QuantumCircuit.compose() doesn't work properly when the arguments are qubit=None, clbit=None

[rum-yasuhiro]

Informations

• Qiskit version:
  • qiskit==0.32.1
  • qiskit-aer==0.9.1
  • qiskit-aqua==0.9.5
  • qiskit-ignis== 0.6.0
  • qiskit-terra==0.18.3
  • qiskit-ibmq-provider==0.18.0
• Python version:
  • 3.8.5

---

• Operating system:
  • macOS Catalina Version 10.15.7 (Darwin Kernel Version 19.6.0)

What is the current behavior?

qiskit/circuit/quantumcircuit.py

    725         if not isinstance(other, QuantumCircuit):
    726             if qubits is None:
--> 727                 qubits = list(range(other.num_qubits))
    728 
    729             if clbits is None:

AttributeError: 'QuantumRegister' object has no attribute 'num_qubits'

other here is supposed to be QuantumCircuit object.

Steps to reproduce the problem

# create qc to be copied, which contain 1-qubit and 1 Hadamard gate.
qr_origin = QuantumRegister(1)
qc_origin = QuantumCircuit(qr_origin)
qc_origin.h(qr_origin[0])

# create empty circuit and apply `compose` method
qr_emp = QuantumRegister(1)
qc_emp = QuantumCircuit(qr_emp)
qc_emp.compose(qc_origin)

What is the expected behavior?

When qubit=None, clbit=None, this function should run successfully.

Suggested solutions

QuantumCircuit object has the attribute "num_qubits".
Or variable "other": QuantumCircuit be converted to DAGCircuit object which has an attribute "num_qubits", execute the process above, then reconverted to QuantumCircuit again.

[jakelishman]

I'm not certain what bug you're talking about here - I don't get any error when I run your code sample on Terra 0.18.3, and the circuit produces what I expect:

In [1]: from qiskit.circuit import QuantumRegister, QuantumCircuit
   ...: # create qc to be copied, which contain 1-qubit and 1 Hadamard gate.
   ...: qr_origin = QuantumRegister(1)
   ...: qc_origin = QuantumCircuit(qr_origin)
   ...: qc_origin.h(qr_origin[0])
   ...:
   ...: # create empty circuit and apply `compose` method
   ...: qr_emp = QuantumRegister(1)
   ...: qc_emp = QuantumCircuit(qr_emp)
   ...: qc_emp.compose(qc_origin).draw()
Out[1]:
      ┌───┐
q1_0: ┤ H ├
      └───┘

[Cryoris]

From the error message it looks like the input to compose was not a circuit but a QuantumRegister. I can also not reproduce this issue so I'll go ahead and close it. Feel free to reopen @rum-yasuhiro if we're missing anything here!