import bpy

from bpy.types import Panel, Operator, PropertyGroup
from bpy.props import EnumProperty, PointerProperty, StringProperty


class MyProperties(PropertyGroup):

    my_enum : EnumProperty(
        name= "Enumerator / Dropdown",
        description= "sample text",
        items= [('OP1', "Append", ""),
                ('OP2', "Remove", "")
        ]
    )
    new_item : StringProperty()
    my_list = []


class ADDONNAME_PT_main_panel(Panel):
    bl_label = "Main Panel"
    bl_idname = "ADDONNAME_PT_main_panel"
    bl_space_type = 'VIEW_3D'
    bl_region_type = 'UI'
    bl_category = "New Tab"

    def draw(self, context):
        layout = self.layout
        scene = context.scene
        mytool = scene.my_tool

        layout.prop(mytool, "my_enum", expand = True)
        layout.prop(mytool, "new_item")
        layout.operator("addonname.myop_operator")


class ADDONNAME_OT_my_op(Operator):
    bl_label = "Submit"
    bl_idname = "addonname.myop_operator"

    def execute(self, context):
        scene = context.scene
        mytool = scene.my_tool
        list = mytool.my_list
        enum = mytool.my_enum
        new_item = mytool.new_item

        a = "alpha"
        b = "beta"

        if enum == 'OP1':
            if a and b not in list:
                list.extend((a, b))
            if new_item != "":
                if new_item not in list:
                    list.append(new_item)
        else:
            list.clear()

        print(list)

        return {'FINISHED'}




classes = [MyProperties, ADDONNAME_PT_main_panel, ADDONNAME_OT_my_op]

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

    bpy.types.Scene.my_tool = PointerProperty(type= MyProperties)

def unregister():
    for cls in classes:
        bpy.utils.unregister_class(cls)
    del bpy.types.Scene.my_tool

if __name__ == "__main__":
    register()