Fix dag visualization for DAGCircuits without registers

qiskit/visualization/dag_visualization.py

@@ -21,6 +21,7 @@
 import tempfile
 
 from qiskit.dagcircuit.dagnode import DAGOpNode, DAGInNode, DAGOutNode
+from qiskit.circuit import Qubit
 from qiskit.utils import optionals as _optionals
 from qiskit.exceptions import InvalidFileError
 from .exceptions import VisualizationError
@@ -173,7 +174,9 @@ def node_attr_func(node):
         edge_attr_func = None
 
     else:
-        bit_labels = {
+        qubit_indices = {bit: index for index, bit in enumerate(dag.qubits)}
+        clbit_indices = {bit: index for index, bit in enumerate(dag.clbits)}
+        register_bit_labels = {
             bit: f"{reg.name}[{idx}]"
             for reg in list(dag.qregs.values()) + list(dag.cregs.values())
             for (idx, bit) in enumerate(reg)
@@ -192,12 +195,20 @@ def node_attr_func(node):
                     n["style"] = "filled"
                     n["fillcolor"] = "lightblue"
                 if isinstance(node, DAGInNode):
-                    n["label"] = bit_labels[node.wire]
+                    if isinstance(node.wire, Qubit):
+                        label = register_bit_labels.get(node.wire, f"q_{qubit_indices[node.wire]}")
+                    else:
+                        label = register_bit_labels.get(node.wire, f"c_{clbit_indices[node.wire]}")
+                    n["label"] = label
                     n["color"] = "black"
                     n["style"] = "filled"
                     n["fillcolor"] = "green"
                 if isinstance(node, DAGOutNode):
-                    n["label"] = bit_labels[node.wire]
+                    if isinstance(node.wire, Qubit):
+                        label = register_bit_labels.get(node.wire, f"q[{qubit_indices[node.wire]}]")
+                    else:
+                        label = register_bit_labels.get(node.wire, f"c[{clbit_indices[node.wire]}]")
+                    n["label"] = label
                     n["color"] = "black"
                     n["style"] = "filled"
                     n["fillcolor"] = "red"
@@ -207,7 +218,11 @@ def node_attr_func(node):
 
         def edge_attr_func(edge):
             e = {}
-            e["label"] = bit_labels[edge]
+            if isinstance(edge, Qubit):
+                label = register_bit_labels.get(edge, f"q_{qubit_indices[edge]}")
+            else:
+                label = register_bit_labels.get(edge, f"c_{clbit_indices[edge]}")
+            e["label"] = label
             return e
 
     dot_str = dag._multi_graph.to_dot(node_attr_func, edge_attr_func, graph_attrs)

releasenotes/notes/fix-dag-drawer-no-reg-6eee9d1f6e4b9261.yaml

@@ -0,0 +1,9 @@
+---
+fixes:
+  - |
+    Fixed an issue with the visualization function :func:`~.dag_drawer` and the
+    :meth:`.DAGCircuit.draw` method where previously the drawer would fail
+    when attempting to generate a visualization for a :class:`~.DAGCircuit`
+    object that contained a :class:`~.Qubit` or :class:`~.Clbit` which wasn't
+    part of a :class:`~QuantumRegister` or :class:`~ClassicalRegister`.
+    Fixed `#7915 <https://github.com/Qiskit/qiskit-terra/issues/7915>`__

test/python/visualization/test_dag_drawer.py

@@ -12,17 +12,22 @@
 
 """Tests for DAG visualization tool."""
 
+import os
+import tempfile
 import unittest
 
-from qiskit import QuantumRegister, QuantumCircuit
-from qiskit.test import QiskitTestCase
+from PIL import Image
+
+from qiskit.circuit import QuantumRegister, QuantumCircuit, Qubit, Clbit
 from qiskit.tools.visualization import dag_drawer
 from qiskit.exceptions import InvalidFileError
 from qiskit.visualization.exceptions import VisualizationError
 from qiskit.converters import circuit_to_dag
+from qiskit.utils import optionals as _optionals
+from .visualization import path_to_diagram_reference, QiskitVisualizationTestCase
 
 
-class TestDagDrawer(QiskitTestCase):
+class TestDagDrawer(QiskitVisualizationTestCase):
     """Qiskit DAG drawer tests."""
 
     def setUp(self):
@@ -49,6 +54,22 @@ def test_dag_drawer_checks_filename_extension(self):
         with self.assertRaisesRegex(InvalidFileError, "Filename extension must be one of: .*"):
             dag_drawer(self.dag, filename="aa.abc")
 
+    @unittest.skipUnless(_optionals.HAS_GRAPHVIZ, "Graphviz not installed")
+    def test_dag_drawer_no_register(self):
+        """Test dag visualization with a circuit with no registers."""
+        qubit = Qubit()
+        clbit = Clbit()
+        qc = QuantumCircuit([qubit, clbit])
+        qc.h(0)
+        qc.measure(0, 0)
+        dag = circuit_to_dag(qc)
+        with tempfile.TemporaryDirectory() as tmpdirname:
+            tmp_path = os.path.join(tmpdirname, "dag.png")
+            dag_drawer(dag, filename=tmp_path)
+            image_ref = path_to_diagram_reference("dag_no_reg.png")
+            image = Image.open(tmp_path)
+            self.assertImagesAreEqual(image, image_ref, 0.2)
+
 
 if __name__ == "__main__":
     unittest.main(verbosity=2)