I'm totally a newbie here who love Odoo so much :)

I got stuck with this problem for a few days in Odoo CE 13.

I want my SHIPPING TYPE field which I made in sale.order can be used in purchase.order as well.

So I did this in shipping_type.py

# -*- coding: utf-8 -*-
    from odoo import models, fields
    
    
    
    class SaleOrder(models.Model):
        _inherit = 'sale.order'
    
        shipping_selection = [
                    ('type1', 'Instant'),
                    ('type2', 'Same Day'),
                    ('type3', 'JNE'),
                    ('type4', 'Tokopedia'),
                    ('type5', 'Pick Up'),
                    ('type6', 'AnterAja-Reguler'),
                    ('type7', 'J&T-Reguler'),
                    ('type8', 'Ninja-Reguler'),
    
                     ]
    
        shipping_type = fields.Selection(shipping_selection,'Shipping Type',)

Then, I was trying to make purchase_shipping_type.py in another addons, and created this:
# -*- coding: utf-8 -*-
from odoo import models, fields


class SaleOrder(models.Model):
    _inherit = ['sale.order']
    shipping_selection = [
                ('type1', 'Instant'),
                ('type2', 'Same Day'),
                ('type3', 'JNE'),
                ('type4', 'Tokopedia'),
                ('type5', 'Pick Up'),
                ('type6', 'AnterAja-Reguler'),
                ('type7', 'J&T-Reguler'),
                ('type8', 'Ninja-Reguler'),
                 ]
    shipping_type = fields.Selection(shipping_selection,'Shipping Type',)

class PurchaseOrder(models.Model):
    _inherit = ['purchase.order']
   
shipping_type_purchase = fields.Selection(string='ShippingType',related=shipping_type.shipping_type_purhase, readonly=True)
This error showed up:
Aug 21 06:29:03 kama-odoo-server odoo13[24202]: NameError: name 'shipping_type' is not defined - - -
How to set up this properly? I really hope someone can help me :) Thanks!