Added classical_predecessors method (#9980)

* Added classical_predecessors method

* add accesed method

* Added classsical_successor method

* Added classical predecessors and successors method

* added tol command

* successors method

* add test classical_successor method

* changed DagOutNode

* Apply suggestions from code review from @atharva-satpute. Thanks!

Co-authored-by: atharva-satpute <55058959+atharva-satpute@users.noreply.github.com>

* add assertRaises on test methods

* correct typo variable name

* change the descrption

* change the text on the example

* change the format of yaml

* new example

* new suggestions

* fix test

* new test example and change the release note

* Fixup release note

* Fix typo

---------

Co-authored-by: Maldoalberto <amaldonador1300@alumno.ipn.mx>
Co-authored-by: Maldoalberto <Alberto.Maldonado.Romo@ibm.com>
Co-authored-by: Luciano Bello <bel@zurich.ibm.com>
Co-authored-by: atharva-satpute <55058959+atharva-satpute@users.noreply.github.com>
Co-authored-by: Jake Lishman <jake.lishman@ibm.com>

qiskit/dagcircuit/dagcircuit.py

@@ -1565,6 +1565,15 @@ class DAGCircuit:
             )
         )
 
+    def classical_predecessors(self, node):
+        """Returns iterator of the predecessors of a node that are
+        connected by a classical edge as DAGOpNodes and DAGInNodes."""
+        return iter(
+            self._multi_graph.find_predecessors_by_edge(
+                node._node_id, lambda edge_data: isinstance(edge_data, Clbit)
+            )
+        )
+
     def ancestors(self, node):
         """Returns set of the ancestors of a node as DAGOpNodes and DAGInNodes."""
         return {self._multi_graph[x] for x in rx.ancestors(self._multi_graph, node._node_id)}
@@ -1589,6 +1598,15 @@ class DAGCircuit:
             )
         )
 
+    def classical_successors(self, node):
+        """Returns iterator of the successors of a node that are
+        connected by a classical edge as DAGOpNodes and DAGInNodes."""
+        return iter(
+            self._multi_graph.find_successors_by_edge(
+                node._node_id, lambda edge_data: isinstance(edge_data, Clbit)
+            )
+        )
+
     def remove_op_node(self, node):
         """Remove an operation node n.
 

releasenotes/notes/add-classical-predecessors-9ecef0561822e934.yaml

@@ -0,0 +1,27 @@
+---
+features:
+  - |
+    Added :meth:`.DAGCircuit.classical_predecessors` and
+    :meth:`.DAGCircuit.classical_successors`, an alternative to select the classical
+    wires without having to go to the inner graph object directly of a node in the DAG.
+    The following example illustrates the new functionality::
+
+        from qiskit import QuantumCircuit, QuantumRegister, ClassicalRegister
+        from qiskit.converters import circuit_to_dag
+        from qiskit.circuit.library import RZGate
+
+        q = QuantumRegister(3, 'q')
+        c = ClassicalRegister(3, 'c')
+        circ = QuantumCircuit(q, c)
+        circ.h(q[0])
+        circ.cx(q[0], q[1])
+        circ.measure(q[0], c[0])
+        circ.rz(0.5, q[1]).c_if(c, 2)
+        circ.measure(q[1], c[0])
+        dag = circuit_to_dag(circ)
+
+        rz_node = dag.op_nodes(RZGate)[0]
+        # Contains the "measure" on clbit 0, and the "wire start" nodes for clbits 1 and 2.
+        classical_predecessors = list(dag.classical_predecessors(rz_node))
+        # Contains the "measure" on clbit 0, and the "wire end" nodes for clbits 1 and 2.
+        classical_successors = list(dag.classical_successors(rz_node))

test/python/dagcircuit/test_dagcircuit.py

@@ -740,6 +740,73 @@ class TestDagNodeSelection(QiskitTestCase):
             or (isinstance(predecessor2, DAGInNode) and isinstance(predecessor1.op, Reset))
         )
 
+    def test_classical_predecessors(self):
+        """The method dag.classical_predecessors() returns predecessors connected by classical edges"""
+
+        #       ┌───┐                         ┌───┐
+        #  q_0: ┤ H ├──■───────────────────■──┤ H ├
+        #       ├───┤┌─┴─┐               ┌─┴─┐├───┤
+        #  q_1: ┤ H ├┤ X ├──■─────────■──┤ X ├┤ H ├
+        #       └───┘└───┘┌─┴─┐┌───┐┌─┴─┐└───┘└───┘
+        #  q_2: ──────────┤ X ├┤ H ├┤ X ├──────────
+        #                 └───┘└───┘└───┘
+        #  c: 5/═══════════════════════════════════
+
+        self.dag.apply_operation_back(HGate(), [self.qubit0], [])
+        self.dag.apply_operation_back(HGate(), [self.qubit1], [])
+        self.dag.apply_operation_back(CXGate(), [self.qubit0, self.qubit1], [])
+        self.dag.apply_operation_back(CXGate(), [self.qubit1, self.qubit2], [])
+        self.dag.apply_operation_back(HGate(), [self.qubit2], [])
+        self.dag.apply_operation_back(CXGate(), [self.qubit1, self.qubit2], [])
+        self.dag.apply_operation_back(CXGate(), [self.qubit0, self.qubit1], [])
+        self.dag.apply_operation_back(HGate(), [self.qubit0], [])
+        self.dag.apply_operation_back(HGate(), [self.qubit1], [])
+        self.dag.apply_operation_back(Measure(), [self.qubit0, self.clbit0], [])
+        self.dag.apply_operation_back(Measure(), [self.qubit1, self.clbit1], [])
+
+        predecessor_measure = self.dag.classical_predecessors(self.dag.named_nodes("measure").pop())
+
+        predecessor1 = next(predecessor_measure)
+
+        with self.assertRaises(StopIteration):
+            next(predecessor_measure)
+
+        self.assertIsInstance(predecessor1, DAGInNode)
+        self.assertIsInstance(predecessor1.wire, Clbit)
+
+    def test_classical_successors(self):
+        """The method dag.classical_successors() returns successors connected by classical edges"""
+
+        #       ┌───┐                         ┌───┐
+        #  q_0: ┤ H ├──■───────────────────■──┤ H ├
+        #       ├───┤┌─┴─┐               ┌─┴─┐├───┤
+        #  q_1: ┤ H ├┤ X ├──■─────────■──┤ X ├┤ H ├
+        #       └───┘└───┘┌─┴─┐┌───┐┌─┴─┐└───┘└───┘
+        #  q_2: ──────────┤ X ├┤ H ├┤ X ├──────────
+        #                 └───┘└───┘└───┘
+        #  c: 5/═══════════════════════════════════
+
+        self.dag.apply_operation_back(HGate(), [self.qubit0], [])
+        self.dag.apply_operation_back(HGate(), [self.qubit1], [])
+        self.dag.apply_operation_back(CXGate(), [self.qubit0, self.qubit1], [])
+        self.dag.apply_operation_back(CXGate(), [self.qubit1, self.qubit2], [])
+        self.dag.apply_operation_back(HGate(), [self.qubit2], [])
+        self.dag.apply_operation_back(CXGate(), [self.qubit1, self.qubit2], [])
+        self.dag.apply_operation_back(CXGate(), [self.qubit0, self.qubit1], [])
+        self.dag.apply_operation_back(HGate(), [self.qubit0], [])
+        self.dag.apply_operation_back(HGate(), [self.qubit1], [])
+        self.dag.apply_operation_back(Measure(), [self.qubit0, self.clbit0], [])
+        self.dag.apply_operation_back(Measure(), [self.qubit1, self.clbit1], [])
+
+        successors_measure = self.dag.classical_successors(self.dag.named_nodes("measure").pop())
+
+        successors1 = next(successors_measure)
+        with self.assertRaises(StopIteration):
+            next(successors_measure)
+
+        self.assertIsInstance(successors1, DAGOutNode)
+        self.assertIsInstance(successors1.wire, Clbit)
+
     def test_is_predecessor(self):
         """The method dag.is_predecessor(A, B) checks if node B is a predecessor of A"""