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
  assert generate_number() < 100000
  assert len(str(generate_number())) == 6
  assert generate_number() != generate_number()
output: import random
  
  def generate_number():
      random_number = random.randint(10000, 99999)
      return random_number