Create __add__ method for Moment (#2665)

* Added __add__ to moment

* Added test.

* remove else

* add test for non-list circuit

* Return NotImplemented if right operand is not Operation

Author: fedimser

Diff for: cirq/ops/moment.py

@@ -193,6 +193,11 @@ def transform_qubits(self: TSelf_Moment,
     def _json_dict_(self):
         return protocols.obj_to_dict_helper(self, ['operations'])
 
+    def __add__(self, other):
+        if isinstance(other, raw_types.Operation):
+            return self.with_operation(other)
+        return NotImplemented
+
 
 def _list_repr_with_indented_item_lines(items: Sequence[Any]) -> str:
     block = '\n'.join([repr(op) + ',' for op in items])

Diff for: cirq/ops/moment_test.py

@@ -261,3 +261,16 @@ def test_immutable_moment():
         circuit = cirq.Circuit(cirq.X(q1))
         moment = circuit.moments[0]
         moment.operations += (cirq.Y(q2),)
+
+
+def test_add():
+    a, b = cirq.LineQubit.range(2)
+    expected_circuit = cirq.Circuit([cirq.CNOT(a, b), cirq.X(a), cirq.Y(b)])
+
+    circuit1 = cirq.Circuit([cirq.CNOT(a, b), cirq.X(a)])
+    circuit1[1] += cirq.Y(b)
+    assert circuit1 == expected_circuit
+
+    circuit2 = cirq.Circuit(cirq.CNOT(a, b), cirq.Y(b))
+    circuit2[1] += cirq.X(a)
+    assert circuit2 == expected_circuit