title: Get first column value in Pandas DataFrame where row matches condition
body: <p>Say I have a pandas dataframe that looks like this:</p>
  
  <pre><code>   color  number
  0    red       3
  1   blue       4
  2  green       2
  3   blue       2
  </code></pre>
  
  <p>I want to get the first value from the number column where the color column has the value <code>'blue'</code> which in this case would return <code>4</code>.</p>
  
  <p>I know this can be done using <code>loc</code> in something like this:</p>
  
  <pre><code>df[df['color'] == 'blue']['number'][0]
  </code></pre>
  
  <p>I'm wondering if there is any more optimal approach given that I only ever need the first occurrence.</p>