title: ValueError: zero length field name in format in Python2.6.6
body: <p>I use this python shell to generate a string:</p>
  
  <pre><code>&gt;&gt;&gt;':'.join("{:x}\n".format(random.randint(0, 2**16 - 1)) for i in range(4))
  </code></pre>
  
  <p>When I run this shell in Python2.7.5, everything goes okay. But it occurs <code>ValueError: zero length field name in format</code> when Python version is <code>2.6.6</code>. What should I run this shell fine when Python version is <code>2.6.6</code>?</p>