Deprecate the condition attribute and related functionality

This commit deprecates the Instruction.condition and c_if() method for
removal in 2.0. This functionality has been superseded by the more
sophisiticated `IfElseOp` for some time now. Removing the condition
property will simplify the Qiskit data model as it is implemented in
a kind of ad-hoc manner that adds additional overhead to manually check
it in many places.

For the unittest modifications the deprecation warning for the .condtion
getter is suppressed for the entire suite because this gets triggered
internally by the transpiler and a lot of other places, including from
rust code as until it is removed we need to use it to check that piece
of the data model.

Fixes Qiskit#9556

qiskit/circuit/barrier.py

@@ -19,6 +19,7 @@
 from __future__ import annotations
 
 from qiskit.exceptions import QiskitError
+from qiskit.utils import deprecate_func
 from .instruction import Instruction
 
 
@@ -44,5 +45,6 @@ def inverse(self, annotated: bool = False):
         """Special case. Return self."""
         return Barrier(self.num_qubits)
 
+    @deprecate_func(since="1.3.0", removal_timeline="in 2.0.0")
     def c_if(self, classical, val):
         raise QiskitError("Barriers are compiler directives and cannot be conditional.")

qiskit/circuit/delay.py

@@ -19,6 +19,7 @@
 from qiskit.circuit.gate import Gate
 from qiskit.circuit import _utils
 from qiskit.circuit.parameterexpression import ParameterExpression
+from qiskit.utils import deprecate_func
 
 
 @_utils.with_gate_array(np.eye(2, dtype=complex))
@@ -42,6 +43,7 @@ def inverse(self, annotated: bool = False):
         """Special case. Return self."""
         return self
 
+    @deprecate_func(since="1.3.0", removal_timeline="in 2.0.0")
     def c_if(self, classical, val):
         raise CircuitError("Conditional Delay is not yet implemented.")
 

qiskit/circuit/instruction.py

@@ -160,6 +160,7 @@ def to_mutable(self):
         return self.copy()
 
     @property
+    @deprecate_func(since="1.3.0", removal_timeline="in 2.0.0", is_property=True, stacklevel=2)
     def condition(self):
         """The classical condition on the instruction."""
         return self._condition
@@ -499,6 +500,7 @@ def inverse(self, annotated: bool = False):
         inverse_gate.definition = inverse_definition
         return inverse_gate
 
+    @deprecate_func(since="1.3.0", removal_timeline="in 2.0.0")
     def c_if(self, classical, val):
         """Set a classical equality condition on this instruction between the register or cbit
         ``classical`` and value ``val``.

qiskit/circuit/instructionset.py

@@ -20,6 +20,7 @@
 from typing import Callable
 
 from qiskit.circuit.exceptions import CircuitError
+from qiskit.utils import deprecate_func
 from .classicalregister import Clbit, ClassicalRegister
 from .operation import Operation
 from .quantumcircuitdata import CircuitInstruction
@@ -105,6 +106,7 @@ def inverse(self, annotated: bool = False):
                 )
         return self
 
+    @deprecate_func(since="1.3.0", removal_timeline="in 2.0.0")
     def c_if(self, classical: Clbit | ClassicalRegister | int, val: int) -> "InstructionSet":
         """Set a classical equality condition on all the instructions in this set between the
         :obj:`.ClassicalRegister` or :obj:`.Clbit` ``classical`` and value ``val``.

qiskit/circuit/singleton.py

@@ -251,6 +251,7 @@ class XGate(Gate, metaclass=_SingletonMeta, overrides=_SingletonGateOverrides):
 
 import functools
 
+from qiskit.utils import deprecate_func
 from .instruction import Instruction
 from .gate import Gate
 from .controlledgate import ControlledGate, _ctrl_state_to_int
@@ -489,6 +490,7 @@ class they are providing overrides for has more lazy attributes or user-exposed
         instruction._params = _frozenlist(instruction._params)
         return instruction
 
+    @deprecate_func(since="1.3.0", removal_timeline="in 2.0.0")
     def c_if(self, classical, val):
         return self.to_mutable().c_if(classical, val)
 

qiskit/circuit/store.py

@@ -16,6 +16,7 @@
 
 import typing
 
+from qiskit.utils import deprecate_func
 from .exceptions import CircuitError
 from .classical import expr, types
 from .instruction import Instruction
@@ -88,6 +89,7 @@ def rvalue(self):
         """Get the r-value :class:`~.expr.Expr` node that is being written into the l-value."""
         return self.params[1]
 
+    @deprecate_func(since="1.3.0", removal_timeline="in 2.0.0")
     def c_if(self, classical, val):
         """:meta hidden:"""
         raise NotImplementedError(

qiskit/qasm2/parse.py

@@ -11,7 +11,7 @@
 # that they have been altered from the originals.
 
 """Python-space bytecode interpreter for the output of the main Rust parser logic."""
-
+import warnings
 import dataclasses
 import math
 from typing import Iterable, Callable
@@ -255,19 +255,34 @@ def from_bytecode(bytecode, custom_instructions: Iterable[CustomInstruction]):
                 CircuitInstruction(gates[gate_id](*parameters), [qubits[q] for q in op_qubits])
             )
         elif opcode == OpCode.ConditionedGate:
+            warnings.warn(
+                "Conditioned gates in qasm2 will be loaded as an IfElseOp starting in Qiskit 2.0",
+                FutureWarning,
+                stacklevel=3,
+            )
             gate_id, parameters, op_qubits, creg, value = op.operands
             gate = gates[gate_id](*parameters).c_if(qc.cregs[creg], value)
             qc._append(CircuitInstruction(gate, [qubits[q] for q in op_qubits]))
         elif opcode == OpCode.Measure:
             qubit, clbit = op.operands
             qc._append(CircuitInstruction(Measure(), (qubits[qubit],), (clbits[clbit],)))
         elif opcode == OpCode.ConditionedMeasure:
+            warnings.warn(
+                "Conditioned measurements in qasm2 will be loaded as an IfElseOp starting in Qiskit 2.0",
+                FutureWarning,
+                stacklevel=3,
+            )
             qubit, clbit, creg, value = op.operands
             measure = Measure().c_if(qc.cregs[creg], value)
             qc._append(CircuitInstruction(measure, (qubits[qubit],), (clbits[clbit],)))
         elif opcode == OpCode.Reset:
             qc._append(CircuitInstruction(Reset(), (qubits[op.operands[0]],)))
         elif opcode == OpCode.ConditionedReset:
+            warnings.warn(
+                "Conditioned resets in qasm2 will be loaded as an IfElseOp starting in Qiskit 2.0",
+                FutureWarning,
+                stacklevel=3,
+            )
             qubit, creg, value = op.operands
             reset = Reset().c_if(qc.cregs[creg], value)
             qc._append(CircuitInstruction(reset, (qubits[qubit],)))

qiskit/qpy/binary_io/circuits.py

@@ -318,6 +318,13 @@ def _read_instruction(
             use_symengine,
             standalone_vars,
         )
+        if condition is not None:
+            warnings.warn(
+                f"The .condition attribute on {gate_name} will be loaded as an IfElseOp "
+                "starting in Qiskit 2.0",
+                FutureWarning,
+                stacklevel=3,
+            )
         inst_obj.condition = condition
         if instruction.label_size > 0:
             inst_obj.label = label
@@ -414,6 +421,12 @@ def _read_instruction(
                 gate = gate_class(*params)
         if condition:
             if not isinstance(gate, ControlFlowOp):
+                warnings.warn(
+                    f"The .condition attribute on {gate} will be loaded as an "
+                    "IfElseOp starting in Qiskit 2.0",
+                    FutureWarning,
+                    stacklevel=3,
+                )
                 gate = gate.c_if(*condition)
             else:
                 gate.condition = condition

qiskit/transpiler/passes/utils/convert_conditions_to_if_ops.py

@@ -22,6 +22,7 @@
 )
 from qiskit.dagcircuit import DAGCircuit
 from qiskit.transpiler.basepasses import TransformationPass
+from qiskit.utils import deprecate_func
 
 
 class ConvertConditionsToIfOps(TransformationPass):
@@ -31,6 +32,10 @@ class ConvertConditionsToIfOps(TransformationPass):
     This is a simple pass aimed at easing the conversion from the old style of using
     :meth:`.InstructionSet.c_if` into the new style of using more complex conditional logic."""
 
+    @deprecate_func(since="1.3.0", removal_timeline="in Qiskit 2.0.0")
+    def __init__(self):
+        super().__init__()
+
     def _run_inner(self, dag):
         """Run the pass on one :class:`.DAGCircuit`, mutating it.  Returns ``True`` if the circuit
         was modified and ``False`` if not."""

releasenotes/notes/deprecate-condition_c_if-9548e5522814fe9c.yaml

@@ -0,0 +1,38 @@
+---
+deprecations_circuits:
+  - |
+    Deprecated the :attr:`.Instruction.condition` and the
+    :meth:`.Instruction.c_if` method. This attribute and method will be removed
+    in Qiskit 2.0 and along with any other uses of this in the Qiskit data
+    model. This functionality has been superseded by the :class:`.IfElseOp`
+    which can be used to describe a classically condition in a circuit. For
+    example, a circuit using :attr:`.Instruction.condition` like::
+
+        from qiskit.circuit import QuantumCircuit
+
+        qc = QuantumCircuit(2, 2)
+        qc.h(0)
+        qc.x(0).c_if(0, 1)
+        qc.z(1.c_if(1, 0)
+        qc.measure(0, 0)
+        qc.measure(1, 1)
+
+    can be rewritten using as::
+
+        qc = QuantumCircuit(2, 2)
+        qc.h(0)
+        with expected.if_test((expected.clbits[0], True)):
+            qc.x(0)
+        with expected.if_test((expected.clbits[1], False)):
+            qc.z(1)
+        qc.measure(0, 0)
+        qc.measure(1, 1)
+
+    The now deprecated :class:`.ConvertConditionsToIfOps` transpiler pass can
+    be used to automate this conversion for existing circuits.
+deprecations_transpiler:
+  - |
+    The transpiler pass :class:`.ConvertConditionsToIfOps` has been deprecated
+    and will be removed in Qiskit 2.0.0. This class is now deprecated because
+    the underlying data model for :attr:`.Instruction.condition` which this
+    pass is converting from has been deprecated and will be removed in 2.0.0.

test/python/circuit/classical/test_expr_constructors.py

@@ -26,7 +26,8 @@ def test_lift_legacy_condition(self):
         clbit = Clbit()
 
         inst = Instruction("custom", 1, 0, [])
-        inst.c_if(cr, 7)
+        with self.assertWarns(DeprecationWarning):
+            inst.c_if(cr, 7)
         self.assertEqual(
             expr.lift_legacy_condition(inst.condition),
             expr.Binary(
@@ -38,7 +39,8 @@ def test_lift_legacy_condition(self):
         )
 
         inst = Instruction("custom", 1, 0, [])
-        inst.c_if(cr, 255)
+        with self.assertWarns(DeprecationWarning):
+            inst.c_if(cr, 255)
         self.assertEqual(
             expr.lift_legacy_condition(inst.condition),
             expr.Binary(
@@ -50,7 +52,8 @@ def test_lift_legacy_condition(self):
         )
 
         inst = Instruction("custom", 1, 0, [])
-        inst.c_if(clbit, False)
+        with self.assertWarns(DeprecationWarning):
+            inst.c_if(clbit, False)
         self.assertEqual(
             expr.lift_legacy_condition(inst.condition),
             expr.Unary(
@@ -61,7 +64,8 @@ def test_lift_legacy_condition(self):
         )
 
         inst = Instruction("custom", 1, 0, [])
-        inst.c_if(clbit, True)
+        with self.assertWarns(DeprecationWarning):
+            inst.c_if(clbit, True)
         self.assertEqual(
             expr.lift_legacy_condition(inst.condition),
             expr.Var(clbit, types.Bool()),

test/python/circuit/library/test_permutation.py

@@ -149,7 +149,8 @@ def test_reverse_ops(self):
     def test_conditional(self):
         """Test adding conditional permutations."""
         qc = QuantumCircuit(5, 1)
-        qc.append(PermutationGate([1, 2, 0]), [2, 3, 4]).c_if(0, 1)
+        with self.assertWarns(DeprecationWarning):
+            qc.append(PermutationGate([1, 2, 0]), [2, 3, 4]).c_if(0, 1)
         self.assertIsNotNone(qc.data[0].operation.condition)
 
     def test_qasm(self):

test/python/circuit/library/test_qft.py

@@ -272,7 +272,8 @@ def test_reverse_ops(self):
     def test_conditional(self):
         """Test adding conditional to a QFTGate."""
         qc = QuantumCircuit(5, 1)
-        qc.append(QFTGate(4), [1, 2, 0, 4]).c_if(0, 1)
+        with self.assertWarns(DeprecationWarning):
+            qc.append(QFTGate(4), [1, 2, 0, 4]).c_if(0, 1)
         self.assertIsNotNone(qc.data[0].operation.condition)
 
     def test_qasm(self):

test/python/circuit/test_circuit_load_from_qpy.py

@@ -138,11 +138,13 @@ def test_qpy_full_path(self):
     def test_circuit_with_conditional(self):
         """Test that instructions with conditions are correctly serialized."""
         qc = QuantumCircuit(1, 1)
-        qc.x(0).c_if(qc.cregs[0], 1)
+        with self.assertWarns(DeprecationWarning):
+            qc.x(0).c_if(qc.cregs[0], 1)
         qpy_file = io.BytesIO()
         dump(qc, qpy_file)
         qpy_file.seek(0)
-        new_circ = load(qpy_file)[0]
+        with self.assertWarns(DeprecationWarning):
+            new_circ = load(qpy_file)[0]
         self.assertEqual(qc, new_circ)
         self.assertDeprecatedBitProperties(qc, new_circ)
 
@@ -408,13 +410,15 @@ def test_multiple_circuits(self):
         """Test multiple circuits can be serialized together."""
         circuits = []
         for i in range(10):
-            circuits.append(
-                random_circuit(10, 10, measure=True, conditional=True, reset=True, seed=42 + i)
-            )
+            with self.assertWarns(DeprecationWarning):
+                circuits.append(
+                    random_circuit(10, 10, measure=True, conditional=True, reset=True, seed=42 + i)
+                )
         qpy_file = io.BytesIO()
         dump(circuits, qpy_file)
         qpy_file.seek(0)
-        new_circs = load(qpy_file)
+        with self.assertWarns(DeprecationWarning):
+            new_circs = load(qpy_file)
         self.assertEqual(circuits, new_circs)
         for old, new in zip(circuits, new_circs):
             self.assertDeprecatedBitProperties(old, new)
@@ -674,12 +678,14 @@ def test_circuit_with_conditional_with_label(self):
         """Test that instructions with conditions are correctly serialized."""
         qc = QuantumCircuit(1, 1)
         gate = XGate(label="My conditional x gate")
-        gate.c_if(qc.cregs[0], 1)
+        with self.assertWarns(DeprecationWarning):
+            gate.c_if(qc.cregs[0], 1)
         qc.append(gate, [0])
         qpy_file = io.BytesIO()
         dump(qc, qpy_file)
         qpy_file.seek(0)
-        new_circ = load(qpy_file)[0]
+        with self.assertWarns(DeprecationWarning):
+            new_circ = load(qpy_file)[0]
         self.assertEqual(qc, new_circ)
         self.assertEqual(
             [x.operation.label for x in qc.data], [x.operation.label for x in new_circ.data]
@@ -755,12 +761,14 @@ def test_single_bit_teleportation(self):
         qc = QuantumCircuit(qr, cr, name="Reset Test")
         qc.x(0)
         qc.measure(0, cr[0])
-        qc.x(0).c_if(cr[0], 1)
+        with self.assertWarns(DeprecationWarning):
+            qc.x(0).c_if(cr[0], 1)
         qc.measure(0, cr[1])
         qpy_file = io.BytesIO()
         dump(qc, qpy_file)
         qpy_file.seek(0)
-        new_circ = load(qpy_file)[0]
+        with self.assertWarns(DeprecationWarning):
+            new_circ = load(qpy_file)[0]
         self.assertEqual(qc, new_circ)
         self.assertEqual(
             [x.operation.label for x in qc.data], [x.operation.label for x in new_circ.data]
@@ -1138,11 +1146,13 @@ def test_qpy_with_for_loop(self):
             qc.h(0)
             qc.cx(0, 1)
             qc.measure(0, 0)
-            qc.break_loop().c_if(0, True)
+            with self.assertWarns(DeprecationWarning):
+                qc.break_loop().c_if(0, True)
         qpy_file = io.BytesIO()
         dump(qc, qpy_file)
         qpy_file.seek(0)
-        new_circuit = load(qpy_file)[0]
+        with self.assertWarns(DeprecationWarning):
+            new_circuit = load(qpy_file)[0]
         self.assertEqual(qc, new_circuit)
         self.assertDeprecatedBitProperties(qc, new_circuit)
 
@@ -1154,11 +1164,13 @@ def test_qpy_with_for_loop_iterator(self):
             qc.h(0)
             qc.cx(0, 1)
             qc.measure(0, 0)
-            qc.break_loop().c_if(0, True)
+            with self.assertWarns(DeprecationWarning):
+                qc.break_loop().c_if(0, True)
         qpy_file = io.BytesIO()
         dump(qc, qpy_file)
         qpy_file.seek(0)
-        new_circuit = load(qpy_file)[0]
+        with self.assertWarns(DeprecationWarning):
+            new_circuit = load(qpy_file)[0]
         self.assertEqual(qc, new_circuit)
         self.assertDeprecatedBitProperties(qc, new_circuit)