title: Pandas: Groupby.transform -> assign specific values to column
body: <p>In general terms, is there a way to assign specific values to a column via groupby.transform(), where the groupby size is known in advance?</p>
  <p>For example:</p>
  <pre><code>df = pd.DataFrame(data = {'A':[10,10,20,20],'B':['abc','def','ghi','jkl'],'GroupID':[1,1,2,2]})
  
  funcDict = {'A':'sum','B':['specific_val_1', 'specific_val_2']}
  
  df = df.groupby('GroupID').transform(funcDict)
  </code></pre>
  <p>where the result would be:</p>
  <div class="s-table-container">
  <table class="s-table">
  <thead>
  <tr>
  <th style="text-align: left;">index</th>
  <th style="text-align: left;">A</th>
  <th style="text-align: left;">B</th>
  </tr>
  </thead>
  <tbody>
  <tr>
  <td style="text-align: left;">1</td>
  <td style="text-align: left;">20</td>
  <td style="text-align: left;">specific_val_1</td>
  </tr>
  <tr>
  <td style="text-align: left;">2</td>
  <td style="text-align: left;">20</td>
  <td style="text-align: left;">specific_val_2</td>
  </tr>
  <tr>
  <td style="text-align: left;">3</td>
  <td style="text-align: left;">40</td>
  <td style="text-align: left;">specific_val_1</td>
  </tr>
  <tr>
  <td style="text-align: left;">4</td>
  <td style="text-align: left;">40</td>
  <td style="text-align: left;">specific_val_2</td>
  </tr>
  </tbody>
  </table>
  </div>