import bpy
from dxfwrite import DXFEngine as dxf

filename = bpy.path.abspath("//exported.dxf")

reverseOrder = False
scale = 10
spacing1 = 200
spacing2 = 100
numSpacing1Slices = 3
startY = 0
addLabels = True
labelsX = 0
labelsY = 0
fontHeight = 8

# It's important to get the formers in the right order, so first
# make a list of the objects we want and mark them with their y
# coordinate to use as a sort key

sliceObjects = []

for obj in bpy.context.scene.objects:
    if obj.select_get():
        if hasattr( obj.data, 'splines' ):
            spline = obj.data.splines[0]
            if spline.type == 'POLY' :
                obj["sliceY"] = spline.points[0].co.y
                sliceObjects.append(obj)

# Sort by ascending y value, ie. starting from the center in most cases
# Change reverse to False if you want the parts stacked in the opposite order
sliceObjects.sort(key=lambda x: x['sliceY'], reverse=reverseOrder)

drawing = dxf.drawing(filename)
drawing.add_layer('0',color=0)

sliceNumber = 1
yOffset = startY
spacing = spacing1

for obj in sliceObjects:
    print(obj["sliceY"])
    for spline in obj.data.splines:
        numpoints = len(spline.points)
        for i in range(numpoints):
            ii = (i + 1) % numpoints
            p1 = spline.points[i]
            p2 = spline.points[ii]
            drawing.add(dxf.line((p1.co.x*scale, p1.co.z*scale+yOffset), (p2.co.x*scale, p2.co.z*scale+yOffset), layer='0'))

    if addLabels:
        text = dxf.text(str(sliceNumber), (labelsX, labelsY+yOffset), height=fontHeight, rotation=0)
        text['layer'] = '0'
        drawing.add(text)

    yOffset = yOffset + spacing
    sliceNumber = sliceNumber + 1
    if sliceNumber >= numSpacing1Slices:
        spacing = spacing2

drawing.save()