Why is this code throwing a 'tuple index out of range' error?

Question

I got this error once yesterday but not since I finalized the function. Now I'm making more of the same buttons for similar but different functions, and now the first one is throwing this:

experiment.blend/bake_script", line 71, in execute
IndexError: tuple index out of range

There isn't any tuple in this code, as far as I can tell.

for mat in obj.data.materials:

                mat.use_nodes = True #Here it is assumed that the materials have been created with nodes, otherwise it would not be possible to assign a node for the Bake, so this step is a bit useless
                bake_one.is_active_output = True
                mat.node_tree.links.new(skin_comp.outputs[0], bake_one.inputs[0])
                linkToDelete = baked_BSDF.inputs[0].links[0]
                mat.node_tree.links.remove(linkToDelete)
                bump_main.mute = True
                texture_node = diffuse
                texture_node.select = True
                nodes.active = texture_node
                texture_node.image = imgbake #Assign the image to the node

The line that's throwing the error is linkToDelete = baked_BSDF.inputs[0].links[0]

There are two mysteries here:

1: Why is it throwing this error today, when it wasn't yesterday?

2: Why does it think it's getting a tuple in this?

Edit: It's this connection in particular, but I'm having the same issue with the new button I'm making too.

It occurred to me that maybe the nodes are listing connections as tuples, but then 0,0 should be within that list...

Answer 1

Many thanks to Gorgious for helping me formulate an elegant solution to this problem.

I'm still not entirely sure why the node connections are listed in a way that requires this, but the solution turns out to be fairly simple. Rather than trying to define all possible connections in a block for the entire script to use, you define them ad-hoc for each function.

Also, to avoid the problem of the script failing if the connection happens to not exist, you can use an if statement to either confirm or skip the step.

Here's the full code of one module:

class WM_OT_Bake_Diffuse(Operator):
    bl_label = "Bake Diffuse"
    bl_idname = "wm.bake_diffuse"
    bl_options = {'REGISTER', 'INTERNAL'}
    
    
    @classmethod
    def poll(cls, context):
        return True
    
    def execute(self, context):
            size = bpy.data.scenes["Scene"].bake_tool.bake_int, bpy.data.scenes["Scene"].bake_tool.bake_int
            obj = bpy.context.active_object
            diffuse = bpy.data.materials["Skin"].node_tree.nodes["DiffuseBake"]
            imgbake = bpy.data.images.new("DiffuseBake", width=size[0], height=size[1])
            BSDF = bpy.data.materials["Skin"].node_tree.nodes["Baked_BSDF"]
                    
                        
        # set bake type and parameters
            for mat in obj.data.materials:

                mat.use_nodes = True #Here it is assumed that the materials have been created with nodes, otherwise it would not be possible to assign a node for the Bake, so this step is a bit useless
                bake_one.is_active_output = True
                mat.node_tree.links.new(skin_comp.outputs[0], bake_one.inputs[0])
                if BSDF.inputs[0].links: # Checks if the connection in question exists, skips the step if it doesn't.
                    linkToDelete = BSDF.inputs[0].links[0]
                    mat.node_tree.links.remove(linkToDelete)
# the rest of this script sets up the bake, bakes the appropriate image, then reconnects everything.
                bump_main.mute = True
                texture_node = diffuse
                texture_node.select = True
                nodes.active = texture_node
                texture_node.image = imgbake #Assign the image to the node
                
            bpy.context.view_layer.objects.active = obj
            bpy.ops.object.bake(type='DIFFUSE', save_mode='EXTERNAL')
            imgbake.filepath_raw = "//bakes/DiffuseBake.png"
            imgbake.file_format = 'PNG'
            imgbake.save()

        # find node and assign the texture to it
            img = bpy.data.images.load("//bakes/DiffuseBake.png", check_existing=True)
            bpy.data.materials["Skin"].node_tree.nodes["DiffuseBake"].image = img
            bpy.data.images["DiffuseBake"].colorspace_settings.name = 'sRGB'
            mat.node_tree.links.new(diffuse.outputs[0], baked_BSDF.inputs[0])
            bump_main.mute = False
            return {'FINISHED'}