Activate addon not working

Question

I was experimenting with python scripting and came across registering addons. According to Blender Add-on Tutorial, the register and unregister functions are called when activating and deactivating the addon. I tried this but it seemed that the register function is ignored during activation!

To replicate, copy following script and save it into the addons folder.

bl_info = {
    "name": "Example",
    "category": "Node",
    "location": "Node Editor",
}
import bpy
from bpy.types import NodeTree, Node, NodeSocket
import nodeitems_utils
from nodeitems_utils import NodeItem, NodeCategory

class MyCustomTree(NodeTree):
    bl_idname = 'CustomTreeType'
    bl_label = "Custom Node Tree"
    bl_icon = 'NODETREE'


class MyCustomSocket(NodeSocket):
    bl_idname = 'CustomSocketType'
    bl_label = "Custom Node Socket"

    my_items = (
        ('DOWN', "Down", "Where your feet are"),
        ('UP', "Up", "Where your head should be"),
        ('LEFT', "Left", "Not right"),
        ('RIGHT', "Right", "Not left"),
    )

    my_enum_prop: bpy.props.EnumProperty(
        name="Direction",
        description="Just an example",
        items=my_items,
        default='UP',
    )

    def draw(self, context, layout, node, text):
        if self.is_output or self.is_linked:
            layout.label(text=text)
        else:
            layout.prop(self, "my_enum_prop", text=text)

    def draw_color(self, context, node):
        return (1.0, 0.4, 0.216, 0.5)

class MyCustomTreeNode:
    @classmethod
    def poll(cls, ntree):
        return ntree.bl_idname == 'CustomTreeType'

class MyCustomNode(MyCustomTreeNode, Node):
    bl_idname = 'CustomNodeType'
    bl_label = "Custom Node"

    my_string_prop: bpy.props.StringProperty()
    my_float_prop: bpy.props.FloatProperty(default=3.1415926)

    def init(self, context):
        self.inputs.new('CustomSocketType', "Hello")
        self.inputs.new('NodeSocketFloat', "World")
        self.inputs.new('NodeSocketVector', "!")

        self.outputs.new('NodeSocketColor', "How")
        self.outputs.new('NodeSocketColor', "are")
        self.outputs.new('NodeSocketFloat', "you")

    def draw_buttons(self, context, layout):
        layout.label(text="Node settings")
        layout.prop(self, "my_float_prop")

    def draw_buttons_ext(self, context, layout):
        layout.prop(self, "my_float_prop")
        layout.prop(self, "my_string_prop")

    def draw_label(self):
        return "Example"

class MyNodeCategory(NodeCategory):
    @classmethod
    def poll(cls, context):
        return context.space_data.tree_type == 'CustomTreeType'


node_categories = [
    MyNodeCategory('SOMENODES', "Some Nodes", items=[
        NodeItem("CustomNodeType"),
    ]),
    MyNodeCategory('OTHERNODES', "Other Nodes", items=[
        NodeItem("CustomNodeType", label="Node A", settings={
            "my_string_prop": repr("Lorem ipsum dolor sit amet"),
            "my_float_prop": repr(1.0),
        }),
        NodeItem("CustomNodeType", label="Node B", settings={
            "my_string_prop": repr("consectetur adipisicing elit"),
            "my_float_prop": repr(2.0),
        }),
    ]),
]

classes = (
    MyCustomTree,
    MyCustomSocket,
    MyCustomNode,
)


def register():
    from bpy.utils import register_class
    for cls in classes:
        register_class(cls)

    nodeitems_utils.register_node_categories('CUSTOM_NODES', node_categories)


def unregister():
    nodeitems_utils.unregister_node_categories('CUSTOM_NODES')

    from bpy.utils import unregister_class
    for cls in reversed(classes):
        unregister_class(cls)


if __name__ == "__main__":
    register()

To determine the success of the addon, after activation, there should be a "Custom Node Tree" editor.

Answer 1

It turns out the blender field should be included in the bl_info dictionary. Otherwise, Blender will just ignore the script. the bl_info should end-up looking like this:

bl_info: {
    "name": "Example",
    "category": "Node",
    "blender": (3,40,0)
}

def register()
    print("Welcome!")

def unregister()
    print("Goodbye!")

Answer 2

If you execute the code you provided from the text editor, you get the error:

Python: Traceback (most recent call last):
  File "\Text", line 121, in <module>
  File "\Text", line 109, in register
NameError: name 'nodeitems_utils' is not defined

Line 109:

nodeitems_utils.register_node_categories('CUSTOM_NODES', node_categories)

nodeitems_utils isn't present in your imports:

import bpy
from bpy.types import NodeTree, Node, NodeSocket
from nodeitems_utils import NodeItem, NodeCategory

You could import the node categories' register/unregisters there and remove nodeitems_utils. when you register/unregister them to make that error go away, and it seems to also unregister when disabling the addon.

import bpy
from bpy.types import NodeTree, Node, NodeSocket
from nodeitems_utils import NodeItem, NodeCategory, register_node_categories, unregister_node_categories

(...)

def register():
    register_node_categories('CUSTOM_NODES', node_categories)
    (...)

def unregister():
    unregister_node_categories('CUSTOM_NODES')
    (...)

But i am not familiar with custom nodes, so I don't know what your script does and how to check its success beyond that.