title: Pandas merge produces no data
body: <p>I have 2 dataframes:</p>
  
  <p>df1:</p>
  
  <pre><code>branch    sessions users
  toledo    10       3
  new york  14       2
  boston    102      43
  seattle   9        7
  </code></pre>
  
  <p>df2:</p>
  
  <pre><code>branch    guests
  toledo    10      
  new york  14       
  boston    102      
  seattle   9    
  </code></pre>
  
  <p>The result I'm looking for merges the "guests" column from df2 to df1 like this:
  df1:</p>
  
  <pre><code>branch    sessions users  guests
  toledo    10       3      10
  new york  14       2      14
  boston    102      43     102
  seattle   9        7      9
  </code></pre>
  
  <p>I've tried <code>concat</code>, <code>join</code> and <code>merge</code> with no luck. </p>
  
  <p>With <code>merge</code></p>
  
  <pre><code>guest_sessions = df2[['branch','guests']].copy()
  pd.merge(df1, guest_sessions, left_index=True, right_index=True)
  </code></pre>
  
  <p>I get this:</p>
  
  <pre><code>branch_x    sessions    users   guests_x    branch_y    guests_y
  </code></pre>
  
  <p>What am I doing wrong?</p>