Deprecate the ActOnArgs._on_* methods (quantumlib#5224)

Now that we have separated out the `QuantumStateRepresentation` from `ActOnArgs`, there is no need for the `_on_*` methods anymore. This PR deprecates those, adds deprecation tests, and updates any old test ActOnArgs to use the new QuantumStateRepresentation internally. @95-martin-orion

cirq-core/cirq/protocols/act_on_protocol_test.py

@@ -20,17 +20,19 @@
 from cirq.ops.raw_types import TSelf
 
 
+class DummyQuantumState(cirq.QuantumStateRepresentation):
+    def copy(self, deep_copy_buffers=True):
+        pass
+
+    def measure(self, axes, seed=None):
+        pass
+
+
 class DummyActOnArgs(cirq.ActOnArgs):
-    def __init__(self, fallback_result: Any = NotImplemented, measurements=None):
-        super().__init__(np.random.RandomState())
-        if measurements is None:
-            measurements = []
-        self.measurements = measurements
+    def __init__(self, fallback_result: Any = NotImplemented):
+        super().__init__(np.random.RandomState(), state=DummyQuantumState())
         self.fallback_result = fallback_result
 
-    def _perform_measurement(self, qubits):
-        return self.measurements  # coverage: ignore
-
     def _act_on_fallback_(
         self,
         action: Any,

cirq-core/cirq/sim/act_on_args.py

@@ -30,7 +30,7 @@
 import numpy as np
 
 from cirq import protocols, value
-from cirq._compat import deprecated
+from cirq._compat import _warn_or_error, deprecated
 from cirq.protocols.decompose_protocol import _try_decompose_into_operations_and_qubits
 from cirq.sim.operation_target import OperationTarget
 
@@ -78,6 +78,8 @@ def __init__(
             }
         )
         self._state = state
+        if state is None:
+            _warn_or_error('This function will require a valid `state` input in cirq v0.16.')
 
     @property
     def prng(self) -> np.random.RandomState:
@@ -147,9 +149,17 @@ def copy(self: TSelf, deep_copy_buffers: bool = True) -> TSelf:
         if self._state is not None:
             args._state = self._state.copy(deep_copy_buffers=deep_copy_buffers)
         else:
+            _warn_or_error(
+                'Pass a `QuantumStateRepresentation` into the `ActOnArgs` constructor. The `_on_`'
+                ' overrides will be removed in cirq v0.16.'
+            )
             self._on_copy(args, deep_copy_buffers)
         return args
 
+    @deprecated(
+        deadline='v0.16',
+        fix='Pass a `QuantumStateRepresentation` into the `ActOnArgs` constructor.',
+    )
     def _on_copy(self: TSelf, args: TSelf, deep_copy_buffers: bool = True):
         """Subclasses should implement this with any additional state copy
         functionality."""
@@ -164,10 +174,18 @@ def kronecker_product(self: TSelf, other: TSelf, *, inplace=False) -> TSelf:
         if self._state is not None and other._state is not None:
             args._state = self._state.kron(other._state)
         else:
+            _warn_or_error(
+                'Pass a `QuantumStateRepresentation` into the `ActOnArgs` constructor. The `_on_`'
+                ' overrides will be removed in cirq v0.16.'
+            )
             self._on_kronecker_product(other, args)
         args._set_qubits(self.qubits + other.qubits)
         return args
 
+    @deprecated(
+        deadline='v0.16',
+        fix='Pass a `QuantumStateRepresentation` into the `ActOnArgs` constructor.',
+    )
     def _on_kronecker_product(self: TSelf, other: TSelf, target: TSelf):
         """Subclasses should implement this with any additional state product
         functionality, if supported."""
@@ -204,6 +222,10 @@ def factor(
             extracted._state = e
             remainder._state = r
         else:
+            _warn_or_error(
+                'Pass a `QuantumStateRepresentation` into the `ActOnArgs` constructor. The `_on_`'
+                ' overrides will be removed in cirq v0.16.'
+            )
             self._on_factor(qubits, extracted, remainder, validate, atol)
         extracted._set_qubits(qubits)
         remainder._set_qubits([q for q in self.qubits if q not in qubits])
@@ -214,6 +236,10 @@ def allows_factoring(self):
         """Subclasses that allow factorization should override this."""
         return self._state.supports_factor if self._state is not None else False
 
+    @deprecated(
+        deadline='v0.16',
+        fix='Pass a `QuantumStateRepresentation` into the `ActOnArgs` constructor.',
+    )
     def _on_factor(
         self: TSelf,
         qubits: Sequence['cirq.Qid'],
@@ -247,10 +273,18 @@ def transpose_to_qubit_order(
         if self._state is not None:
             args._state = self._state.reindex(self.get_axes(qubits))
         else:
+            _warn_or_error(
+                'Pass a `QuantumStateRepresentation` into the `ActOnArgs` constructor. The `_on_`'
+                ' overrides will be removed in cirq v0.16.'
+            )
             self._on_transpose_to_qubit_order(qubits, args)
         args._set_qubits(qubits)
         return args
 
+    @deprecated(
+        deadline='v0.16',
+        fix='Pass a `QuantumStateRepresentation` into the `ActOnArgs` constructor.',
+    )
     def _on_transpose_to_qubit_order(self: TSelf, qubits: Sequence['cirq.Qid'], target: TSelf):
         """Subclasses should implement this with any additional state transpose
         functionality, if supported."""

cirq-core/cirq/sim/act_on_args_container_test.py

@@ -11,21 +11,40 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
-from typing import Any, Dict, List, Optional, Sequence
+from typing import Any, Dict, Optional, Sequence
 
 import cirq
 
 
+class EmptyQuantumState(cirq.QuantumStateRepresentation):
+    def copy(self, deep_copy_buffers=True):
+        return self
+
+    def measure(self, axes, seed=None):
+        return [0] * len(axes)
+
+    @property
+    def supports_factor(self):
+        return True
+
+    def kron(self, other):
+        return self
+
+    def factor(self, axes, *, validate=True, atol=1e-07):
+        return self, self
+
+    def reindex(self, axes):
+        return self
+
+
 class EmptyActOnArgs(cirq.ActOnArgs):
     def __init__(self, qubits, classical_data):
         super().__init__(
+            state=EmptyQuantumState(),
             qubits=qubits,
             classical_data=classical_data,
         )
 
-    def _perform_measurement(self, qubits: Sequence[cirq.Qid]) -> List[int]:
-        return [0] * len(qubits)
-
     def _act_on_fallback_(
         self,
         action: Any,
@@ -34,10 +53,6 @@ def _act_on_fallback_(
     ) -> bool:
         return True
 
-    @property
-    def allows_factoring(self):
-        return True
-
 
 q0, q1, q2 = qs3 = cirq.LineQubit.range(3)
 qs2 = cirq.LineQubit.range(2)

cirq-core/cirq/sim/act_on_args_test.py

@@ -20,13 +20,21 @@
 from cirq.sim import act_on_args
 
 
-class DummyArgs(cirq.ActOnArgs):
-    def __init__(self):
-        super().__init__(qubits=cirq.LineQubit.range(2))
+class DummyQuantumState(cirq.QuantumStateRepresentation):
+    def copy(self, deep_copy_buffers=True):
+        pass
 
-    def _perform_measurement(self, qubits):
+    def measure(self, axes, seed=None):
         return [5, 3]
 
+    def reindex(self, axes):
+        return self
+
+
+class DummyArgs(cirq.ActOnArgs):
+    def __init__(self):
+        super().__init__(state=DummyQuantumState(), qubits=cirq.LineQubit.range(2))
+
     def _act_on_fallback_(
         self,
         action: Any,
@@ -95,3 +103,49 @@ def test_field_getters():
     assert args.qubit_map == {q: i for i, q in enumerate(cirq.LineQubit.range(2))}
     with cirq.testing.assert_deprecated('always returns False', deadline='v0.16'):
         assert not args.ignore_measurement_results
+
+
+def test_on_methods_deprecated():
+    class OldStyleArgs(cirq.ActOnArgs):
+        def _act_on_fallback_(self, action, qubits, allow_decompose=True):
+            pass
+
+    with cirq.testing.assert_deprecated('state', deadline='v0.16'):
+        args = OldStyleArgs()
+    with cirq.testing.assert_deprecated('_on_', deadline='v0.16', count=2):
+        _ = args.copy()
+    with cirq.testing.assert_deprecated('_on_', deadline='v0.16', count=2):
+        _ = args.kronecker_product(args)
+    with cirq.testing.assert_deprecated('_on_', deadline='v0.16', count=2):
+        _ = args.factor([])
+    with cirq.testing.assert_deprecated('_on_', deadline='v0.16', count=2):
+        _ = args.transpose_to_qubit_order([])
+
+
+def test_on_methods_deprecated_if_implemented():
+    class OldStyleArgs(cirq.ActOnArgs):
+        def _act_on_fallback_(self, action, qubits, allow_decompose=True):
+            pass
+
+        def _on_copy(self, args, deep_copy_buffers=True):
+            pass
+
+        def _on_kronecker_product(self, other, target):
+            pass
+
+        def _on_factor(self, qubits, extracted, remainder, validate=True, atol=1e-07):
+            pass
+
+        def _on_transpose_to_qubit_order(self, qubits, target):
+            pass
+
+    with cirq.testing.assert_deprecated('state', deadline='v0.16'):
+        args = OldStyleArgs()
+    with cirq.testing.assert_deprecated('_on_', deadline='v0.16'):
+        _ = args.copy()
+    with cirq.testing.assert_deprecated('_on_', deadline='v0.16'):
+        _ = args.kronecker_product(args)
+    with cirq.testing.assert_deprecated('_on_', deadline='v0.16'):
+        _ = args.factor([])
+    with cirq.testing.assert_deprecated('_on_', deadline='v0.16'):
+        _ = args.transpose_to_qubit_order([])