restrict identity check to other.gate

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, gate_operation
+from cirq.ops import identity, raw_types, pauli_gates, pauli_string
 from cirq.ops.pauli_string import PauliString, _validate_qubit_mapping
 from cirq.value.linear_dict import _format_terms
 
@@ -570,10 +570,11 @@ 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)):
-            return NotImplemented
+            if hasattr(other, 'gate') and isinstance(other.gate, identity.IdentityGate):
+                other = PauliString(other)
+            else:
+                return NotImplemented
         result = self.copy()
         result += other
         return result