input: 
  from typing import List
  import random
  
  def generate_number() -> int:
      """ Generate a unique 6 digit number.
      >>> len(str(generate_number())) == 6
      True
      >>> generate_number() != generate_number()
      True
      """
      return random.randint(100000, 999999)
  
output:     while True:
          num = random.randint(100000, 999999)
          if len(str(num)) == 6:
              return num