with open(filename) as f:
        while 1:
            if f.readline().strip() == find_string:
                #действия.

with open(filename) as infile:
    for line in infile:
        do_something_with(line)

>>> with open('/etc/passwd', encoding='utf-8') as fin:
...   for line in fin:
...     print(line, end='')
...     if line.startswith('root'):
...       break
...   print('sep')
...   print(next(fin), end='')
... 
root:x:0:0:root:/root:/bin/bash
sep
bin:x:1:1:bin:/bin:/sbin/nologin
>>>

with open("a.dat","rb") as f:
    for i in f:
        print("{0}:{1}".format(f.tell(),i))
>>> 
2:b'1\n'
4:b'2\n'
6:b'3\n'
8:b'4\n'
10:b'5\n'
11:b'6'
with open("a.dat","r",encoding="utf8") as f:
    for i in f:
        print("{0}:{1}".format(f.tell(),i))
>>> 
Traceback (most recent call last):
  File "<string>", line 301, in runcode
  File "<editor selection>", line 3, in <module>
OSError: telling position disabled by next() call