        result = Circuit(other)
-        return result.__iadd__(self)
+        result = self.copy()
+        result._moments[:0] = Circuit(other)._moments
+        result._device.validate_circuit(result)
+        return result
 
     def __imul__(self, repetitions: int):
         if not isinstance(repetitions, int):

diff --git a/cirq/circuits/circuit_test.py b/cirq/circuits/circuit_test.py
@@ -277,6 +277,17 @@ def test_radd_op_tree():
         [cirq.Moment([cirq.X(a)]),
          cirq.Moment([cirq.Y(b)])])
 
+    # Preserves device.
+    c = cirq.Circuit(device=cirq.google.Bristlecone)
+    c2 = [] + c
+    assert c2.device is cirq.google.Bristlecone
+    assert c2 == c
+
+    # Validates versus device.
+    c = cirq.Circuit(device=cirq.google.Bristlecone)
+    with pytest.raises(ValueError, match='Unsupported qubit'):
+        _ = [cirq.X(cirq.NamedQubit('a'))] + c
+
 
 def test_bool():
     assert not cirq.Circuit()