Add new transpiler exception class for too many qubits

qiskit/compiler/transpiler.py

@@ -28,7 +28,7 @@
 from qiskit.pulse import Schedule, InstructionScheduleMap
 from qiskit.transpiler import Layout, CouplingMap, PropertySet
 from qiskit.transpiler.basepasses import BasePass
-from qiskit.transpiler.exceptions import TranspilerError
+from qiskit.transpiler.exceptions import TranspilerError, CircuitTooWideForTarget
 from qiskit.transpiler.instruction_durations import InstructionDurations, InstructionDurationsType
 from qiskit.transpiler.passes.synthesis.high_level_synthesis import HLSConfig
 from qiskit.transpiler.preset_passmanagers import generate_preset_pass_manager
@@ -455,7 +455,7 @@ def _check_circuits_coupling_map(circuits, cmap, backend):
         # If coupling_map is not None or num_qubits == 1
         num_qubits = len(circuit.qubits)
         if max_qubits is not None and (num_qubits > max_qubits):
-            raise TranspilerError(
+            raise CircuitTooWideForTarget(
                 f"Number of qubits ({num_qubits}) in {circuit.name} "
                 f"is greater than maximum ({max_qubits}) in the coupling_map"
             )

qiskit/transpiler/__init__.py

@@ -1251,6 +1251,8 @@
 .. autoexception:: TranspilerAccessError
 .. autoexception:: CouplingError
 .. autoexception:: LayoutError
+.. autoexception:: CircuitTooWideForTarget
+
 """
 
 # For backward compatibility
@@ -1263,7 +1265,13 @@
 from .passmanager import PassManager, StagedPassManager
 from .passmanager_config import PassManagerConfig
 from .propertyset import PropertySet  # pylint: disable=no-name-in-module
-from .exceptions import TranspilerError, TranspilerAccessError, CouplingError, LayoutError
+from .exceptions import (
+    TranspilerError,
+    TranspilerAccessError,
+    CouplingError,
+    LayoutError,
+    CircuitTooWideForTarget,
+)
 from .fencedobjs import FencedDAGCircuit, FencedPropertySet
 from .basepasses import AnalysisPass, TransformationPass
 from .coupling import CouplingMap

qiskit/transpiler/exceptions.py

@@ -49,3 +49,7 @@ def __init__(self, *msg):
     def __str__(self):
         """Return the message."""
         return repr(self.msg)
+
+
+class CircuitTooWideForTarget(TranspilerError):
+    """Error raised if the circuit is too wide for the target."""

releasenotes/notes/new-exception-too-wide-3231c1df15952445.yaml

@@ -0,0 +1,9 @@
+---
+features:
+  - |
+    Added a new exception class :exc:`.CircuitToWideForTarget` which
+    subclasses :exc:`.TranspilerError`. It's used in places where a
+    :exc:`.TranspilerError` was previously raised when the error was that
+    the number of circuit qubits was larger than the target backend's qubits.
+    The new class enables more differentiating between this error condition and
+    other :exc:`.TranspilerError`\s.

test/python/compiler/test_transpiler.py

@@ -87,7 +87,7 @@
 from qiskit.test import QiskitTestCase, slow_test
 from qiskit.tools import parallel
 from qiskit.transpiler import CouplingMap, Layout, PassManager, TransformationPass
-from qiskit.transpiler.exceptions import TranspilerError
+from qiskit.transpiler.exceptions import TranspilerError, CircuitTooWideForTarget
 from qiskit.transpiler.passes import BarrierBeforeFinalMeasurements, GateDirection, VF2PostLayout
 from qiskit.transpiler.passmanager_config import PassManagerConfig
 from qiskit.transpiler.preset_passmanagers import generate_preset_pass_manager, level_0_pass_manager
@@ -987,7 +987,7 @@ def test_check_circuit_width(self):
 
         qc = QuantumCircuit(15, 15)
 
-        with self.assertRaises(TranspilerError):
+        with self.assertRaises(CircuitTooWideForTarget):
             transpile(qc, coupling_map=cmap)
 
     @data(0, 1, 2, 3)