title: sum of a number's digits using a built-in function
body: <p>I have written a Python function that takes an integer as input and returns the sum of all that number's digits:</p>
  
  <pre><code>def digit_sum(n):
      n_lst = list( str(n) )
      n_sum = 0
      for i in range( len(n_lst) ):
          n_sum += int( n_lst[i] )
      return n_sum
  
  print( digit_sum(1234) )            # 10
  </code></pre>
  
  <p>Now I wonder if there is a more concise way of doing this perhaps, using a built-in function or list comprehensions?</p>