How to jump to a particular line in a huge text file?

You can't jump ahead without reading in the file at least once, since you don't know where the line breaks are. You could do something like:

# Read in the file once and build a list of line offsets
line_offset = []
offset = 0
for line in file:
    line_offset.append(offset)
    offset += len(line)
file.seek(0)

# Now, to skip to line n (with the first line being line 0), just do
file.seek(line_offset[n])

linecache:

The linecache module allows one to get any line from a Python source file, while attempting to optimize internally, using a cache, the common case where many lines are read from a single file. This is used by the traceback module to retrieve source lines for inclusion in the formatted traceback...

You don't really have that many options if the lines are of different length... you sadly need to process the line ending characters to know when you've progressed to the next line.

You can, however, dramatically speed this up AND reduce memory usage by changing the last parameter to "open" to something not 0.

0 means the file reading operation is unbuffered, which is very slow and disk intensive. 1 means the file is line buffered, which would be an improvement. Anything above 1 (say 8 kB, i.e. 8192, or higher) reads chunks of the file into memory. You still access it through for line in open(etc):, but python only goes a bit at a time, discarding each buffered chunk after its processed.

I'm probably spoiled by abundant ram, but 15 M is not huge. Reading into memory with readlines() is what I usually do with files of this size. Accessing a line after that is trivial.

Since there is no way to determine the lenght of all lines without reading them, you have no choice but to iterate over all lines before your starting line. All you can do is to make it look nice. If the file is really huge then you might want to use a generator based approach:

from itertools import dropwhile

def iterate_from_line(f, start_from_line):
    return (l for i, l in dropwhile(lambda x: x[0] < start_from_line, enumerate(f)))

for line in iterate_from_line(open(filename, "r", 0), 141978):
    DoSomethingWithThisLine(line)

Note: the index is zero based in this approach.

If you don't want to read the entire file in memory .. you may need to come up with some format other than plain text.

of course it all depends on what you're trying to do, and how often you will jump across the file.

For instance, if you're gonna be jumping to lines many times in the same file, and you know that the file does not change while working with it, you can do this:
First, pass through the whole file, and record the "seek-location" of some key-line-numbers (such as, ever 1000 lines),
Then if you want line 12005, jump to the position of 12000 (which you've recorded) then read 5 lines and you'll know you're in line 12005 and so on

None of the answers are particularly satisfactory, so here's a small snippet to help.

class LineSeekableFile:
    def __init__(self, seekable):
        self.fin = seekable
        self.line_map = list() # Map from line index -> file position.
        self.line_map.append(0)
        while seekable.readline():
            self.line_map.append(seekable.tell())

    def __getitem__(self, index):
        # NOTE: This assumes that you're not reading the file sequentially.  
        # For that, just use 'for line in file'.
        self.fin.seek(self.line_map[index])
        return self.fin.readline()

Example usage:

In: !cat /tmp/test.txt

Out:
Line zero.
Line one!

Line three.
End of file, line four.

In:
with open("/tmp/test.txt", 'rt') as fin:
    seeker = LineSeekableFile(fin)    
    print(seeker[1])
Out:
Line one!

This involves doing a lot of file seeks, but is useful for the cases where you can't fit the whole file in memory. It does one initial read to get the line locations (so it does read the whole file, but doesn't keep it all in memory), and then each access does a file seek after the fact.

I offer the snippet above under the MIT or Apache license at the discretion of the user.

If you know in advance the position in the file (rather the line number), you can use file.seek() to go to that position.

Edit: you can use the linecache.getline(filename, lineno) function, which will return the contents of the line lineno, but only after reading the entire file into memory. Good if you're randomly accessing lines from within the file (as python itself might want to do to print a traceback) but not good for a 15MB file.

Do the lines themselves contain any index information? If the content of each line was something like "<line index>:Data", then the seek() approach could be used to do a binary search through the file, even if the amount of Data is variable. You'd seek to the midpoint of the file, read a line, check whether its index is higher or lower than the one you want, etc.

Otherwise, the best you can do is just readlines(). If you don't want to read all 15MB, you can use the sizehint argument to at least replace a lot of readline()s with a smaller number of calls to readlines().

Here's an example using readlines(sizehint) to read a chunk of lines at a time. DNS pointed out that solution. I wrote this example because the other examples here are single-line oriented.

def getlineno(filename, lineno):
    if lineno < 1:
        raise TypeError("First line is line 1")
    f = open(filename)
    lines_read = 0
    while 1:
        lines = f.readlines(100000)
        if not lines:
            return None
        if lines_read + len(lines) >= lineno:
            return lines[lineno-lines_read-1]
        lines_read += len(lines)

print getlineno("nci_09425001_09450000.smi", 12000)

@george brilliantly suggested mmap, which presumably uses the syscall mmap. Here's another rendition.

import mmap

LINE = 2  # your desired line

with open('data.txt','rb') as i_file, mmap.mmap(i_file.fileno(), length=0, prot=mmap.PROT_READ) as data:
  for i,line in enumerate(iter(data.readline, '')):
    if i!=LINE: continue
    pos = data.tell() - len(line)
    break

  # optionally copy data to `chunk`
  i_file.seek(pos)
  chunk = i_file.read(len(line))

print(f'line {i}')
print(f'byte {pos}')
print(f'data {line}')
print(f'data {chunk}')