'list indices must be integers, not tuple' error in newbie script

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Hi expers!!

Im a new user of SAGE. Right now im writing a script that plot 'N' random lines of lenght=1 in a rectangle of b*h center in axes. The code is shhow below:

N=float(raw_input('Número de rectas aleatóreos (N)?:\n'))
b=float(raw_input('Base del rectángulo (b>1)?:\n'))
h=float(raw_input('Altura del cuadrado (h>1)?:\n'))

uniforme_x=RealDistribution('uniform',[-b/2,b/2])
uniforme_y=RealDistribution('uniform',[-h/2,h/2])

lista_x=[uniforme_x.get_random_element() for j in srange(N)]
lista_y=[uniforme_y.get_random_element() for j in srange(N)]

lista_xy = zip(lista_x , lista_y)

lineas=[]

d2=1*1*2

for j in lista_xy:
    x1 = lista_xy[j][0]
    y1 = lista_xy[j][1]
    while d2>1*1:
        x2 = uniforme_x.get_random_element()
        y2 = uniforme_x.get_random_element()
        d2=(x1-x2)*(x1-x2)+(y1-y2)*(y1-y2)
    lineas.append(line([lista_xy[j],(x2,y2)]))

grafico=sum(lineas)

grafico.show()

When i execute this code I get the error messege 'list indices must be integers, not tuple'.

Whats the problem?

Waiting for your answers.

Thanks a lot!