title: Pandas: create new column in one dataframe with values based on matching key from another dataframe
body: <p>I have two pandas dataframes, the first dataframe has two columns assumed to be the key and value and the second dataframe contains only the keys and I want to add a new column in the second dataframe the values for this column should be the values for the matching keys from the first dataframe</p>
  
  <pre><code>df = pd.DataFrame({'vi' : ['a','b','c','d','e'],'s':[2,5,7,0,1]})  
  tf = pd.DataFrame({'vi' : ['b','d','c','a','e']})    
  df    
  
  vi  s  
   a  2  
   b  5  
   c  7  
   d  0  
   e  1  
  
  tf  
  
  vi    
   b  
   d  
   c  
   a  
   e  
  
  result tf  
  
  vi  s  
   b  5  
   d  0  
   c  7  
   a  2  
   e  1
  </code></pre>