Extrude circle along a path - output stl file

# install euclid and numpy packages
from euclid import Vector3, Matrix4
import numpy as np
import math

execfile("stl-writer.py")

# number of points on the circle
PRECISE=100
# number of circles to be placed across the path
NUM_CIRCLES = 500

# connect corresponding points between two instances of same polygon
def get_mesh(poly1, poly2, precision):
  faces = []
  for i in range(precision):
    p1 = i
    p2 = (i + 1) % precision
    face = [poly1[p1], poly1[p2], poly2[p2], poly2[p1]]
    faces.append(face)
    # print str(i) + " " + str(face)
  # print faces
  return faces


# draw a catmull spline
def spline_4p(t, p_1, p0, p1, p2 ):
    """ Catmull-Rom
        (Ps can be numpy vectors or arrays too: colors, curves ...)
    """
        # wikipedia Catmull-Rom -> Cubic_Hermite_spline
        # 0 -> p0,  1 -> p1,  1/2 -> (- p_1 + 9 p0 + 9 p1 - p2) / 16
    # assert 0 <= t <= 1
    return (
          t*((2-t)*t - 1)     * p_1
        + (t*t*(3*t - 5) + 2) * p0
        + t*((4 - 3*t)*t + 1) * p1
        + (t-1)*t*t           * p2 ) / 2


# control points, note first and last points are repeated
P = np.array([[0, 0, 0], [0, 0, 0], [1, 1, 2], [0.5, 0, 3], [-1, 2, 4], [-0.5, 0, 6], [-0.5, 0, 6]])

# calculate the path with NUM_CIRCLE points along it
path = np.array(P[0])
k=NUM_CIRCLES/(len(P) - 3)
print k
for j in range( 1, len(P)-2 ):  # skip the ends
    for t in range(1,k):  # t: 0 .1 .2 .. .9
        p = spline_4p( 1.0*t/k, P[j-1], P[j], P[j+1], P[j+2] )
        path = np.vstack((path, p))

#path = np.vstack((path, P[2:4]))

cylinder = []

for i in range(3,NUM_CIRCLES-3):
  shape1 = [Vector3(math.cos(x) + path[i-1][0], math.sin(x) + path[i-1][1], path[i-1][2]) for x in np.linspace(0,2*math.pi,PRECISE,endpoint=False)]
  shape2 = [Vector3(math.cos(x) + path[i][0], math.sin(x) + path[i][1], path[i][2]) for x in np.linspace(0,2*math.pi,PRECISE,endpoint=False)]
  faces = get_mesh(shape1, shape2, PRECISE)
  cylinder.extend(faces)

with open('cylinder.stl', 'wb') as fp:
  writer = ASCII_STL_Writer(fp)
  faces = []
  for face in cylinder:
    faces.append([face[0].xyz, face[1].xyz, face[2].xyz, face[3].xyz])
  #print faces
  writer.add_faces(faces)
  writer.close()