How can I read the properties of a node group in a blend file?

Question

Is it possible with Python to detect if a nodegroup is embedded in a frame in another blend file without adding it first?

I have several blend files with the following structure:

The main group always contains a certain name (in this example "_TEST"), and in it are other nodegroups. Some of them are embedded in frames, some are not.

And from another file, before I add/process a certain nodegroup from these files, I want to check if it is embedded in a frame or not.

Basically reading the file works fine, and I can add the nodes, but in this step I always get only the name of the group, but not its properties, like parent:

with bpy.data.libraries.load(filepath) as (data_from, data_to):
    for group_name in data_from.node_groups:
        print(group_name)

If I have a node group in the same blend file, it works fine with the following code:

maingroup = bpy.data.node_groups['_TEST'].nodes
for n in maingroup:
    if n.parent == maingroup['Frame']:
        print("Node '" + n.node_tree.name + "' is framed")

But when I try to read the information of a node group that is in another file, unfortunately it doesn't work.

Any suggestions for a solution?

Answer 1

Here's another approach using subprocess to spawn a separate process so you don't have to directly load and store the data into the current blend file's bpy.data, but unfortunately it is a bit slow. I'm not sure how frequently you will need to invoke this. If the call is just made once to make all the checks, then I guess it might suffice for your addon. But nevertheless it might still be interesting.

import subprocess

return_keyword = "RETURN_VALUE:"

def check_blend_file(file_path, group_name):

    script = """
import bpy
 
def load_test():
    file_path = "{}"
    group_name = "{}"
    with bpy.data.libraries.load(file_path) as (data_from, data_to):
        data_to.node_groups.append(group_name)
    maingroup = bpy.data.node_groups[group_name].nodes             
    for n in maingroup:
        if n.parent == maingroup['Frame']:
            return "Node '" + n.node_tree.name + "' is framed"
    return group_name + " not found"

result = load_test()
print("{}", result)
    """.format(file_path, group_name, return_keyword)

    command = ["blender", "--background", "--python-expr", script]

    result = subprocess.check_output(command, stderr=subprocess.STDOUT, text=True)
    return_value = "No result found"

    for line in result.splitlines():
        if return_keyword in line:
           return_value = line.replace(return_keyword, "")

    return return_value

return_value = check_blend_file("C:/Users/harry/Downloads/test.blend", "_TEST")

print("Return value:", return_value)

Answer 2

Thanks to @MarkusVonBroady for his contribution to the previous script version, here is the final script that should work. Since the libraries.load only loads object names, by adding the names we want to load to data_to, i.e. data_to.node_groups.append('_TEST'), only then will the data we specify get loaded into bpy.data after the with block exists, that is when the Context Manager's __exit__ method gets called. This way you can control what data to load and you can do your checks.

import bpy
 
def load_test():
    file_path = "C:/Users/harry/Downloads/test.blend"
    with bpy.data.libraries.load(file_path) as (data_from, data_to):
        data_to.node_groups.append('_TEST')
    maingroup = bpy.data.node_groups['_TEST'].nodes             
    for n in maingroup:
        if n.parent == maingroup['Frame']:
            print("Node '" + n.node_tree.name + "' is framed")
    return True
 
success = load_test() # succeeds and test for frame works