How to duplicate vertices and modify their uvs with script?

Question

I'm trying to write something similar to the array modifier and am trying to modify the uvs of duplicated vertices, just like the modifier does, and add 1 on the X axis for every iteration. This code fails:

for i in range(8):
    bpy.ops.mesh.duplicate()
    bpy.ops.transform.shrink_fatten(value=-0.1, use_even_offset=False)

    for loop in ac_ob.data.loops:
        if actv_ob.data.vertices[loop.vertex_index].select is True:
            actv_ob.data.uv_layers.active.data[loop.index].uv += Vector([1, 0])

with the error:

ac_ob.data.uv_layers.active.data[loop.index].uv += Vector([1, 0])
IndexError: bpy_prop_collection[index]: index 0 out of range, size 0

location: <unknown location>:-1

The object does have a uv map. What am I doing wrong here?

Answer 1

Consider this shader, it uses the typical Array modifier $u$ coordinate offset of $1$ to make every 2nd element metallic:

Here's a Python script that efficiently duplicates the objects and for each removes the UV map, creates a new one, copies the data from the previous one, but increments the UV coordinates as specified in first two lines:

u_increment = 1.0
v_increment = 0.0

import bpy, numpy as np
from bpy import context as C

orig = C.object
offset = orig.dimensions[0]
uv_orig = orig.data.uv_layers['UVMap'].uv
size = len(uv_orig)
arr = np.empty(size*2, dtype=float)
uv_orig.foreach_get('vector', arr)

for i in range(4):
    ob = orig.copy()  # linked duplicate
    ob.data = ob.data.copy()  # need a copy to change UV
    ob.location.x += offset * (i+1)
    C.collection.objects.link(ob)
    uvs = ob.data.uv_layers
    uvs.remove(uvs[0])  # I don't understand why a new UV map has to be created
    uv_new = uvs.new()  # it seems like a mesh object reuses the copied UV otherwise
    arr.reshape((size,2))[:,:] += (u_increment, v_increment)
    uv_new.uv.foreach_set('vector', arr)

The front line is the Array modifier, and the next line is the effect of duplicating one Suzanne using the above script: