How to read the properties of an data object into a file.blend without loading it into the current project with Python?

Question

I would like to set some rules before loading a date object, into the current project. But I don't really know how to do it and if it is possible to do it.

So I give an example:

I would like to import (Then load) an object, only if it has a material in at least bpy.data.objects['object_name'].data.materials[:]

Unfortunately I realized that the bpy.data.libraries.load() method may not do what I have in mind.

Below, I report what I tried to do (which obviously does not work) but forste explains better what I would like to do:

with bpy.data.libraries.load(my_path, link=False) as (data_from, data_to):
    objs_to_load=[]
    for obj in data_from.objects:
        if obj.data.materials[:]: # <--Incorrect example on purpose
            objs_to_load.append(obj)
        
    data_to.objects = objs_to_load

Clearly in the example above there is an error, I cannot read obj.data.materials, as it results in a string. The problem is that I don't want to import the object, as I don't want to fill the memory with useless objects for my purpose.

Anyone have any idea how to do it if it is possible?

Answer 1

One obvious but inefficient way is to run a new Blender process in the background that opens the file and grabs whatever data you need. The data will need to be serialized by the background process, passed back to your script, then deserialized.

Afterwards you can use the data with libraries.load.

Here's a simple example:

import bpy
import subprocess

my_script = """
import bpy
print("$ BEGIN")
for ob in bpy.data.objects:
    if ob.material_slots:
        print("+", ob.name)
print("$ END")
"""

my_path = "Test.blend"

result = subprocess.run(
    [
        bpy.app.binary_path,
        "-b",
        my_path,
        "--python-exit-code", "1",
        "--python-expr",
        "exec(%s)" % repr(my_script),
    ],
    capture_output=True,
    check=True,
    text=True,
)

# Deserialize object names
lines = result.stdout.splitlines()
lines = lines[lines.index("$ BEGIN") + 1:lines.index("$ END")]
ob_names = [line.removeprefix("+ ") for line in lines]

print(ob_names)