Bezier curve is straight

Question

I would like to draw a Bezier curve, I use 4 control points, but my line is still straight.

I have a class which handle points. I use pyglet to draw the output.

def Bezier(P0,P1,P2,P3, t):
    t2 = t*t
    t3 = t2 * t
    mt = 1-t
    mt2 = mt * mt
    mt3 = mt2 * mt

    P0.mulP(mt)

    P1.mulP(3)
    P1.mulP(mt2)
    P1.mulP(t)

    P2.mulP(mt)
    P2.mulP(3)
    P2.mulP(t2)

    P3.mulP(t3)

    P0.addP(P1)
    P0.addP(P2)
    P0.addP(P3)

    return P0

Edit: I'm still playing around with this issue. I have a function to calculate the coordinates of the Bezier curve, it shows the same thing...

def Bezier3deg(P0,P1,P2,t):
    ReP = points.point()

    t1 = (1 - t) * (1 - t)
    P0.mulP(t1)

    t2 = 2 * (1-t) * t
    P1.mulP(t2)

    t3 = t*t
    P2.mulP(t3)

    ReP.addP(P0)
    ReP.addP(P1)
    ReP.addP(P2)
    return ReP

Answer 1

Okay, I figure it out, that the problem was my python skill. The Bezier function always changed the original coordinates of the control points. After I fixed it, the curves are fine.