title: Hashable, immutable
body: <p>From a recent SO question (see <a href="https://stackoverflow.com/questions/2671211/create-a-dictionary-in-python-which-is-indexed-by-lists">Create a dictionary in python which is indexed by lists</a>) I realized I probably had a wrong conception of the meaning of hashable and immutable objects in python.</p>
  
  <ul>
  <li>What does hashable mean in practice?</li>
  <li>What is the relation between hashable and immmutable?</li>
  <li>Are there mutable objects that are hashable or immutable objects that are not hashable? </li>
  </ul>