title: replace empty cells by specific length in Python Pandas
body: <p>I'm trying to replace empty cells by certain count for each group in the data. Say data looks like the table below. In the first group, length of strings is 5 and hence row 3 should be filled with "NEW_STRING" across all 5 columns. However for group 2, length of strings is 3 and row 7 should be filled with "NEW_STRING" across the first 3 columns.</p>
  
  <pre><code>GROUP   | col_1 | col_2 | col_3 | col_4 | col_5
  ------------------------------------------------
  GROUP_1 | STR_1 | STR_2 | STR_3 | STR_4 | STR_5
  GROUP_1 | STR_1 | STR_2 | STR_3 | STR_4 | STR_5
  GROUP_1 |       |       |       |       |
  GROUP_1 | STR_1 | STR_2 | STR_3 | STR_4 | STR_5
  GROUP_2 | STR_1 | STR_2 | STR_3 |       |
  GROUP_2 | STR_1 | STR_2 | STR_3 |       |
  GROUP_2 |       |       |       |       |
  </code></pre>
  
  <p>Output should be like,</p>
  
  <pre><code>GROUP   | col_1  | col_2 | col_3 | col_4 | col_5
  -------------------------------------------------
  GROUP_1 | STR_1  | STR_2 | STR_3 | STR_4 | STR_5
  GROUP_1 | STR_1  | STR_2 | STR_3 | STR_4 | STR_5
  GROUP_1 | NEW_STR|NEW_STR|NEW_STR|NEW_STR|NEW_STR
  GROUP_1 | STR_1  | STR_2 | STR_3 | STR_4 | STR_5
  GROUP_2 | STR_1  | STR_2 | STR_3 |       |
  GROUP_2 | STR_1  | STR_2 | STR_3 |       |
  GROUP_2 | NEW_STR|NEW_STR|NEW_STR|       |
  </code></pre>
  
  <p>Kind help is greatly appreciated!!!</p>