title: What is the fastest way to flatten arbitrarily nested lists in Python?
body: <p>What is fastest solution to flatten lists which contain other lists of arbitrary length?</p>
  <p>For example <code>[1, 2, [3, 4, [5],[]], [6]]</code> would become <code>[1,2,3,4,5,6]</code>.</p>
  <p>There can be arbitrarily many levels. Some of the list objects can be strings, which mustn't be flattened into their sequential characters in the output list.</p>