How to fix a lost connection error to MySQL?

Question

MySQL

I have a bot connected to mysql database. When you enter the /start command, it gives an error

mysql.connector.errors.OperationalError: 2055 Lost connection to MySQL server at '' , system error: 9 Bad file descriptor

When you enter the start command, a check is made for the presence of the user in the database.

bot.py

if(not db.check_user(message.from_user.id)):
  db.add_user(message.from_user.id, message.from_user.username)
  bot.send_message(message.chat.id, "<b>Добро пожаловать, </b>"+usname, reply_markup=keyboard, parse_mode="HTML", disable_web_page_preview = True)
else:
  bot.send_message(message.chat.id, "<b>Добро пож&nbsp;алов&quot;ать, </b>"+usname, reply_markup=keyboard, parse_mode="HTML", disable_web_page_preview = True)

sqlighter.py

def add_user(self, user_id, username):
  sql = "INSERT INTO `usersdb` (`user_id`, `username`) VALUES (%s,%s)"
  val = user_id, username
  self.user_cursor.execute(sql, val)
  self.user_mydb.commit()
  self.user_mydb.close()

def check_user(self, user_id):
  sql = "SELECT * FROM usersdb WHERE user_id = {0}".format(user_id)
  self.user_cursor.execute(sql)
  data = self.user_cursor.fetchone()
  return data

Answer 1

self.user_mydb.close()
What do you think this call does?