title: Python nested loop - get next N lines
body: <p>I'm new to Python and trying to do a nested loop. I have a very large file (1.1 million rows), and I'd like to use it to create a file that has each line along with the next N lines, for example with the next 3 lines:</p>
  
  <pre><code>1    2
  1    3
  1    4
  2    3
  2    4
  2    5
  </code></pre>
  
  <p>Right now I'm just trying to get the loops working with rownumbers instead of the strings since it's easier to visualize. I came up with this code, but it's not behaving how I want it to:</p>
  
  <pre><code>with open('C:/working_file.txt', mode='r', encoding = 'utf8') as f: 
  for i, line in enumerate(f):
       line_a = i
       lower_bound = i + 1
       upper_bound = i + 4
       with open('C:/working_file.txt', mode='r', encoding = 'utf8') as g:
          for j, line in enumerate(g):
              while j &gt;= lower_bound and j &lt;= upper_bound:
                  line_b = j
                  j = j+1
                  print(line_a, line_b)
  </code></pre>
  
  <p>Instead of the output I want like above, it's giving me this: </p>
  
  <pre><code>990     991
  990     992
  990     993
  990     994
  990     992
  990     993
  990     994
  990     993
  990     994
  990     994
  </code></pre>
  
  <p>As you can see the inner loop is iterating multiple times for each line in the outer loop. It seems like there should only be one iteration per line in the outer loop. What am I missing?</p>
  
  <p>EDIT: My question was answered below, here is the exact code I ended up using: </p>
  
  <pre><code>from collections import deque
  from itertools import cycle
  log = open('C:/example.txt', mode='w', encoding = 'utf8') 
  try:
      xrange 
  except NameError: # python3
      xrange = range
  
  def pack(d):
      tup = tuple(d)
      return zip(cycle(tup[0:1]), tup[1:])
  
  def window(seq, n=2):
      it = iter(seq)
      d = deque((next(it, None) for _ in range(n)), maxlen=n)
      yield pack(d)
      for e in it:
          d.append(e)
          yield pack(d)
  
  for l in window(open('c:/working_file.txt', mode='r', encoding='utf8'),100):
      for a, b in l:
          print(a.strip() + '\t' + b.strip(), file=log)
  </code></pre>