title: python logging string formatting
body: <p>I am using python's log <a href="http://docs.python.org/library/logging.html#formatter-objects" rel="noreferrer">formatter</a> to format <a href="http://docs.python.org/library/logging.html#logrecord-attributes" rel="noreferrer">log records</a> and i have a fmt value of</p>
  
  <pre><code>fmt = "[%(filename)s:%(lineno)s] %(message)s"
  </code></pre>
  
  <p>What i would like is that "[file.py:20]" to be stretched to 10 characters wide (for example). If it was one value that would have been easy but is there any way to stretch this entire structure to a specified length?
  I want something like:</p>
  
  <pre><code>tmp = "[%(filename)s:%(lineno)s]"
  fmt = "%(tmp)10s %(message)s"
  </code></pre>
  
  <p>I would like to know if this is possible using string formatting or if I can trick python's formatter somehow to get what i want..</p>