qc.draw('text', idle_wires=False) results in ValueError : min() arg is an empty sequence

[sumit-kale]

Environment

• Qiskit version: 1.2.0
• Python version: 3.10.14
• Operating system: MacOS (M1)

What is happening?

For a parametrised quantum circuit qc.draw('text', idle_wires=False) results in ValueError: min() arg is an empty sequence
This issue only comes in Qiskit version: 1.2.0 and the same code works fine in Qiskit version: 1.1.0.

Additionally qc.draw('mpl', idle_wires=False) prints the quantum circuit but the number of qubits gets reduced. We haven't trinspiled the circuit yet and didn't mention anything about qubit reuse.

This is the full traceback!

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
File /opt/miniconda3/envs/qiskitenv/lib/python3.10/site-packages/IPython/core/formatters.py:347, in BaseFormatter.__call__(self, obj)
    [345](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/qiskitenv/lib/python3.10/site-packages/IPython/core/formatters.py:345)     method = get_real_method(obj, self.print_method)
    [346](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/qiskitenv/lib/python3.10/site-packages/IPython/core/formatters.py:346)     if method is not None:
--> [347](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/qiskitenv/lib/python3.10/site-packages/IPython/core/formatters.py:347)         return method()
    [348](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/qiskitenv/lib/python3.10/site-packages/IPython/core/formatters.py:348)     return None
    [349](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/qiskitenv/lib/python3.10/site-packages/IPython/core/formatters.py:349) else:

File /opt/miniconda3/envs/qiskitenv/lib/python3.10/site-packages/qiskit/visualization/circuit/text.py:762, in TextDrawing._repr_html_(self)
    [755](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/qiskitenv/lib/python3.10/site-packages/qiskit/visualization/circuit/text.py:755) def _repr_html_(self):
    [756](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/qiskitenv/lib/python3.10/site-packages/qiskit/visualization/circuit/text.py:756)     return (
    [757](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/qiskitenv/lib/python3.10/site-packages/qiskit/visualization/circuit/text.py:757)         '<pre style="word-wrap: normal;'
    [758](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/qiskitenv/lib/python3.10/site-packages/qiskit/visualization/circuit/text.py:758)         "white-space: pre;"
    [759](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/qiskitenv/lib/python3.10/site-packages/qiskit/visualization/circuit/text.py:759)         "background: #fff0;"
    [760](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/qiskitenv/lib/python3.10/site-packages/qiskit/visualization/circuit/text.py:760)         "line-height: 1.1;"
    [761](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/qiskitenv/lib/python3.10/site-packages/qiskit/visualization/circuit/text.py:761)         'font-family: &quot;Courier New&quot;,Courier,monospace">'
--> [762](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/qiskitenv/lib/python3.10/site-packages/qiskit/visualization/circuit/text.py:762)         f"{self.single_string()}</pre>"
    [763](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/qiskitenv/lib/python3.10/site-packages/qiskit/visualization/circuit/text.py:763)     )

File /opt/miniconda3/envs/qiskitenv/lib/python3.10/site-packages/qiskit/visualization/circuit/text.py:779, in TextDrawing.single_string(self)
    [776](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/qiskitenv/lib/python3.10/site-packages/qiskit/visualization/circuit/text.py:776)     return self._single_string
    [777](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/qiskitenv/lib/python3.10/site-packages/qiskit/visualization/circuit/text.py:777) try:
    [778](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/qiskitenv/lib/python3.10/site-packages/qiskit/visualization/circuit/text.py:778)     self._single_string = (
--> [779](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/qiskitenv/lib/python3.10/site-packages/qiskit/visualization/circuit/text.py:779)         "\n".join(self.lines()).encode(self.encoding).decode(self.encoding)
...
-> [1079](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/qiskitenv/lib/python3.10/site-packages/qiskit/visualization/circuit/text.py:1079)     if min(qubit_indices) > wire_map[ctrl_qubit[0]]:
   [1080](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/qiskitenv/lib/python3.10/site-packages/qiskit/visualization/circuit/text.py:1080)         top_box.append(ctrl_qubit)
   [1081](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/qiskitenv/lib/python3.10/site-packages/qiskit/visualization/circuit/text.py:1081)     elif max(qubit_indices) < wire_map[ctrl_qubit[0]]:

ValueError: min() arg is an empty sequence

How can we reproduce the issue?

# Minimal code

from qiskit import QuantumCircuit, ClassicalRegister, QuantumRegister
from qiskit.circuit import ParameterVector
import numpy as np

nx=2

thetas = ParameterVector('θ',length=1)
phis = ParameterVector('φ',length=nx+1)  

qr = QuantumRegister(nx+1)
cr = ClassicalRegister(1)
qc = QuantumCircuit(qr,cr)

for i in range(nx):
    qc.ry(thetas[0], i)
qc.barrier()

for i in range(nx):
    j=nx-i # start from the highest power
    qc.ry(phis[j], j)
    qc.rz(np.pi/2,j-1)
    qc.cx(j-1,j)
            
qc.barrier()
qc.ry(phis[0], 0)  
qc.measure(0, 0)

print(qc.draw('text',idle_wires=False))
print(qc.draw('mpl',idle_wires=False))

What should happen?

The user should be able to print the quantum circuit without getting the ValueError and the circuit with 'mpl' should not reduce the number of qubits.

Any suggestions?

No response

[t-imamichi]

It has been fixed by the main branch already. But the fix is not ported to the latest release 1.2.1, unfortunately.

# main branch

      ┌──────────┐ ░ ┌─────────┐                        ░ ┌──────────┐┌─┐
q0_0: ┤ Ry(θ[0]) ├─░─┤ Rz(π/2) ├────────────────────■───░─┤ Ry(φ[0]) ├┤M├
      ├──────────┤ ░ ├─────────┤      ┌──────────┐┌─┴─┐ ░ └──────────┘└╥┘
q0_1: ┤ Ry(θ[0]) ├─░─┤ Rz(π/2) ├───■──┤ Ry(φ[1]) ├┤ X ├─░──────────────╫─
      └──────────┘ ░ ├─────────┴┐┌─┴─┐└──────────┘└───┘ ░              ║
q0_2: ─────────────░─┤ Ry(φ[2]) ├┤ X ├──────────────────░──────────────╫─
                   ░ └──────────┘└───┘                  ░              ║
c0: 1/═════════════════════════════════════════════════════════════════╩═
                                                                       0

# qiskit 1.2.1

Traceback (most recent call last):
  File "/Users/ima/tasks/4_2024/qiskit/terra/tmp/13128.py", line 30, in <module>
    print(qc.draw('text',idle_wires=False))
  File "/Users/ima/envs/qiskit312/lib/python3.12/site-packages/qiskit/visualization/circuit/text.py", line 753, in __str__
    return self.single_string()
           ^^^^^^^^^^^^^^^^^^^^
  File "/Users/ima/envs/qiskit312/lib/python3.12/site-packages/qiskit/visualization/circuit/text.py", line 779, in single_string
    "\n".join(self.lines()).encode(self.encoding).decode(self.encoding)
              ^^^^^^^^^^^^
  File "/Users/ima/envs/qiskit312/lib/python3.12/site-packages/qiskit/visualization/circuit/text.py", line 827, in lines
    layers = self.build_layers()
             ^^^^^^^^^^^^^^^^^^^
  File "/Users/ima/envs/qiskit312/lib/python3.12/site-packages/qiskit/visualization/circuit/text.py", line 1287, in build_layers
    layer, current_cons, current_cons_cond, connection_label = self._node_to_gate(
                                                               ^^^^^^^^^^^^^^^^^^^
  File "/Users/ima/envs/qiskit312/lib/python3.12/site-packages/qiskit/visualization/circuit/text.py", line 1181, in _node_to_gate
    controls_array = TextDrawing.controlled_wires(
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ima/envs/qiskit312/lib/python3.12/site-packages/qiskit/visualization/circuit/text.py", line 1079, in controlled_wires
    if min(qubit_indices) > wire_map[ctrl_qubit[0]]:
       ^^^^^^^^^^^^^^^^^^
ValueError: min() iterable argument is empty

[Cryoris]

With @mrossinek we had quite the bug hunt for this:

• Avoid operator creation in transpiler #12826 introduces this bug on main and 1.2 (since it was backported)
• [DAGCircuit Oxidation] Port DAGCircuit to Rust #12550 fixes this on main

After some surgical insertions of print statements it turns out that the DAGCircuit.layers method is the culprit, from what we understood because it adds nodes to a layer via reference instead of a copy, as before.

[mrvee-qC-bee]

@kifumi -san for your awareness!