title: Union of values of two dictionaries merged by key
body: <p><br>
  I have two dictionaries:</p>
  
  <pre><code>d1 = {'a':('x','y'),'b':('k','l')}
  d2 = {'a':('m','n'),'c':('p','r')}
  </code></pre>
  
  <p>How do I merge these two dictionaries to get such result:</p>
  
  <pre><code>d3 = {'a':('x','y','m','n'),'b':('k','l'),'c':('p','r')}
  </code></pre>
  
  <p>This works when values of the dictionary are simple type like int or str:</p>
  
  <pre><code>d3 = dict([(i,a[i]+b[i]) for i in set(a.keys()+b.keys())])
  </code></pre>
  
  <p>But I have no idea how deal with lists as values...</p>
  
  <p>Notice: my question is different to <a href="https://stackoverflow.com/questions/38987/how-can-i-merge-two-python-dictionaries-in-a-single-expression">How to merge two Python dictionaries in a single expression?</a> cause I don't want to update values but add them</p>