title: Python 3: using %s and .format()
body: <p>I have finally switched from <code>%</code> to the <code>.format()</code> string formatting operator in my 2.x code in order to make it easier to migrate to 3.x in future. It was a bit surprising to find out that not only the <code>%</code>-style formatting remains in Py3, but it is widely used in the standard library code. It seems logical, because writing <code>'(%s)' % variable</code> is a bit shorter and maybe easier to comprehend than <code>'({})'.format(variable)</code>. But I'm still in doubt. Is it proper (pythonic?) to use both approaches in the code? 
  Thank you.</p>