Self is not passed to the class method in Python

I am trying the following python code:

import sqlite3
class database:

    def __init__(self):
        self.conn = sqlite3.connect("warehousedb.db")
        self.cursor = self.conn.cursor()
        self.cursor.execute(
            "CREATE TABLE IF NOT EXISTS `admin` (admin_id INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL, username TEXT, password TEXT)")
        self.cursor.execute(
            "CREATE TABLE IF NOT EXISTS `product` (product_id INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL, product_name TEXT, product_qty TEXT, product_price TEXT)")
        self.cursor.execute("SELECT * FROM `admin` WHERE `username` = 'admin' AND `password` = 'admin'")

        if self.cursor.fetchone() is None:
            self.cursor.execute("INSERT INTO `admin` (username, password) VALUES('admin', 'admin')")
            self.conn.commit()


    def __del__(self):
        self.cursor.close()
        self.conn.close()

    def Execute_SQL(self, sql):
        self.cursor.execute(sql)
        self.conn.commit()

    def Get_SQL(self, sql):
        self.cursor.execute(sql)
        return self.cursor.fetchall()

    def Get_SQL_One_Rec(self, sql):
        self.cursor.execute(sql)
        return self.cursor.fetchone()

Then when trying this code:

db = database()    
st = "SELECT * FROM `admin` WHERE `username` = '" + USERNAME.get() + "' AND `password` = '" + PASSWORD.get() + "'"
rec = db.Get_SQL_One_Rec(st)

I am getting the following error:

rec = db.Get_SQL_One_Rec(st) TypeError: Get_SQL_One_Rec() missing 1 required positional argument: 'sql'

I can see from the Python Documentation that the Self object is automatically passed, so why I am getting this error?

from Self is not passed to the class method in Python