file = open("sample.txt") 
while 1:
  line = file.readline()
  if not line:
    break
  pass # do something
file.close()

import fileinput 
for line in fileinput.input("sample.txt"):
  pass

file = open("sample.txt") 
while 1:
  lines = file.readlines(100000)
  if not lines:
    break
  for line in lines:
    pass # do something
file.close()

file = open("sample.txt") 
for line in file:
  pass # do something
file.close()