input: from typing import List
  
  def generate_unique_number() -> int:
      """ Generate a unique 6-digit number.
      >>> generate_unique_number()
      12345
      >>> generate_unique_number()
      67890
      """
      import random
      return random.randint(100000, 999999)
output:     unique_numbers = set()
      while True:
          num = random.randint(100000, 999999)
          if num not in unique_numbers:
              unique_numbers.add(num)
              return num