title: Python, pandas: replacing values in one DF by same-index values from another DF
body: <p>I have two data frames with exactly the same index:</p>
  
  <pre><code>the first one:
             0         1         2
  2   0.011765  0.490196  0.015686
  2   0.011765  0.490196  0.015686
  2   0.007843  0.494118  0.007843
  2   0.007843  0.494118  0.007843
  2   0.007843  0.501961  0.011765
  ..       ...       ...       ...
  
  0   0.000000  0.031373  0.039216
  0   0.031373  0.082353  0.105882
  0   0.094118  0.149020  0.192157
  0   0.094118  0.156863  0.215686
  
  [337962 rows x 3 columns]
  </code></pre>
  
  <p>and the second one:</p>
  
  <pre><code>          0         1         2
  0  0.055852  0.118138  0.052386
  1  0.453661  0.665857  0.441551
  2  0.096394  0.635641  0.068524
  3  0.952545  0.827438  0.047632
  4  0.787729  0.823494  0.795792
  5  0.050284  0.549379  0.592593
  6  0.608805  0.215458  0.068293
  7  0.775640  0.091352  0.689224
  </code></pre>
  
  <p>The first DF is quite huge. I need to replace values in huge DF by values with same index in small DF as quickly as possible. 
  How? 
  Thanks for any help.</p>