from collections import deque

class HistorialLineas:
    def __init__(self, lineas, longitud = 5):
        self.lineas = lineas
        self.historial = deque(maxlen=longitud)

    def __iter__(self):
        for numero_linea, linea in enumerate(self.lineas, 1):
            self.historial.append((numero_linea, linea))
            yield linea

    def remover(self):
        self.historial.clear()


if __name__ == '__main__':
    with open('python.txt', 'r') as f:
        lineas = HistorialLineas(f)

        for linea in lineas:
            if 'Software' in linea:
                for numero_linea, historial_linea in lineas.historial:
                    print('{}:{}'.format(numero_linea, historial_linea), end='')