title: Pandas get the row ID based on condition
body: <p>I have the following DataFrame:</p>
  
  <pre><code>Date          best    a    b    c    d
  1990-01-01    a       5    4    7    2
  1991-01-02    c       10   1    2    0
  1992-01-03    d       2    1    4    12
  1993-01-04    a       5    8    11   6
  </code></pre>
  
  <p>I wish to get the row ID where <code>df['Date' == '1992-01-03']</code>.</p>
  
  <p>The expected return is <code>2</code>.</p>
  
  <p>Any ideas?</p>