How can I open a 'save folder' dialog for this (3.6)?

Question

I'm new to writing addons for Blender and also to writing code in python, most of this is 'snippets' I found elsewhere, so I have little understanding of what this code is actually doing, beyond what it's supposed to be:

It opens a 'Save File' dialog (not useable in this case) which just results in 'access denied'
The purpose of the script is to find every object in every scene, and while preserving object hierarchies to export each as a separate FBX file.

The entire Traceback is this:

Python: Traceback (most recent call last):
  File "C:\Users\\\*\*\*\AppData\Roaming\Blender Foundation\Blender\3.6\scripts\addons\BatchFBXExportUnity.py", line 91, in execute
    batch_fbx_export_objects(self.directory_path + "/" if self.directory_path else "")
  File "C:\Users\\\*\*\*\AppData\Roaming\Blender Foundation\Blender\3.6\scripts\addons\BatchFBXExportUnity.py", line 48, in batch_fbx_export_objects
    bpy.ops.export_scene.fbx(
  File "C:\Program Files\WindowsApps\BlenderFoundation.Blender_3.6.1.0_x64__ppwjx1n5r4v9t\Blender\3.6\scripts\modules\bpy\ops.py", line 113, in \_\_call\_\_
    ret = _op_call(self.idname_py(), None, kw)
RuntimeError: Error: Python: Traceback (most recent call last):
  File "C:\Program Files\WindowsApps\BlenderFoundation.Blender_3.6.1.0_x64__ppwjx1n5r4v9t\Blender\3.6\scripts\addons\io_scene_fbx\__init__.py", line 661, in execute
    return export_fbx_bin.save(self, context, \*\*keywords)
  File "C:\Program Files\WindowsApps\BlenderFoundation.Blender_3.6.1.0_x64__ppwjx1n5r4v9t\Blender\3.6\scripts\addons\io_scene_fbx\export_fbx_bin.py", line 3514, in save
    ret = save_single(operator, context.scene, depsgraph, filepath, \*\*kwargs_mod)
  File "C:\Program Files\WindowsApps\BlenderFoundation.Blender_3.6.1.0_x64__ppwjx1n5r4v9t\Blender\3.6\scripts\addons\io_scene_fbx\export_fbx_bin.py", line 3409, in save_single
    encode_bin.write(filepath, root, FBX_VERSION)
  File "C:\Program Files\WindowsApps\BlenderFoundation.Blender_3.6.1.0_x64__ppwjx1n5r4v9t\Blender\3.6\scripts\addons\io_scene_fbx\encode_bin.py", line 314, in write
    with open(fn, 'wb') as f:
PermissionError: [Errno 13] Permission denied: '000000.fbx'
Location: C:\Program Files\WindowsApps\BlenderFoundation.Blender_3.6.1.0_x64__ppwjx1n5r4v9t\Blender\3.6\scripts\modules\bpy\ops.py:113
bpy.ops.object.batch_fbx_export(filepath="C:\\Users\\\*\*\*\\Documents\\Modules\\untitled.fbx")

And here is the code:

bl_info = {
    "name": "Batch FBX Exporter for Unity",
    "description": "Exports each object in the scene as an FBX file",
    "author": "Stellar Wolf Entertainment",
    "version": (0, 0, 1),
    "blender": (3, 6, 0),
    "location": "File > Export > Batch > Unity FBX",
    "warning": "This addon is still in development and may not behave as expected.",
    "category": "Import-Export",
}

import bpy
from bpy.props import StringProperty
from bpy_extras.io_utils import ExportHelper

def move_to_origin(obj):
    original_location = obj.location.copy()
    obj.location = (0, 0, 0)
    return original_location

def move_back_to_original_location(obj, original_location):
    # Move the object back to its original location
    obj.location = original_location

def get_object_hierarchy(obj):
    # Recursive function to get the entire hierarchy of an object
    hierarchy = [obj]

    if obj.children:
        for child in obj.children:
            hierarchy.extend(get_object_hierarchy(child))

    return hierarchy

def batch_fbx_export_objects(directory_path):
    for scene in bpy.data.scenes:
        bpy.context.window.scene = scene

        for obj in bpy.data.objects:
            if obj.type == "MESH":
                object_hierarchy = get_object_hierarchy(obj)

                for obj_in_hierarchy in object_hierarchy:
                    obj_in_hierarchy.select_set(True)
                original_locations = [move_to_origin(obj_in_hierarchy) for obj_in_hierarchy in object_hierarchy]

                file_path = bpy.path.abspath(directory_path + obj.name + ".fbx")
                bpy.ops.export_scene.fbx(
                    filepath=file_path,
                    use_selection=True,
                    axis_forward='-Z',
                    axis_up='Y',
                    global_scale=1.0,
                    apply_unit_scale=True,
                    apply_scale_options='FBX_SCALE_ALL',
                    bake_space_transform=True,
                    object_types={'MESH'},
                    use_mesh_modifiers=True,
                    mesh_smooth_type='FACE',
                    use_mesh_edges=False,
                    use_tspace=True,
                    use_custom_props=False,
                    use_armature_deform_only=False,
                    bake_anim=False,
                    bake_anim_use_all_actions=False,
                    bake_anim_use_nla_strips=False,
                    bake_anim_use_all_bones=False,
                    path_mode='AUTO'
                )

                for obj_in_hierarchy, original_location in zip(object_hierarchy, original_locations):
                    move_back_to_original_location(obj_in_hierarchy, original_location)

                for obj_in_hierarchy in object_hierarchy:
                    obj_in_hierarchy.select_set(False)

class BatchFBXExportOperator(bpy.types.Operator, ExportHelper):
    bl_idname = "object.batch_fbx_export"
    bl_label = "Batch FBX Export"
    bl_options = {'REGISTER', 'UNDO'}
    filename_ext = ".fbx"

    
    directory_path: StringProperty(
        name="Directory Path",
        description="Select the directory where the files will be saved",
        subtype='DIR_PATH'
    )

    def execute(self, context):
        batch_fbx_export_objects(self.directory_path + "/" if self.directory_path else "")
        return {'FINISHED'}

def menu_func(self, context):
    layout = self.layout
    layout.separator()
    layout.operator(BatchFBXExportOperator.bl_idname)

def register():
    bpy.utils.register_class(BatchFBXExportOperator)
    bpy.types.TOPBAR_MT_file_export.append(menu_func)

def unregister():
    bpy.utils.unregister_class(BatchFBXExportOperator)
    bpy.types.TOPBAR_MT_file_export.remove(menu_func)

if __name__ == "__main__":
    register()

Answer 1

I tried your script on my side, and it seems to work without error. Since the error is about a denied access, maybe check if you have the rights permissions on that folder, or try another one.

That being said, your script have other issues:

1. The FBX were exported in the parent directory of what I selected. This is partially due to the method you use to set the output path, which also leaded to bad user experience as the dialog window is for setting up a file output, not a folder output
2. If I understand, you want to export objects with their children in one file? In that case, you need to make a list of objects, and when you export a parent and its children: remove them from the list so that you don't export the children again.

Getting the output path

Since ExportHelper is made specifically to create a file output path, I don't see the point of using it here.

You could use fileselect_add() instead with a few tweaks to make if work for a folder.

I basically used the code from this answer

Object walking

When you cycle through object, Blender doesn't "see" them them way you see them in the outliner by default.

To "see" like Blender, you can switch your outliner to Blender-File Data display type:

Now you might understand the issue with your method:

It will walk through each object alphabetically with no notion of relationships. So, when you have an object at the top of the list that is the children of another object, it will be exported at least twice; firstly as the parent, then as the children of the other object.

I couldn't think of an ideal way of doing this without having issues with other scenarios, so I decided to just assume you will only have mesh objects at the top of your hierarchy and went with this approach:

get a list of all mesh objects
    for those who have no parents:
        get their children
        add the result in a list

        change position & export operations

This would not work with rigged models, for example, as the meshes are children of the armature. But since you disabled animation in your export option, I assumed this isn't an issue.

But otherwise, I suggest you consider just getting the selected objects, programmatically select the hierarchy (there's a nice operator just for it) and then export.

Final code

I made a point to add lots of print() to help you see what happens in the console, I also added lots of comments to explain the code a bit.

bl_info = {
    "name": "Batch FBX Exporter for Unity",
    "description": "Exports each object in the scene as an FBX file",
    "author": "Stellar Wolf Entertainment",
    "version": (0, 0, 1),
    "blender": (3, 6, 0),
    "location": "File > Export > Batch > Unity FBX",
    "warning": "This addon is still in development and may not behave as expected.",
    "category": "Import-Export",
}

import bpy
from bpy.props import StringProperty, BoolProperty
from bpy.types import Operator

def move_to_origin(obj):
    original_location = obj.location.copy()
    obj.location = (0, 0, 0)
    return original_location

def move_back_to_original_location(obj, original_location):
    # Move the object back to its original location
    obj.location = original_location

def batch_fbx_export_objects(directory_path):
    for scene in bpy.data.scenes:
        bpy.context.window.scene = scene

        # Get a list of all mesh objects in the scene
        mesh_objects = [obj for obj in bpy.data.objects if obj.type == 'MESH']
        print("Found mesh objects:")
        for i in mesh_objects:
            print(f"  - '{i.name}'")

        for obj in mesh_objects:
            bpy.ops.object.select_all(action='DESELECT')
            # filter to only top parents
            if obj.parent == None:
                top_parent_name = obj.name
                print("Found object_hierarchy to export:")
                print(f"  - '{top_parent_name}'")
                
                object_hierarchy = [obj]
                
                # check for children and add them in the object_hierarchy list
                children = [child for child in obj.children if child.type == 'MESH']
                for child in children:
                    print(f"    - '{child.name}'")
                    object_hierarchy.append(child)

                # select each of object_hierarchy
                for obj_in_hierarchy in object_hierarchy:
                    obj_in_hierarchy.select_set(True)
                    
                original_locations = [move_to_origin(obj_in_hierarchy) for obj_in_hierarchy in object_hierarchy]


                file_path = bpy.path.abspath(f"{directory_path}{top_parent_name}.fbx")
                bpy.ops.export_scene.fbx(
                    filepath=file_path,
                    use_selection=True,
                    axis_forward='-Z',
                    axis_up='Y',
                    global_scale=1.0,
                    apply_unit_scale=True,
                    apply_scale_options='FBX_SCALE_ALL',
                    bake_space_transform=True,
                    object_types={'MESH'},
                    use_mesh_modifiers=True,
                    mesh_smooth_type='FACE',
                    use_mesh_edges=False,
                    use_tspace=True,
                    use_custom_props=False,
                    use_armature_deform_only=False,
                    bake_anim=False,
                    bake_anim_use_all_actions=False,
                    bake_anim_use_nla_strips=False,
                    bake_anim_use_all_bones=False,
                    path_mode='AUTO'
                )

                for obj_in_hierarchy, original_location in zip(object_hierarchy, original_locations):
                    move_back_to_original_location(obj_in_hierarchy, original_location)

                for obj_in_hierarchy in object_hierarchy:
                    obj_in_hierarchy.select_set(False)
            else:
                parent = obj.parent
                print(f"X Skipping '{obj.name}' as it has a parent: '{parent.name}'")

class BatchFBXExportOperator(bpy.types.Operator):
    # EDIT: this adds a tooltip to the operarot, visible when hovering it in the export menu.
    """Export the scene's meshes to FBX for Unity"""
    bl_idname = "object.batch_fbx_export"
    bl_label = "Batch FBX Export"
    # EDIT: you can t undo the export, so no need to flag this operator as undoable
    bl_options = {'REGISTER'}
    # EDIT: no need for extension if not using ExportHelper

    
    directory: StringProperty(
        name="Output Path",
        description="Where to export the FBX files"
        # subtype='DIR_PATH' is not needed to specify the selection mode.
        # But this will be anyway a directory path.
        )

    # Filters folders
    filter_folder: BoolProperty(
        default=True,
        options={"HIDDEN"}
        )

    def execute(self, context):
        print(f"Selected path '{self.directory}'")
        if self.directory:
            batch_fbx_export_objects(self.directory)
            # EDIT: letting the user know things are done is always good.
            self.report({'INFO'}, 'Export finished!')
        else:
            self.report({'WARNING'}, 'No valid path provided')
        return {'FINISHED'}

    def invoke(self, context, event):
        # Open browser, take reference to 'self' read the path to selected
        # file, put path in predetermined self fields.
        # See: https://docs.blender.org/api/current/bpy.types.WindowManager.html#bpy.types.WindowManager.fileselect_add
        context.window_manager.fileselect_add(self)
        # Tells Blender to hang on for the slow user input
        return {'RUNNING_MODAL'}

def menu_func(self, context):
    layout = self.layout
    layout.separator()
    layout.operator(BatchFBXExportOperator.bl_idname)

def register():
    bpy.utils.register_class(BatchFBXExportOperator)
    bpy.types.TOPBAR_MT_file_export.append(menu_func)

def unregister():
    bpy.utils.unregister_class(BatchFBXExportOperator)
    bpy.types.TOPBAR_MT_file_export.remove(menu_func)


#### THIS IS FOR TESTING FROM THE EDITOR ONLY
#### REMOVE FROM FINAL ADDON
# force invoke register if started from editor
if __name__ == "__main__":
    register()

    # run the operator from the invoke function
    bpy.ops.object.batch_fbx_export("INVOKE_DEFAULT")