title: How to replace the white space in a string in a pandas dataframe?
body: <p>Suppose I have a pandas dataframe like this:</p>
  
  <pre><code>    Person_1     Person_2     Person_3 
  0   John Smith   Jane Smith   Mark Smith 
  1   Harry Jones  Mary Jones   Susan Jones
  </code></pre>
  
  <p>Reproducible form:</p>
  
  <pre><code>df = pd.DataFrame([['John Smith', 'Jane Smith', 'Mark Smith'],
                 ['Harry Jones', 'Mary Jones', 'Susan Jones'],
                columns=['Person_1', 'Person_2', 'Person_3'])
  </code></pre>
  
  <p>What is the nicest way to replace the whitespace between the first and last name in each name with an underscore _ to get:</p>
  
  <pre><code>    Person_1     Person_2     Person_3 
  0   John_Smith   Jane_Smith   Mark_Smith 
  1   Harry_Jones  Mary_Jones   Susan_Jones
  </code></pre>
  
  <p>Thank you in advance!</p>