restricting the casting in PauliString's add method to GateOperation

Author: NoureldinYosri

cirq-core/cirq/ops/linear_combinations.py

@@ -30,7 +30,7 @@
 from cirq import linalg, protocols, qis, value
 from cirq._doc import document
 from cirq.linalg import operator_spaces
-from cirq.ops import identity, raw_types, pauli_gates, pauli_string
+from cirq.ops import identity, raw_types, pauli_gates, pauli_string, gate_operation
 from cirq.ops.pauli_string import PauliString, _validate_qubit_mapping
 from cirq.value.linear_dict import _format_terms
 
@@ -570,11 +570,14 @@ def __iadd__(self, other):
         return self
 
     def __add__(self, other):
+        if isinstance(other, (gate_operation.GateOperation)):
+            other = PauliString(other)
         if not isinstance(other, (numbers.Complex, PauliString, PauliSum)):
-            try:
-                other = PauliString(other)
-            except:
-                return NotImplemented
+            return NotImplemented
+            # try:
+            #     other = PauliSum.from_pauli_strings([PauliString(other)])
+            # except:
+            #     return NotImplemented
         result = self.copy()
         result += other
         return result

cirq-core/cirq/ops/linear_combinations_test.py

@@ -1139,8 +1139,8 @@ def test_paulisum_validation():
     try:
         ps = cirq.PauliSum()
         ps += cirq.I(cirq.LineQubit(0))
-    except e:
-        raise Exeption("Failed to add identity to PauliSum.", e)
+    except:
+        raise Exception("Failed to add identity to PauliSum.")
     assert ps == cirq.PauliSum(cirq.LinearDict({frozenset(): complex(1)}))