Deprecate the condition attribute and related functionality

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)
     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` attribute and the
+    :meth:`.Instruction.c_if` method. They will be removed
+    in Qiskit 2.0, along with any uses in the Qiskit data
+    model. This functionality has been superseded by the :class:`.IfElseOp` class
+    which can be used to describe a classical condition in a circuit. For
+    example, a circuit using  :meth:`.Instruction.c_if` 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 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)