title: How to get the common index of two pandas dataframes?
body: <p>I have two pandas DataFrames df1 and df2 and I want to transform them in order that they keep values only for the index that are common to the 2 dataframes.</p>
  
  <p><em>df1</em></p>
  
  <pre><code>                      values 1
  0                            
  28/11/2000          -0.055276
  29/11/2000           0.027427
  30/11/2000           0.066009
  01/12/2000           0.012749
  04/12/2000           0.113892
  </code></pre>
  
  <p><em>df2</em></p>
  
  <pre><code>                       values 2
  
  24/11/2000            -0.004808
  27/11/2000            -0.001812
  28/11/2000            -0.026316
  29/11/2000             0.015222
  30/11/2000            -0.024480
  </code></pre>
  
  <p>become</p>
  
  <p><em>df1</em></p>
  
  <pre><code>                     value 1
  
  28/11/2000          -0.055276
  29/11/2000           0.027427
  30/11/2000           0.066009
  </code></pre>
  
  <p><em>df2</em></p>
  
  <pre><code>                       value 2
  
  28/11/2000            -0.026316
  29/11/2000             0.015222
  30/11/2000            -0.024480
  </code></pre>