ChatGPT gave me Qiskit code that used += to append a circuit?

Question

I asked ChatGPT for a Qiskit implementation of Deutsch's algorithm. I pasted the code it returned below. Under where it says "# Apply the oracle gate", it tries to use "circuit += oracle".

Can "+=" actually be used in this fashion or some way similar?

from qiskit import QuantumCircuit, Aer, execute

# Define a function to create the Deutsch circuit
def deutsch_circuit(oracle):
    # Create a quantum circuit with two qubits and one classical bit
    circuit = QuantumCircuit(2, 1)

    # Initialize the input qubit to |0>
    circuit.x(1)

    # Apply a Hadamard gate to both qubits
    circuit.h(0)
    circuit.h(1)

    # Apply the oracle gate
    circuit += oracle

    # Apply another Hadamard gate to the first qubit
    circuit.h(0)

    # Measure the first qubit and store the result in the classical bit
    circuit.measure(0, 0)

    # Return the circuit
    return circuit

# Define the oracle for the constant function f(x) = 0
def constant_oracle():
    circuit = QuantumCircuit(2)

    # Do nothing

    # Return the oracle
    return circuit

# Define the oracle for the balanced function f(x) = x
def balanced_oracle():
    circuit = QuantumCircuit(2)

    # Apply a CNOT gate with the first qubit as control and the second qubit as target
    circuit.cx(0, 1)

    # Return the oracle
    return circuit

# Run the Deutsch algorithm with the constant function
circuit = deutsch_circuit(constant_oracle())
backend = Aer.get_backend('qasm_simulator')
result = execute(circuit, backend).result()
print(result.get_counts())

# Run the Deutsch algorithm with the balanced function
circuit = deutsch_circuit(balanced_oracle())
backend = Aer.get_backend('qasm_simulator')
result = execute(circuit, backend).result()
print(result.get_counts())
```

Answer 1

The operator += used to be part of public interface of QuantumCircuit. It's deprecated in version 0.17 of Qiskit Terra with the recommendation to use compose() method instead which is more flexible w.r.t circuit register compatibility. And it's removed in version 0.23.