title: Pandas: Map DF with multiple columns to another
body: <p>I am trying to figure out an efficient way to map <code>df2</code> to <code>df1</code>.  What makes this a bit trickier, is the <em>key</em> can sometimes be a tuple of 2+ <code>keys</code>.</p>
  <p>Keys (the a, b, c's) are indeed strings.</p>
  <pre><code>df1 = pd.DataFrame(data={'Index':[1,2,3,4,5,6,7,8],'key':['a','a','b',('a','c'),'c',('a','b','c'),'b','a']}
  
  df2 = pd.DataFrame(data = {'Index':[a,b,c],'Val1':[1,2,3],'Val2':[.1,.2,.3],'Val3':[10,20,30],'Val4':[1,2,3],'Val5':[2,4,6]})
  </code></pre>
  <p>Basically, I am trying to map <code>df2</code> (via the <em>Index</em>) to the <em>key</em> in <code>df1</code>.  The end result would have <code>columns = [key, Val1, Val2, Val3, Val4, Val5]</code>, and in the result that the <code>type(key) == tuple</code>, I would like a tuple of the 2+ matched Vals.</p>
  <p>Any help is appreciated!
  Thanks!</p>