title: Operation on every pair of element in a list
body: <p>Using Python, I'd like to compare every possible pair in a list.</p>
  
  <p>Suppose I have</p>
  
  <pre><code>my_list = [1,2,3,4]
  </code></pre>
  
  <p>I'd like to do an operation (let's call it foo) on every combination of 2 elements from the list.</p>
  
  <p>The final result should be the same as</p>
  
  <pre><code>foo(1,1)
  foo(1,2)
  ...
  foo(4,3)
  foo(4,4)
  </code></pre>
  
  <p>My first thought was to iterate twice through the list manually, but that doesn't seem very pythonic.</p>