title: Python: which is a fast way to find index in pandas dataframe?
body: <p>I have a dataframe like the following</p>
  
  <pre><code>df = 
      a   ID1         ID2         Proximity
  0   0   900000498   NaN         0.000000
  1   1   900000498   900004585   3.900000
  2   2   900000498   900005562   3.900000
  3   3   900000498   900008613   0.000000
  4   4   900000498   900012333   0.000000
  5   5   900000498   900019524   3.900000
  6   6   900000498   900019877   0.000000
  7   7   900000498   900020141   3.900000
  8   8   900000498   900022133   3.900000
  9   9   900000498   900022919   0.000000
  </code></pre>
  
  <p>I want to find for a given couple <code>ID1-ID2</code> the corresponding <code>Proximity</code> value.
  For instance given the input <code>[900000498, 900022133]</code> I want as output <code>3.900000</code></p>