mathutils.md

The mathutils.geometry submodule has a collection of methods that perform common geometric tasks. Listed here

These functions tend to be self explanatory, and if you're stuck you can google them and i'm sure you'll get several code snippets to digest. Some functions accept a Matrix so you don't have to do extra multiplication and inverting, others are less sophisticated and let you take care of having the right transforms :)

My personal favourites are

• interpolate_bezier
• intersect_line_line
• intersect_line_plane
• intersect_ray_tri

Some food for thought.

Example 1. intersect_line_plane

This function expects (vector_a, vector_b, plane_coordinate, plane_normal, flip)

import bpy
import bmesh
import mathutils

def extend_vertex(limit_axis='x', coordinate=4.2, system='local'):

    obj = bpy.context.edit_object
    me = obj.data
    bm = bmesh.from_edit_mesh(me)
    verts = bm.verts
    try:
        v1, v2 = [v for v in bm.verts if v.select]
    except:
        print('need two vertices selected, or one edge')
        bm.free()
        return

    plane_idx = {'x': 0, 'y': 1, 'z': 2}.get(limit_axis)
    plane_co, plane_no = [0,0,0], [0,0,0]
    plane_no[plane_idx] = 1
    plane_co[plane_idx] = coordinate

    intersect_l_p = mathutils.geometry.intersect_line_plane
    
    if system == 'local':
        new_co = intersect_l_p(v1.co, v2.co, plane_co, plane_no, False)
        A_len = (v1.co-new_co).length
        B_len = (v2.co-new_co).length
    else:
        mw = obj.matrix_world
        new_co = intersect_l_p(mw * v1.co, mw * v2.co, plane_co, plane_no, False)
        A_len = ((mw*v1.co)-new_co).length
        B_len = ((mw*v2.co)-new_co).length
        new_co = mw.inverted() * new_co
        
    if A_len < B_len:
        v1.co = new_co
    else:
        v2.co = new_co

    bmesh.update_edit_mesh(me, True)


extend_vertex(limit_axis='x', coordinate=-2, system='global')