Fix dag visualization with Var wires (#12848)

* Fix dag visualization with Var wires

This commit fixes the dag visualization for DAGs with classical
variables. The Var type was not handled in the attribute callback
functions for nodes and edges. This was causing the visualizer to fail
if the dag contained these types. This fixes this by adding explict
handling for the Var types and using the name attribute of the Var
object.

* Add release note and test

qiskit/visualization/dag_visualization.py

@@ -174,10 +174,13 @@ def node_attr_func(node):
                         label = register_bit_labels.get(
                             node.wire, f"q_{dag.find_bit(node.wire).index}"
                         )
-                    else:
+                    elif isinstance(node.wire, Clbit):
                         label = register_bit_labels.get(
                             node.wire, f"c_{dag.find_bit(node.wire).index}"
                         )
+                    else:
+                        label = str(node.wire.name)
+
                     n["label"] = label
                     n["color"] = "black"
                     n["style"] = "filled"
@@ -187,10 +190,12 @@ def node_attr_func(node):
                         label = register_bit_labels.get(
                             node.wire, f"q[{dag.find_bit(node.wire).index}]"
                         )
-                    else:
+                    elif isinstance(node.wire, Clbit):
                         label = register_bit_labels.get(
                             node.wire, f"c[{dag.find_bit(node.wire).index}]"
                         )
+                    else:
+                        label = str(node.wire.name)
                     n["label"] = label
                     n["color"] = "black"
                     n["style"] = "filled"
@@ -203,8 +208,10 @@ def edge_attr_func(edge):
             e = {}
             if isinstance(edge, Qubit):
                 label = register_bit_labels.get(edge, f"q_{dag.find_bit(edge).index}")
-            else:
+            elif isinstance(edge, Clbit):
                 label = register_bit_labels.get(edge, f"c_{dag.find_bit(edge).index}")
+            else:
+                label = str(edge.name)
             e["label"] = label
             return e
 

releasenotes/notes/fix-var-wires-4ebc40e0b19df253.yaml

@@ -0,0 +1,8 @@
+---
+fixes:
+  - |
+    Fixed an issue with :func:`.dag_drawer` and :meth:`.DAGCircuit.draw`
+    when attempting to visualize a :class:`.DAGCircuit` instance that contained
+    :class:`.Var` wires. The visualizer would raise an exception trying to
+    do this which has been fixed so the expected visualization will be
+    generated.

test/python/visualization/test_dag_drawer.py

@@ -16,12 +16,14 @@
 import tempfile
 import unittest
 
-from qiskit.circuit import QuantumRegister, ClassicalRegister, QuantumCircuit, Qubit, Clbit
+from qiskit.circuit import QuantumRegister, ClassicalRegister, QuantumCircuit, Qubit, Clbit, Store
 from qiskit.visualization import dag_drawer
 from qiskit.exceptions import InvalidFileError
 from qiskit.visualization import VisualizationError
 from qiskit.converters import circuit_to_dag, circuit_to_dagdependency
 from qiskit.utils import optionals as _optionals
+from qiskit.dagcircuit import DAGCircuit
+from qiskit.circuit.classical import expr, types
 from .visualization import path_to_diagram_reference, QiskitVisualizationTestCase
 
 
@@ -108,6 +110,17 @@ def test_dag_drawer_with_dag_dep(self):
             image = Image.open(tmp_path)
             self.assertImagesAreEqual(image, image_ref, 0.1)
 
+    @unittest.skipUnless(_optionals.HAS_GRAPHVIZ, "Graphviz not installed")
+    @unittest.skipUnless(_optionals.HAS_PIL, "PIL not installed")
+    def test_dag_drawer_with_var_wires(self):
+        """Test visualization works with var nodes."""
+        a = expr.Var.new("a", types.Bool())
+        dag = DAGCircuit()
+        dag.add_input_var(a)
+        dag.apply_operation_back(Store(a, a), (), ())
+        image = dag_drawer(dag)
+        self.assertIsNotNone(image)
+
 
 if __name__ == "__main__":
     unittest.main(verbosity=2)