title: Sum values of column based on the unique values of another column
body: <p>I have a data frame of</p>
  
  <pre><code>Column1  Column2
  1          20
  2          25
  3          30
  2          40
  4          18
  1          24
  </code></pre>
  
  <p>and I want to sum Column2 based on the unique values of Column1. We can find sum based on a specific value such as 1 using this way:</p>
  
  <pre><code>df.loc[df['Column1'] == 1, 'Column2'].sum()
  </code></pre>
  
  <p>which correctly gives us 44. But how we can do it for all unique values in Column1 such that it produces this one</p>
  
  <pre><code>Column1  Column2
  1          44
  2          65
  3          30
  4          18
  </code></pre>