title: Increment numbers in list from a certain point
body: <p>I have a list of numbers, e.g. <code>[50,100,150,200,250]</code>. I need to increment (or decrement) each number from a specified index and by a specified amount. I have been able to do this in two ways:</p>
  
  <pre><code>from itertools import islice
  
  l = [50,100,150,200,250]
  start_increment_index = 3
  
  l[start_increment_index:] = [e+100 for e in l[start_increment_index:]]
  print (l)
  
  l = [50,100,150,200,250]
  
  l[start_increment_index:] = [e+100 for e in islice(l,start_increment_index,len(l))]
  print (l)
  </code></pre>
  
  <p>Both print: <code>[50, 100, 150, 300, 350]</code>.</p>
  
  <p>However, my real list contains millions of numbers and this operation is performed repeatedly with different indexes and different increments/decrements. Would there be a faster way of doing this using a Python list? I have been considering writing my own C/C++ extension to deal with this.</p>
  
  <p>Edit: Would this be a useful module for Python in general? Having a function written in C which can take parameters <code>(python_list_object, increment_amount, start_index, end_index)</code>?</p>