Avoid mutating user backend config (#1076)

Author: shiavm006

qumat/qumat.py

@@ -60,12 +60,12 @@ def __init__(self, backend_config):
                 "Please provide a dictionary with backend-specific options "
             )
 
-        self.backend_config = backend_config
-        self.backend_name = backend_config["backend_name"]
+        self.backend_config = backend_config.copy()
+        self.backend_name = self.backend_config["backend_name"]
         self.backend_module = import_module(
             f".{self.backend_name}_backend", package="qumat"
         )
-        self.backend = self.backend_module.initialize_backend(backend_config)
+        self.backend = self.backend_module.initialize_backend(self.backend_config)
         self.circuit = None
         self.num_qubits = None
         self.parameters = {}

testing/qumat/test_parameter_binding.py

@@ -220,3 +220,35 @@ def test_partially_bound_parameters_error(self, backend_name):
         # This should raise ValueError
         with pytest.raises(ValueError, match="unbound parameters"):
             qumat.execute_circuit(parameter_values={"theta0": math.pi})
+
+    @pytest.mark.parametrize("backend_name", TESTING_BACKENDS)
+    def test_execute_circuit_does_not_mutate_backend_config(self, backend_name):
+        """Test that execute_circuit does not mutate the user's backend_config across all backends."""
+        backend_config = get_backend_config(backend_name).copy()
+        original_config = backend_config.copy()
+
+        qumat = QuMat(backend_config)
+        qumat.create_empty_circuit(num_qubits=1)
+        qumat.apply_rx_gate(0, "theta")
+        qumat.execute_circuit(parameter_values={"theta": math.pi})
+
+        assert backend_config == original_config, (
+            f"backend_config was mutated in {backend_name}; "
+            "parameter_values or other keys must not be added."
+        )
+
+    @pytest.mark.parametrize("backend_name", TESTING_BACKENDS)
+    def test_get_final_state_vector_does_not_mutate_backend_config(self, backend_name):
+        """Test that get_final_state_vector does not mutate the user's backend_config across all backends."""
+        backend_config = get_backend_config(backend_name).copy()
+        original_config = backend_config.copy()
+
+        qumat = QuMat(backend_config)
+        qumat.create_empty_circuit(num_qubits=1)
+        qumat.apply_rx_gate(0, "theta")
+        qumat.bind_parameters({"theta": math.pi / 2})
+        qumat.get_final_state_vector()
+
+        assert backend_config == original_config, (
+            f"backend_config was mutated by get_final_state_vector in {backend_name}."
+        )