si = Application

def frange(x, y, jump):
    '''
    Like range() but with float values.
    '''
    while x < y:
        yield x
        x += jump

def evenPositions(crv, quantity):
    '''
    Get an amount of evenly spaced positions on a given curve.
    '''
    subcrv = crv.ActivePrimitive.Geometry.Curves(0)
    percs = [num for num in frange(0.0, 100.0, 100.0 / quantity)]
    percs.append(100)
    ppos = [ subcrv.EvaluatePosition( subcrv.GetUFromPercentage(percs[i]) )[0] for i in xrange(len(percs)) ]
    return ppos

def main():
    size = float( Application.XSIInputBox("What particle size do you plan on using?", "So I know the spacing...", 1) )

    newCurves = []
    si.OpenUndo("Make evenly spread linear curves")
    for crv in si.Selection:
        quantity = int(crv.ActivePrimitive.Geometry.Curves(0).Length / (size*2.0))
        positions = evenPositions(crv, quantity)

        positions = ["(%s,%s,%s)" % (a.X,a.Y,a.Z) for a in positions]
        newCurve = si.CreateCurve( 1, 0, ",".join(positions), False)('Value')

        newCurve.Name = crv.Name+'_evenlySpaced'
        newCurves.append(newCurve)
    si.SelectObj(newCurves)
    si.CloseUndo()

main()