input: Write a function to generate a unique 6 digit number.
  
  def generate_number():
      random_number = random.randint(10000, 99999)
      return random_number
  
  assert generate_number() > 9999 and generate_number() < 100000
  assert generate_number() != generate_number()
  assert generate_number() != generate_number()
output: 
  import random
  
  def generate_number():
      random_number = random.randint(100000, 999999)
      return random_number
  