title: Explanation of pylint bad-format-string
body: <p>In the following file:</p>
  
  <pre><code>"""hello I am the module spam.py"""
  from __future__ import unicode_literals
  'hello {world}'.format(world='potato')
  </code></pre>
  
  <p>We have the following pylint violation for <code>bad-format-string</code>:</p>
  
  <pre><code>wim@SDFA100461C:/tmp$ pylint --reports=n spam.py
  No config file found, using default configuration
  ************* Module spam
  W:  3, 0: Invalid format string (bad-format-string)
  </code></pre>
  
  <p>I don't understand this suggestion, pylint devs say the check is about <a href="http://legacy.python.org/dev/peps/pep-3101/">PEP 3101</a> style but I don't see anything in the PEP which is violated here.  </p>
  
  <p>What is the problem? What does pylint wish us to do about it?  </p>
  
  <p>Version numbers below.</p>
  
  <pre><code>wim@SDFA100461C:/tmp$ pylint --version
  No config file found, using default configuration
  pylint 1.3.0, 
  astroid 1.2.0, common 0.62.1
  Python 2.7.6 (default, Mar 22 2014, 22:59:56) 
  [GCC 4.8.2]
  </code></pre>