Python: rewinding one line in file when iterating with f.next()

Question

Python\'s f.tell doesn\'t work as I expected when you iterate over a file with f.next():

>>> f=open(\".bash_profile\", \"r\")
>>> f.tell()
         


        

Answer 1

itertools.tee is probably the least-bad approach -- you can't "defeat" the buffering done by iterating on the file (nor would you want to: the performance effects would be terrible), so keeping two iterators, one "one step behind" the other, seems the soundest solution to me.

import itertools as it

with open('a.txt') as f:
  f1, f2 = it.tee(f)
  f2 = it.chain([None], f2)
  for thisline, prevline in it.izip(f1, f2):
    ...