I think it is a bug but I'm not really sure. Basically, what I'm trying to do is the following: I have an Oracle Instruction which I want to append to my QuantumCircuit, looks as follows:

# initialising registers for readability
[control, train_register] = circ.qregs


# create and append oracle
oracle = create_oracle(train_register, control)  # returns an Instruction
circ.append(oracle, [train_register, control])

If my train_register and control are both QuantumRegisters with length 1, this works perfectly fine. But I want to make this work for variable length of these registers, I get the error qiskit.circuit.exceptions.CircuitError: 'The amount of qubit arguments does not match the instruction expectation.'.

This is logical, since this error occurs because qiskit checks the length of my provided qargs in QuantumCircuit.append(), and I provided two registers in a list with both one qubit, so the length of the list happens to be the same as the number of qubits.

However, I want this to work for variable length. If I know what the length of my QuantumRegisters are, say len(train_register) is 3 and len(control) is 1 I can use

circ.append(oracle, [0, 1, 2, 3])

Or to make it work for unspecified lengths, I can write something like:

indices_to_append_to = list(np.arange(len(train_register) + len(control)))
circ.append(oracle, indices_to_append_to)

But I just really like the idea of just plugging in the names of the qregs as qargs. Is there something to make this work like that?


1 Answer 1


If you pass a list of qubits instead of a list of register this works. For this do

circ.append(oracle, train_register[:] + control[:])

where train_register and control are QuantumRegisters.

  • $\begingroup$ Thank you very much. I was almost a hundred percent certain I checked that possibility, but I think the way I did it was [train_register[:] + control[:]], which doens't work. $\endgroup$ Jun 19, 2020 at 11:42

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