input: Write a function to find Armstrong numbers between 0 and 1000.
  assert find_armstrong_numbers() == [0, 1, 153, 370, 371, 407]
output: def find_armstrong_numbers():
      armstrong_numbers = []
      for num in range(1001):
          digits = [int(d) for d in str(num)]
          if num == sum(d**len(digits) for d in digits):
              armstrong_numbers.append(num)
      return armstrong_numbers