title: Python search string and replace character in next line
body: <p>in a text file I want to search for a line containing specific text and at the next line I want to substitute beginning of a line with #(comment it out) inplace:</p>
  
  <p>Example - before:</p>
  
  <pre><code>#test search 123
  text to be commented out
  #test 123
  </code></pre>
  
  <p>Example - wanted:</p>
  
  <pre><code>#test search 123
  #text to be commented out
  #test 123
  </code></pre>
  
  <p>I can do it via sed:</p>
  
  <pre><code>sed -i '/^#test search 123/!b; n; s/^/#/' test_file
  </code></pre>
  
  <p>but i was wondering if I'm able to do it natively in python. </p>