title: List of lists vs dictionary
body: <p>In Python, are there any advantages / disadvantages of working with a list of lists versus working with a dictionary, more specifically when doing numerical operations with them? I'm writing a class of functions to solve simple matrix operations for my linear algebra class. I was using dictionaries, but then I saw that <code>numpy</code> uses list of lists instead, so I guess there must be some advantages in it.</p>
  
  <p>Example: <code>[[1,2,3],[4,5,6],[7,8,9]]</code> as opposed to <code>{0:[1,2,3],1:[4,5,6],2:[7,8,9]}</code></p>