Change field of a node in a group I am trying to make a script that will change the value of a specific node within a specific node group. I have seen from this question how to target a node within a material, but I can't see how to change a node within a group.

The target node group exists within several materials, but the materials vary in different scenes, so I don't want to have the script change a specific material unless it has to be that way.

(Note: I know a more common way to solve this would be to use a Driver and Custom Property to change the value of the node, and then have the script change the property. However, I'm running into some issues getting my custom properties to update their drivers reliably, so I'm exploring other options too.)
You can directly access node group data from 

node_group = bpy.data.node_groups['NodeGroup']


The node group name is the one chosen in the group node



Each node group then has it's own nodes -

node_group.nodes


You can find a node groups nodes from a material (don't confuse node name with node group name)

bpy.data.material['Material'].node_tree.nodes['Group'].node_tree.nodes




You can get the node group name from the materials group node in node_tree.name

for n in mat.node_tree.nodes:
    if n.type == 'GROUP':
        node_group_name = n.node_tree.name


See this answer for an example of creating a node group using python.

Once you have the group nodes you can access them the same as normal material nodes.

node_group.nodes['Diffuse BSDF'].inputs['Color'].default_value = (1.0,0.0,0.0,1.0)