def generador_fibonacci():
    fib1 = 0
    fib2 = 1

    while True:
        yield fib2
        fib1, fib2 = fib2, fib1 + fib2

with open('fibonaccis-2.txt', 'xt') as f:
    contador = 1

    for fib in generador_fibonacci():
        f.write(str(fib))

        if contador == 10:
            break

        contador += 1