Is It Possible to Select Faces (in Edit Mode) Based on the Normals of the Faces Connected to It?

Question

I understand how weirdly specific this is, but is there no way to select faces based on the normals they're facing AND the adjacent faces' normals?

What I want is a little complicated, but if I were to have a subdivided cube with perfectly flat faces, would I be able to select every face on the X positive normal but make an exception for any face that's connected to a different face with a Z positive normal?

I've tried putting specific selections into different vertex groups, but I'ven't been getting much luck using that work around. Could I not use python to make a custom search and dot in my own parameters?

[EDIT 1]

Alright, thinking a little better this time, I realised that what I was asking was odd because of the examples I used. So, I'm editing this to better clarify, using a model that correlates much better to the issue I'm having.

I suppose there'd be 2 options to what I'm asking: "select adjacent faces of normal based on the faces' normals" or "limit selection of faces based on the connected faces' normals." Though in all fairness, I'd only profit from the 2nd case. Giving examples now.

In the first case, "select adjacent faces of normal based on faces' normals," I've set up an example. In this case, I asked to select all the Z+ normals AND select adjacent faces that have the Y- normal. As you can see, only the faces that are making contact with the Z+ faces are selected.

In the second case, "limit selection of faces based on the connected faces' normals," I've another example. Again, I ask to select all Z+ faces, but this time, avoid any faces that are on the Y- normal.

Is there any way to do this? Not a big deal if there isn't, I just spent the better part of my day trying to do something like the earlier 2nd example with only commands and couldn't.

Answer 1

Select +Z faces and neighboring -Y faces if they go down:

import bpy, bmesh
from mathutils import Vector

up = Vector((0, 0, 1))
side = Vector((0, -1, 0))

def main():
    me = bpy.context.active_object.data
    bm = bmesh.from_edit_mesh(me)
    for face in bm.faces:
        if face.normal == up:
            face.select = True
            conditional_select_neighbors(face)
    bmesh.update_edit_mesh(me)


def conditional_select_neighbors(face):
    for loop in face.loops:  # each edge has a 'loop' associated with it for each face
        loop2 = loop.link_loop_radial_next  # radial_next gets the loop of the same edge but another face
        is_side = loop2.face.normal == side  # as R. Betts spotted, this is not enough
        loop3 = loop2.link_loop_next.link_loop_next
        is_lower = loop.vert.co.z > loop3.vert.co.z  
        if is_side and is_lower:
            loop2.face.select = True


if __name__ == "__main__":
    main()

Select +Z faces without -Y neighbors:

import bpy, bmesh
from mathutils import Vector

up = Vector((0, 0, 1))
side = Vector((0, -1, 0))

def main():
    me = bpy.context.active_object.data
    bm = bmesh.from_edit_mesh(me)
    for face in bm.faces:
        if face.normal == up:
            conditional_select(face)
    bmesh.update_edit_mesh(me)


def conditional_select(face):
    face.select = True
    for loop in face.loops:  # each edge has a 'loop' associated with it for each face
        loop2 = loop.link_loop_radial_next  # radial_next gets the loop of the same edge but another face
        if loop2.face.normal == side:  # in this case this is enough
            face.select = False


if __name__ == "__main__":
    main()