title: Create unique ID from the existing two columns, python
body: <p>My question is: how to efficiently sign data unique id numbers from existing id columns? For example: I have two columns [household_id], and [person_no]. I try to make a new column, the query would be: household_id + '_' + person_no. </p>
  
  <p>here is a sample: </p>
  
  <pre><code>hh_id       pno  
   682138    1   
   365348    1     
   365348    2
  </code></pre>
  
  <p>try to get: </p>
  
  <pre><code>unique_id
  682138_1
  365348_1
  365348_2
  </code></pre>
  
  <p>and add this unique_id as a new column. 
  I am applying Python. My data is very large. Any efficient way to do it would be great. Thanks!</p>