title: How to sum values grouped by a categorical column in pandas?
body: <p>I have data which has a categorical column that groups the data and other columns likes this in a dataframe <code>df</code>.</p>
  
  <pre><code>id      subid      value
  1       10         1.5
  1       20         2.5
  1       30         7.0 
  2       10         12.5
  2       40         5
  </code></pre>
  
  <p>What I need is a column that has the average value for each <code>subid</code> within each <code>id</code>. For example <code>df</code> could be:</p>
  
  <pre><code>id      subid      value     id_sum    proportion
  1       10         1.5       11.0      0.136
  1       20         2.5       11.0      0.227
  1       30         7.0       11.0      0.636
  2       10         12.5      17.5      0.714
  2       40         5         17.5      0.285
  </code></pre>
  
  <p>Now, I tried getting the id_sum column by doing:</p>
  
  <pre><code>df['id_sum'] = df.groupby('id')['value'].sum()
  </code></pre>
  
  <p>But this does not seem to work as hoped. My end goal is to get the <code>proportion</code> column. What is the correct way of getting that?</p>