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() != generate_number()  # Ensure two calls produce different numbers
  assert len(str(generate_number())) == 6  # Ensure the length is exactly 6 digits
output: import random
  
  def generate_number():
      return random.randint(100000, 999999)