Python Random Access File

Since lines can be of arbitrary length, you really can't get at a random line (whether you mean "a line whose number is actually random" or "a line with an arbitrary number, selected by me") without traversing the whole file.

If kinda-sorta-random is enough, you can seek to a random place in the file and then read forward until you hit a line terminator. But that's useless if you want to find (say) line number 1234, and will sample lines non-uniformly if you actually want a randomly chosen line.

You can use linecache:

import linecache
print linecache.getline(your_file.txt, randomLineNumber) # Note: first line is 1, not 0

This seems like just the sort of thing mmap was designed for. A mmap object creates a string-like interface to a file:

>>> f = open("bonnie.txt", "wb")
>>> f.write("My Bonnie lies over the ocean.")
>>> f.close()
>>> f.open("bonnie.txt", "r+b")
>>> mm = mmap(f.fileno(), 0)
>>> print mm[3:9]
Bonnie

In case you were wondering, mmap objects can also be assigned to:

>>> print mm[24:]
ocean.
>>> mm[24:] = "sea.  "
>>> print mm[:]
My Bonnie lies over the sea.