title: Append column to pandas dataframe
body: <p>This is probably easy, but I have the following data:</p>
  
  <p>In data frame 1:</p>
  
  <pre><code>index dat1
  0     9
  1     5
  </code></pre>
  
  <p>In data frame 2:</p>
  
  <pre><code>index dat2
  0     7
  1     6
  </code></pre>
  
  <p>I want a data frame with the following form:</p>
  
  <pre><code>index dat1  dat2
  0     9     7
  1     5     6
  </code></pre>
  
  <p>I've tried using the <code>append</code> method, but I get a cross join (i.e. cartesian product).</p>
  
  <p>What's the right way to do this?</p>