Deprecate passing None to DAGCircuit appenders

This has been against the documented typing of the functions for some
time, but some scheduling methods have mistakenly been passing `None`.
There is no need to support `None` here; the empty tuple `()` is equally
immutable and a CPython singleton, so there are neither mutability nor
memory benefits to supporting both, and removing `None` as an input is a
simple way to remove a(n admittedly cheap) branch from the appender
methods.

qiskit/dagcircuit/dagcircuit.py

@@ -24,6 +24,7 @@
 import copy
 import itertools
 import math
+import warnings
 from typing import Dict, Generator, Any, List
 
 import numpy as np
@@ -661,8 +662,16 @@ def apply_operation_back(self, op, qargs=(), cargs=()):
             DAGCircuitError: if a leaf node is connected to multiple outputs
 
         """
-        qargs = tuple(qargs) if qargs is not None else ()
-        cargs = tuple(cargs) if cargs is not None else ()
+        if qargs is None:
+            _warn_none_args()
+            qargs = ()
+        else:
+            qargs = tuple(qargs)
+        if cargs is None:
+            _warn_none_args()
+            cargs = ()
+        else:
+            cargs = tuple(cargs)
 
         all_cbits = set(self._bits_in_operation(op)).union(cargs)
 
@@ -695,6 +704,17 @@ def apply_operation_front(self, op, qargs=(), cargs=()):
         Raises:
             DAGCircuitError: if initial nodes connected to multiple out edges
         """
+        if qargs is None:
+            _warn_none_args()
+            qargs = ()
+        else:
+            qargs = tuple(qargs)
+        if cargs is None:
+            _warn_none_args()
+            cargs = ()
+        else:
+            cargs = tuple(cargs)
+
         all_cbits = set(self._bits_in_operation(op)).union(cargs)
 
         self._check_condition(op.name, getattr(op, "condition", None))
@@ -2071,3 +2091,12 @@ def draw(self, scale=0.7, filename=None, style="color"):
         from qiskit.visualization.dag_visualization import dag_drawer
 
         return dag_drawer(dag=self, scale=scale, filename=filename, style=style)
+
+
+def _warn_none_args():
+    warnings.warn(
+        "Passing 'None' as the qubits or clbits of an operation to 'DAGCircuit' methods"
+        " is deprecated since Qiskit 0.45 and will be removed in Qiskit 1.0.  Instead, pass '()'.",
+        DeprecationWarning,
+        stacklevel=3,
+    )

qiskit/transpiler/passes/scheduling/padding/base_padding.py

@@ -198,7 +198,7 @@ def _apply_scheduled_op(
         t_start: int,
         oper: Instruction,
         qubits: Qubit | Iterable[Qubit],
-        clbits: Clbit | Iterable[Clbit] | None = None,
+        clbits: Clbit | Iterable[Clbit] = (),
     ):
         """Add new operation to DAG with scheduled information.
 

releasenotes/notes/dag-deprecate-none-args-e6ef868148da275d.yaml

@@ -0,0 +1,7 @@
+---
+deprecations:
+  - |
+    Passing ``None`` as the ``qargs`` or ``cargs`` arguments to :meth:`.DAGCircuit.apply_operation_back`
+    or :meth:`~.DAGCircuit.apply_operation_front` is deprecated and will be removed in Qiskit 1.0.
+    This has been explicitly against the typing documentation for some time, but silently accepted
+    by Qiskit.  Instead, simply pass ``()`` rather than ``None``.

test/python/dagcircuit/test_dagcircuit.py

@@ -488,6 +488,18 @@ def test_apply_operation_back(self):
         self.assertEqual(len(list(self.dag.nodes())), 16)
         self.assertEqual(len(list(self.dag.edges())), 17)
 
+    def test_apply_operation_rejects_none(self):
+        """Test that the ``apply_operation_*`` methods warn when given ``None``."""
+        noop = Instruction("noop", 0, 0, [])
+        with self.assertWarnsRegex(DeprecationWarning, "Passing 'None'"):
+            self.dag.apply_operation_back(noop, None, ())
+        with self.assertWarnsRegex(DeprecationWarning, "Passing 'None'"):
+            self.dag.apply_operation_back(noop, (), None)
+        with self.assertWarnsRegex(DeprecationWarning, "Passing 'None'"):
+            self.dag.apply_operation_front(noop, None, ())
+        with self.assertWarnsRegex(DeprecationWarning, "Passing 'None'"):
+            self.dag.apply_operation_front(noop, (), None)
+
     def test_edges(self):
         """Test that DAGCircuit.edges() behaves as expected with ops."""
         x_gate = XGate()