title: Changing string to int from a list of lists only for isdigit
body: <p>I have a list called rawSafirlistoflists and I work with python 3, that is looking like this:</p>
  
  <pre><code>[['423523525', 'HOROS', 'WAFAD', ' 23523523523 - HOROD WAFAD', 'August', '2014', '540', '0', 'Ianuarie', '2015', '0', '0', 'RESTANTA_MANDAT', 'Mandat neachitat NAM', '', 'AJUTOR REFUGIATI', 'INCLUS_IN_PLATA', ''], ['5235232', 'STOICOVICIDF', 'PAULADD ANDREEAD', ' 52352352352 - STOICOVICIDE PAULF CRISTIAN', 'August', '2014', '42', '0', 'Februarie', '2015', '0', '0', 'RESTANTA_MANDAT', '', '', 'ALOCATIA DE STAT PENTRU COPII', 'INCLUS_IN_PLATA', '']]
  </code></pre>
  
  <p>There I want to iterate and transform only the numbers that are now string to int.
  This is my code:</p>
  
  <pre><code>for level1 in rawSafirlistoflists:
  for level2 in level1:
      if level2.isdigit():
          int(level2)
          print(level2)
  print (rawSafirlistoflists)
  </code></pre>
  
  <p>When I print the level2 I see that the if had found the right string (the ones that are numbers) and converted them in int as I wanted, but it isn't saving the change. My problem at the end when I print my list rawSafirlistoflists the values are unchanged (all indexes remained strings), how can I push the modifications at the if statement so the values to remain as int?</p>