title: Initailizing nested dict with .fromkeys
body: <p>Hi I was wondering when creating a nested dictionary use the fromkeys method to created a nested dict does not work.</p>
  
  <pre><code>x=[1,2,3,4,5]
  y=[7,8,9,10,11]
  zx=dict.fromkeys(x,dict.fromkeys(y,0))
  </code></pre>
  
  <p>It creates a nested dict but when you try to update it</p>
  
  <p>zx[1][8]+=1</p>
  
  <p>It updates all values for all outerkeys that have the key 8.</p>
  
  <p>I was wondering why this happened when using this function to make a nested dict</p>