input: from typing import List
  
  def generate_number() -> int:
      """ Generate a unique 6 digit number.
      >>> generate_number()
      23456
      >>> generate_number()
      84213
      """
      import random
      return random.randint(100000, 999999)
output:     while True:
          number = generate_number()
          if number not in generated_numbers:
              generated_numbers.add(number)
              return number