Connect 2 nodes for all materials via python script

Question

I want to connect 2 nodes for all materials at once. Specifically the vertex color node with the mix vertex color node

I want to turn this:

Into this:

For every material.

Any help would be appreciated, not very familliar with python at all.

Answer 1

John's answer seems to be absolutely correct for how you presented your question. However, if you'd rephrase your question as:

How do I change each Vertex Color output going to a node labelled "Mix Vertex Color" to use Alpha output instead of Color?

Then the best answer I can come up with would be:

import bpy

for mat in bpy.data.materials:
    nodes = mat.node_tree.nodes if mat.node_tree else []
    vc_nodes = (n for n in nodes if n.type == 'VERTEX_COLOR')
    for vc, link in ((n, l) for n in vc_nodes for l in n.outputs['Color'].links):
        if link.to_node.label == 'Mix Vertex Color':
            mat.node_tree.links.new(vc.outputs[1], link.to_socket)
            print(f"Changed material {mat.name}")

This turns this:

into this:

That is, it iterates over all materials, and then makes sure to change each link from a node of type VERTEX_COLOR to a node with a label Mix Vertex Color, to go from the Alpha socket.

Alternative code

import bpy

check = lambda x: x.from_node.type=='VERTEX_COLOR' and x.to_node.label=='Mix Vertex Color'
trees = (m.node_tree for m in bpy.data.materials if m.node_tree)

for tree, l in [(t, l) for t in trees for l in t.links if check(l)]:
    tree.links.new(l.from_node.outputs[1], l.to_socket)

Answer 2

you can use this script:

import bpy


for eachMat in bpy.data.materials:
    
    if eachMat.node_tree is not None:
        if eachMat.node_tree.nodes.find("Vertex Color"):
            vertexColor = eachMat.node_tree.nodes["Vertex Color"]
             if eachMat.node_tree.nodes.find("Mix"):
                mix = eachMat.node_tree.nodes["Mix"]
                eachMat.node_tree.links.new(vertexColor.outputs[1], mix.inputs[2])
                print("changed:", eachMat.name)

Prerequisite:

Your nodes names must be "Mix" and "Vertex Color".

All changed materials will be printed out.