Question

Suppose you open a file, and do an seek() somewhere in the file, how do you know the current file line ?

(I personally solved with an ad-hoc file class that maps the seek position to the line after scanning the file, but I wanted to see other hints and to add this question to stackoverflow, as I was not able to find the problem anywhere on google)

Answer 1

Here’s how I would approach the problem, using as much laziness as possible:

from random import randint
from itertools import takewhile, islice

file = "/etc/passwd"
f = open(file, "r")

f.seek(randint(10,250))
pos = f.tell()

print "pos=%d" % pos

def countbytes(iterable):
    bytes = 0
    for item in iterable:
        bytes += len(item)
        yield bytes

print 1+len(list(takewhile(lambda x: x <= pos, countbytes(open(file, "r")))))

For a slightly less readable but much more lazy approach, use enumerate and dropwhile:

from random import randint
from itertools import islice, dropwhile

file = "/etc/passwd"
f = open(file, "r")

f.seek(randint(10,250))
pos = f.tell()

print "pos=%d" % pos

def countbytes(iterable):
    bytes = 0
    for item in iterable:
        bytes += len(item)
        yield bytes

print list(
        islice(
            dropwhile(lambda x: x[1] <= pos, enumerate(countbytes(open(file, "r"))))
            , 1))[0][0]+1