Binary search a line in a file having all lines formatted the same way

bsfile.py

from __future__ import print_function
from time import time

class BSFile(object):
    def __init__(self, filename, key_func):
        self.filename = filename
        self.key_func = key_func

    def find(self, key):
        '''
        Get file iterator from first line for which key(line) >= <given_key>
        Catch: it will never start from first line
        '''
        f = open(self.filename, 'r')
        f.seek(0, 2) # seek to end
        lo, mid, hi = 0, 0, f.tell()-1
        f.seek(0) # return back to start

        while lo < hi:
            mid = (lo+hi) // 2
            f.seek(mid)
            if mid != 0:
                if not f.readline(): # just to be sure not to consume half line
                    return f # EOF
                mid = f.tell()

            if mid >= hi:
                lo = mid
            else:
                line = f.readline()
                if not line:
                    return f # EOF
                if key > self.key_func(line):
                    lo = mid
                else:
                    hi = mid - 1

        f.seek(mid)
        return f

if __name__ == '__main__':
    bsf = BSFile('test.txt', lambda line: int(line.split()[0]))
    for i in range(20):
        print('*** KEY: %02d ***' % i)
        with bsf.find(i) as reader:
            for line in reader:
                print(line.strip())
            print()

output

*** KEY: 00 ***
1 2
2 3
3 4
5 5
5 6
6 7
9 8
14 123123
14 12324
14 1324
14 34324234
17 asdfsad

*** KEY: 01 ***
1 2
2 3
3 4
5 5
5 6
6 7
9 8
14 123123
14 12324
14 1324
14 34324234
17 asdfsad

*** KEY: 02 ***
2 3
3 4
5 5
5 6
6 7
9 8
14 123123
14 12324
14 1324
14 34324234
17 asdfsad

*** KEY: 03 ***
3 4
5 5
5 6
6 7
9 8
14 123123
14 12324
14 1324
14 34324234
17 asdfsad

*** KEY: 04 ***
5 5
5 6
6 7
9 8
14 123123
14 12324
14 1324
14 34324234
17 asdfsad

*** KEY: 05 ***
5 5
5 6
6 7
9 8
14 123123
14 12324
14 1324
14 34324234
17 asdfsad

*** KEY: 06 ***
6 7
9 8
14 123123
14 12324
14 1324
14 34324234
17 asdfsad

*** KEY: 07 ***
14 12324
14 1324
14 34324234
17 asdfsad

*** KEY: 08 ***
14 12324
14 1324
14 34324234
17 asdfsad

*** KEY: 09 ***
14 12324
14 1324
14 34324234
17 asdfsad

*** KEY: 10 ***
14 12324
14 1324
14 34324234
17 asdfsad

*** KEY: 11 ***
14 12324
14 1324
14 34324234
17 asdfsad

*** KEY: 12 ***
14 12324
14 1324
14 34324234
17 asdfsad

*** KEY: 13 ***
14 12324
14 1324
14 34324234
17 asdfsad

*** KEY: 14 ***
14 12324
14 1324
14 34324234
17 asdfsad

*** KEY: 15 ***
17 asdfsad

*** KEY: 16 ***
17 asdfsad

*** KEY: 17 ***
17 asdfsad

*** KEY: 18 ***

*** KEY: 19 ***

test.txt

1 1
1 2
2 3
3 4
5 5
5 6
6 7
9 8
14 123123
14 12324
14 1324
14 34324234
17 asdfsad