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

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())
$$$$


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.