title: Python: How to do math using f-string
body: <p>I'm attempting to write my own implementation of 99 bottles of beer on the wall using python 3.6's new f-string feature, but I'm stuck: </p>
  
  <pre><code>def ninety_nine_bottles():
      for i in range(10, 0, -1):
          return (f'{i} bottles of beer on the wall, {i} of beer! You take one down, pass it around, {} bottles of beer on the wall')
  </code></pre>
  
  <p>How do I decrement 'i' in the last pair of brackets? I've tried i-=1 to no avail (syntax error)...</p>