I have this field in my model:

class Embed_Ingredients (models.Model): 
     _name = 'afribon_productapp.embed_ingredients' 
     _description = 'a description' 

    GLOBAL_IMPORT_OPTIONS = [('less', '<1 ton'), ('greater', '> 1 ton')]  

    procurement = fields.Selection (
        [('imported', 'Imported'), ('local', 'Local')],
        default = 'imported',
        required = True
    ) 

    freight_greater_or_less = fields.Selection ( 
        GLOBAL_IMPORT_OPTIONS, 
        string = "Weight", 
        default = 'less' 
    ) 



    @ api.onchange (' procurement ') 
     def procurement_change (self): 
     ' '' 
        When user changes procument to `local`, add a new value to the global variable GLOBAL_IMPORT_OPTIONS 
    '' ' 
     if self.procurement ==' local ':   
        self.GLOBAL_IMPORT_OPTIONS.append ((' full_load_container ',' Full Load Container '))         
        print (self.GLOBAL_IMPORT_OPTIONS)
                
        
           
        

When the 'procurement' selection changes to local, I would like to add a new value into the  GLOBAL_IMPORT_OPTIONS variable

When I print the value of GLOBAL_IMPORT_OPTIONS in the terminal, I can see that the value has been appended into the list, but on the form view, I cannot see the new value in the selection dropdown. 

Thanks!