output.txt

$ uname -a
Linux computer 2.6.32-431.5.1.el6.i686 #1 SMP Wed Feb 12 09:07:48 CET 2014 i686 i686 i386 GNU/Linux

$ python
Python 2.6.6 (r266:84292, Jan 23 2014, 10:37:44) 
[GCC 4.4.7 20120313 (Red Hat 4.4.7-4)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> 

$ python pyseektest.py 
f.read '0'
file tell 1
os tell 1

os.read '1'
file tell 1
os tell 2

f.read '2'
file tell 2
os tell 3

os.read '3'
file tell 2
os tell 4

pyseektest.py

#/usr/bin/env python
# -*- coding: utf-8 -*-

import os

def main():
    with open('test.txt', 'wb') as f:
        f.write("01234567890")

    f = open('test.txt', 'rb', 0)
    print "f.read", repr(f.read(1))
    print "file tell", f.tell()
    print "os tell", os.lseek(f.fileno(), 0, os.SEEK_CUR)
    print

    print "os.read", repr(os.read(f.fileno(), 1))
    print "file tell", f.tell()
    print "os tell", os.lseek(f.fileno(), 0, os.SEEK_CUR)
    print

    print "f.read", repr(f.read(1))
    print "file tell", f.tell()
    print "os tell", os.lseek(f.fileno(), 0, os.SEEK_CUR)
    print

    print "os.read", repr(os.read(f.fileno(), 1))
    print "file tell", f.tell()
    print "os tell", os.lseek(f.fileno(), 0, os.SEEK_CUR)
    print

    f.close()

if __name__ == '__main__':
    main()