title: Modify a large list without any loops in python
body: <p>My list is:</p>
  
  <pre><code>a=[1,2,3,4]
  </code></pre>
  
  <p>Now I want my list to be:</p>
  
  <pre><code>a=[-1,-2,-3,-4]
  </code></pre>
  
  <p>How can I change my list this way without using any loops?</p>
  
  <p>Update: this may be a large list, on the order of 10000 elements.</p>