Can't append multiple times with self-defined oracle

Question

import cirq

def set_io_qubits(qubit_count):
    input_qubits = [cirq.GridQubit(i, 0) for i in range(qubit_count)]
    return (input_qubits)

def debug_oracle(input_qubits):
    yield (cirq.H(input_qubits[0]))

def make_grover_circuit(input_qubits, oracle):
    c = cirq.Circuit()

c.append(oracle)
    c.append(oracle)
    c.append(oracle)

c.append(cirq.measure(*input_qubits, key='result'))

return c

def main():
    input_qubits = set_io_qubits(2)

oracle = debug_oracle(input_qubits)

circuit = make_grover_circuit(input_qubits, oracle)
    print('Circuit:')
    print(circuit)

if __name__ == '__main__':
    main()

I add c.append(oracle) three times in make_grover_circuit. However, when I print the circuit, it seems there is only one oracle. Is it a bug? Or just I can't .append my oracle more than once? Here is my result of the program.

/Library/Frameworks/Python.framework/Versions/3.7/bin/python3 /Users/a123/Desktop/Cirq/test.py
Circuit:
(0, 0): ───H───M('result')───
               │
(1, 0): ───────M─────────────

Process finished with exit code 0

Craig Gidney · Answer

The issue is that you are using the result of a generator method multiple times:
def gen():
    yield 1
g = gen()
print(list(g))  # prints [1]
print(list(g))  # prints []

This is a common mistake to make in Python. The solution is to store a list of the items instead of the generator:
def gen():
    yield 1

gen_items_safe_to_reuse = list(gen())

Can't append multiple times with self-defined oracle

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