How do I choose a specific vertex to be extruded with Python?

Question

I am a newbie here. I tried to write python script by copying and pasting commands from the top scripting window in Blender. I met a problem when trying to extrude an object programmatically. Here is my sample code:

bpy.ops.mesh.primitive_cube_add(
      radius=2.5, # The initial radius is the radius of starting point
      view_align=False, 
      enter_editmode=True, 
      location=(1,2,3), 
      layer=mylayers)

bpy.ops.mesh.merge(type='CENTER')
bpy.ops.object.modifier_add(type='SKIN')
bpy.ops.object.modifier_add(type='SUBSURF')
bpy.context.object.modifiers["Subsurf"].levels = 4

bpy.ops.mesh.extrude_region_move(MESH_OT_extrude_region={"mirror":False},
                             TRANSFORM_OT_translate={"value":(0, 0, 0),
                             "constraint_axis":(False, False, False), 
                             "constraint_orientation":'GLOBAL', 
                             "mirror":False, 
                             "proportional":'DISABLED', 
                             "proportional_edit_falloff":'SMOOTH', 
                             "proportional_size":1, 
                             "snap":False, 
                             "snap_target":'CLOSEST', 
                             "snap_point":(0, 0, 0), 
                             "snap_align":False, 
                             "snap_normal":(0, 0, 0), 
                             "gpencil_strokes":False, 
                             "texture_space":False, 
                             "remove_on_cancel":False, 
                             "release_confirm":False})

I first merged the mesh of a cube into a single point in edit mode and added a skin modifier to the point, then tried to extrude this point to create an object similar to this:

However, I realized that there is no such keyword to select the vertex that I want to extrude out the bar from. So I was wondering if anyone knows how to select or activate a specific point (by vertex coordinate?) for extrusion?

Answer 1

A bmesh approach.

In as much as the operator calls are written in INFO area, and can be used to make scripts I always recommend using API class methods when possible.

Here I've created a new object at (1, 2, 3) using a bmesh and the bmesh extrude individual vert bmesh.ops.extrude_vert_indiv(...) which returns the newly created vert at the other end. For example sake I've added 3 random length branches in random directions.

Because of the random in all directions will get some rubbish result. For practical use some kind of tree data structure.

Added the modifiers using API method Object.modifiers.new(name, type)

a test run

import bpy
import bmesh
from mathutils import Vector
from random import randint, uniform
context = bpy.context
scene = context.scene
me = bpy.data.meshes.new("Thing")
ob = bpy.data.objects.new("Thing", me)

bm = bmesh.new()
root = bm.verts.new()
for i in range(3): # tree branches
    v = root
    for l in range(randint(1, 4)):
        ret = bmesh.ops.extrude_vert_indiv(bm, verts=[v])
        for v in ret['verts']:
            v.co += Vector([uniform(-1, 1) for axis in "xyz"])
        bm.to_mesh(me)

ob.location = (1, 2, 3)
skin = ob.modifiers.new(name="Skin", type='SKIN')
sub = ob.modifiers.new(name="Sub", type='SUBSURF')
sub.levels = 2
scene.objects.link(ob)

Used the extrude operator to match the question. Could simply add a vertex at next location and use that to create edge.

newvert = bm.verts.new((2, 2, 2))
newedge = bm.edges.new([root, newvert])