title: Check and increase the quantity of characters inside a cell using pandas
body: <p>I am trying to modify a cell inside a dataframe on panda but I am having a few quantity of problems:</p>
  
  <p>For example, if I read a string with lower quantity of 4 digits, I need to increase '000'</p>
  
  <pre><code>travel_data['b'] = travel_data['b'].astype(str)
              travel_data['b'] = np.where(travel_data['b'].str.len() &lt; 4, '000' + str(travel_data['b']), str(travel_data['b']))
  </code></pre>
  
  <p>But on this is not working, do you have any other idea?</p>
  
  <p>Raphael</p>