[BUG] unnecessary classical registers created when exporting to qasm without measurements

[natestemen]

Expected behavior

No additional classical registers are created when measure_all=False when calling qml.to_openqasm.

Actual behavior

Classical registers are created/added to the QASM code.

Additional information

Out of curiosity, why is altering the users program the default? I'd expect the user to have to signal that they want to add measurements.

Source code

import pennylane as qml

dev = qml.device("default.qubit", wires=2)

@qml.qnode(dev)
def bell_state():
    qml.Hadamard(wires=0)
    qml.CNOT(wires=[0, 1])
    return 0

print(qml.to_openqasm(bell_state, measure_all=False)())

>>> OPENQASM 2.0;
include "qelib1.inc";
qreg q[2];
creg c[2];
h q[0];
cx q[0],q[1];

System information

qml.about() actually fails since I'm using pennylane installed via uv (i.e. no pip installed), but I'm on version 0.43.1 and python 3.12.0 on a mac.

Existing GitHub issues

• I have searched existing GitHub issues to make sure the issue does not already exist.

[CatalinaAlbornoz]

Hi Nate, thanks for opening this issue!

I'm not sure what the issue is but we'll investigate. In the meantime, could you please try using PennyLane v0.44?
The bug with the uv package manager was resolved back in October from what I see in this issue, so it should be fixed in the latest release.

[natestemen]

Confirming I'm still seeing the issue 0.44. Here is the about info as well:

Name: pennylane
Version: 0.44.0
Summary: PennyLane is a cross-platform Python library for quantum computing, quantum machine learning, and quantum chemistry. Train a quantum computer the same way as a neural network.
Home-page: 
Author: 
License: 
Location: [/opt/homebrew/Caskroom/miniforge/base/envs/plane/lib/python3.13/site-packages](https://file+.vscode-resource.vscode-cdn.net/opt/homebrew/Caskroom/miniforge/base/envs/plane/lib/python3.13/site-packages)
Platform info:           macOS-14.8.3-arm64-arm-64bit-Mach-O
Python version:          3.13.12
Numpy version:           2.4.2
Scipy version:           1.17.0
JAX version:             None
Installed devices:
- lightning.qubit (pennylane_lightning-0.44.0)
- default.clifford (pennylane-0.44.0)
- default.gaussian (pennylane-0.44.0)
- default.mixed (pennylane-0.44.0)
- default.qubit (pennylane-0.44.0)
- default.qutrit (pennylane-0.44.0)
- default.qutrit.mixed (pennylane-0.44.0)
- default.tensor (pennylane-0.44.0)
- null.qubit (pennylane-0.44.0)
- reference.qubit (pennylane-0.44.0)

[CatalinaAlbornoz]

Thanks for confirming Nate and for opening the PR!