Encap public fields in cirq.sim (#5320)

* Hide public fields from Step/Trial Reesult

* Also encap noise field

cirq-core/cirq/sim/simulator.py

@@ -789,7 +789,19 @@ class StepResult(Generic[TSimulatorState], metaclass=abc.ABCMeta):
 
     def __init__(self, sim_state: TSimulatorState) -> None:
         self._sim_state = sim_state
-        self.measurements = sim_state.log_of_measurement_results
+        self._measurements = sim_state.log_of_measurement_results
+
+    @property
+    def measurements(self) -> Mapping[str, Sequence[int]]:
+        return self._measurements
+
+    @measurements.setter  # type: ignore
+    @_compat.deprecated(
+        deadline="v0.16",
+        fix="The mutators of this class are deprecated, instantiate a new object instead.",
+    )
+    def measurements(self, measurements: Mapping[str, Sequence[int]]):
+        self._measurements = measurements
 
     def _simulator_state(self) -> TSimulatorState:
         """Returns the simulator state of the simulator after this step.
@@ -986,23 +998,47 @@ class SimulationTrialResult(Generic[TSimulatorState]):
     def __init__(
         self,
         params: 'cirq.ParamResolver',
-        measurements: Dict[str, np.ndarray],
+        measurements: Mapping[str, np.ndarray],
         final_simulator_state: TSimulatorState,
     ) -> None:
         """Initializes the `SimulationTrialResult` class.
 
         Args:
             params: A ParamResolver of settings used for this result.
-            measurements: A dictionary from measurement gate key to measurement
+            measurements: A mapping from measurement gate key to measurement
                 results. Measurement results are a numpy ndarray of actual
                 boolean measurement results (ordered by the qubits acted on by
                 the measurement gate.)
             final_simulator_state: The final simulator state.
         """
-        self.params = params
-        self.measurements = measurements
+        self._params = params
+        self._measurements = measurements
         self._final_simulator_state = final_simulator_state
 
+    @property
+    def params(self) -> 'cirq.ParamResolver':
+        return self._params
+
+    @params.setter  # type: ignore
+    @_compat.deprecated(
+        deadline='v0.16',
+        fix='The mutators of this class are deprecated, instantiate a new object instead.',
+    )
+    def params(self, params: 'cirq.ParamResolver'):
+        self._params = params
+
+    @property
+    def measurements(self) -> Mapping[str, np.ndarray]:
+        return self._measurements
+
+    @measurements.setter  # type: ignore
+    @_compat.deprecated(
+        deadline="v0.16",
+        fix="The mutators of this class are deprecated, instantiate a new object instead.",
+    )
+    def measurements(self, measurements: Mapping[str, np.ndarray]):
+        self._measurements = measurements
+
     def __repr__(self) -> str:
         return (
             f'cirq.SimulationTrialResult(params={self.params!r}, '

cirq-core/cirq/sim/simulator_base.py

@@ -110,9 +110,21 @@ def __init__(
         """
         self._dtype = dtype
         self._prng = value.parse_random_state(seed)
-        self.noise = devices.NoiseModel.from_noise_model_like(noise)
+        self._noise = devices.NoiseModel.from_noise_model_like(noise)
         self._split_untangled_states = split_untangled_states
 
+    @property
+    def noise(self) -> 'cirq.NoiseModel':
+        return self._noise
+
+    @noise.setter  # type: ignore
+    @_compat.deprecated(
+        deadline="v0.16",
+        fix="The mutators of this class are deprecated, instantiate a new object instead.",
+    )
+    def noise(self, noise: 'cirq.NoiseModel'):
+        self._noise = noise
+
     def _create_partial_act_on_args(
         self,
         initial_state: Any,

cirq-core/cirq/sim/simulator_base_test.py

@@ -441,3 +441,9 @@ def _create_simulator_trial_result(self):
     sim = DeprecatedSim()
     with cirq.testing.assert_deprecated(deadline='v0.16'):
         sim.simulate_moment_steps(cirq.Circuit())
+
+
+def test_deprecated_setters():
+    sim = CountingSimulator()
+    with cirq.testing.assert_deprecated(deadline='v0.16'):
+        sim.noise = cirq.ConstantQubitNoiseModel(cirq.Z)

cirq-core/cirq/sim/simulator_test.py

@@ -555,3 +555,14 @@ def _create_simulator_trial_result(self):
     sim = DeprecatedSim()
     with cirq.testing.assert_deprecated(deadline='v0.16'):
         sim.simulate_moment_steps(cirq.Circuit())
+
+
+def test_deprecated_setters():
+    step = FakeStepResult()
+    result = cirq.SimulationTrialResult(cirq.ParamResolver(), {}, 0)
+    with cirq.testing.assert_deprecated(deadline='v0.16'):
+        step.measurements = {}
+    with cirq.testing.assert_deprecated(deadline='v0.16'):
+        result.measurements = {}
+    with cirq.testing.assert_deprecated(deadline='v0.16'):
+        result.params = cirq.ParamResolver()