Create vertex groups for each mesh before joining objects in Blender

SmartJoin.py

bl_info = {
    "name": "Smart Join",
    "author": "Jeroen Trines",
    "version": (1, 0),
    "blender": (3, 1, 0),
    "location": "View3D > Object > Smart Join",
    "description": "Create vertex groups for each mesh before joining objects in Blender",
    "warning": "",
    "doc_url": "",
    "category": "Object",
}

import bpy

def isMesh(data):
    return hasattr(data, 'type') and data.type == 'MESH'

def getSelectedMeshes(context):
    return list(filter(isMesh, context.selected_objects))

def prependObjectNameToVertexGroups(mesh):
    for vg in mesh.vertex_groups:
        if vg.name.startswith(mesh.name.lower()):
            continue
        vg.name = f"{mesh.name.lower()}_{vg.name}"


def lockAllVertexGroups(mesh):
    for vg in mesh.vertex_groups:
        vg.lock_weight = True
        

def addObjectNameAsVertexGroup(mesh):
    # https://docs.blender.org/api/current/bpy.path.html?highlight=case#bpy.path.clean_name
    cleaned = bpy.path.clean_name(name=mesh.name)
    vg = mesh.vertex_groups.new(name=cleaned)
    vg.add(range(len(mesh.data.vertices)), 1.0, 'REPLACE')


def main(context):
    for m in getSelectedMeshes(context):
        prependObjectNameToVertexGroups(m)
        addObjectNameAsVertexGroup(m)
        lockAllVertexGroups(m)
    bpy.ops.object.join()


class SmartJoinOperator(bpy.types.Operator):
    """Tooltip"""
    bl_idname = "object.smart_join"
    bl_label = "Smart Join"

    @classmethod
    def poll(cls, context):
        return len([o for o in context.selected_objects if o.type == 'MESH']) > 1

    def execute(self, context):
        main(context)
        return {'FINISHED'}

def menu_func(self, context):
    self.layout.operator(SmartJoinOperator.bl_idname, text=SmartJoinOperator.bl_label)

# Register and add to the "object" menu (required to also use F3 search "Simple Object Operator" for quick access)
def register():
    bpy.utils.register_class(SmartJoinOperator)
    bpy.types.VIEW3D_MT_object.append(menu_func)


def unregister():
    bpy.utils.unregister_class(SmartJoinOperator)
    bpy.types.VIEW3D_MT_object.remove(menu_func)


if __name__ == "__main__":
    register()