title: Find rare characters with Python
body: <p>Assume I have a huge <code>.txt</code> file full of random characters and I want to find out the "rare ones". Is there some module (something at all, actually) in Python (possibly, version <code>3.x</code>, but I have also a machine using Python <code>2.7</code>, in case it's better) written for this purpose? In case of positive answer, where can I find some basic explanation of its functioning?
  Thank you very much.</p>