Moebiu ring in STL file

import math

out = "moebius.stl"

f = open(out, 'w')

n = 20
m = 40

r1 = 5.0
r2 = 1.0
r3 = 2.0

dtheta = 2.0 * math.pi / m
dphi = 2.0 * math.pi / n

f.write("solid Something (Meshed)\n")
for i in range(0, m):
    theta = 2.0 * math.pi * i * 1.0 / m
    ct = math.cos(theta)
    st = math.sin(theta)
    ct1 = math.cos(theta + dtheta)
    st1 = math.sin(theta + dtheta)
    ct3 = math.cos(3*theta)
    st3 = math.sin(3*theta)
    ct31 = math.cos(3*theta + dtheta)
    st31 = math.sin(3*theta + dtheta)
    for j in range(0, n):
        phi = math.pi * j * 2.0 / n
        cp = math.cos(phi)
        sp = math.sin(phi)
        cp1 = math.cos(phi + dphi)
        sp1 = math.sin(phi + dphi)
        f.write("\tfacet normal %.6f %.6f %.6f\n" % (ct * cp, st * cp, sp))
        f.write("\t\touter loop\n")
        f.write("\t\t\tvertex %.6f %.6f %.6f\n" % ((r1 + r2*cp + r3*ct3) * ct, (r1 + r2*cp + r3*ct3) * st, r2*sp + r3*st3))
        f.write("\t\t\tvertex %.6f %.6f %.6f\n" % ((r1 + r2*cp1 + r3*ct3) * ct, (r1 + r2*cp1 + r3*ct3) * st, r2*sp1 + r3*st3))
        f.write("\t\t\tvertex %.6f %.6f %.6f\n" % ((r1 + r2*cp + r3*ct31) * ct1, (r1 + r2*cp + r3*ct31) * st1, r2*sp + r3*st31))
        f.write("\t\tendloop\n")
        f.write("\tendfacet\n")
        f.write("\tfacet normal %.6f %.6f %.6f\n" % (ct * cp, st * cp, sp))
        f.write("\t\touter loop\n")
        f.write("\t\t\tvertex %.6f %.6f %.6f\n" % ((r1 + r2*cp1 + r3*ct31) * ct1, (r1 + r2*cp1 + r3*ct31) * st1, r2*sp1 + r3*st31))
        f.write("\t\t\tvertex %.6f %.6f %.6f\n" % ((r1 + r2*cp1 + r3*ct3) * ct, (r1 + r2*cp1 + r3*ct3) * st, r2*sp1 + r3*st3))
        f.write("\t\t\tvertex %.6f %.6f %.6f\n" % ((r1 + r2*cp + r3*ct31) * ct1, (r1 + r2*cp + r3*ct31) * st1, r2*sp + r3*st31))
        f.write("\t\tendloop\n")
        f.write("\tendfacet\n")

f.write("endsolid Mesh\n")

f.close()