title: Join first elements of a list of strings in python3
body: <p>Consider the following string that represents a line from a tab delimited txt file:</p>
  
  <pre><code>line = 'pf\t2\t0\t9\t0\t9\t9\n'
  </code></pre>
  
  <p>I would like to join the first two elements from this string using an underscore and then write the line back to file. I am using the following simple script to do it:</p>
  
  <pre><code>newLabel = '_'.join(line.split('\t')[:2])
  newLine = line.split('\t')
  newLine[:2] = newLabel
  </code></pre>
  
  <p>What I would expect is the following:</p>
  
  <pre><code>['pf_2', '0', '9', '0', '9', '9\n']
  </code></pre>
  
  <p>Instead I am getting:</p>
  
  <pre><code>['p', 'f', '_', '2', '0', '9', '0', '9', '9\n']
  </code></pre>
  
  <p>Maybe I am missing something obvious here but why does python split the joined string again? What is the best way to achieve what I want?
  Thanks!</p>