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