title: Adding column to .CSV file in Python and calculate values
body: <p>I have checked many solutions but I have been unable to apply any to my problem.</p>
  
  <p>I have a .csv file, like this:</p>
  
  <pre><code>    Header_A;Header_B
      0;1
      1;4
      5;6
      6;7
      9;8
  </code></pre>
  
  <p>Now I want to pythonically add another column "Header_C" to it and calculate its values (x) from the addition from the first 2 columns per definition, so something like</p>
  
  <pre><code>    def add(a, b):
          x = a + b
          return x
  </code></pre>
  
  <p>where x will be the value of column Header_C and a, b are the sums of columns Header_A and Header_B.</p>
  
  <p>The result should look like this:</p>
  
  <pre><code>    Header_A;Header_B;Header_C
      0;1;1
      1;4;5
      5;6;11
      6;7;13
      9;8;17
  </code></pre>
  
  <p>If possible without installing additional modules. Output can be a new .csv file.</p>
  
  <p>Thanks a lot!</p>