Read a file in reverse order using python

Question

How to read a file in reverse order using python? I want to read a file from last line to first line.

Answer 1

You can also use python module file_read_backwards.

After installing it, via pip install file_read_backwards (v1.2.1), you can read the entire file backwards (line-wise) in a memory efficient manner via:

#!/usr/bin/env python2.7

from file_read_backwards import FileReadBackwards

with FileReadBackwards("/path/to/file", encoding="utf-8") as frb:
    for l in frb:
         print l

It supports "utf-8","latin-1", and "ascii" encodings.

Support is also available for python3. Further documentation can be found at http://file-read-backwards.readthedocs.io/en/latest/readme.html

Answer 2

Here you can find my my implementation, you can limit the ram usage by changing the "buffer" variable, there is a bug that the program prints an empty line in the beginning.

And also ram usage may be increase if there is no new lines for more than buffer bytes, "leak" variable will increase until seeing a new line ("\n").

This is also working for 16 GB files which is bigger then my total memory.

import os,sys
buffer = 1024*1024 # 1MB
f = open(sys.argv[1])
f.seek(0, os.SEEK_END)
filesize = f.tell()

division, remainder = divmod(filesize, buffer)
line_leak=''

for chunk_counter in range(1,division + 2):
    if division - chunk_counter < 0:
        f.seek(0, os.SEEK_SET)
        chunk = f.read(remainder)
    elif division - chunk_counter >= 0:
        f.seek(-(buffer*chunk_counter), os.SEEK_END)
        chunk = f.read(buffer)

    chunk_lines_reversed = list(reversed(chunk.split('\n')))
    if line_leak: # add line_leak from previous chunk to beginning
        chunk_lines_reversed[0] += line_leak

    # after reversed, save the leakedline for next chunk iteration
    line_leak = chunk_lines_reversed.pop()

    if chunk_lines_reversed:
        print "\n".join(chunk_lines_reversed)
    # print the last leaked line
    if division - chunk_counter < 0:
        print line_leak

Answer 3

import sys
f = open(sys.argv[1] , 'r')
for line in f.readlines()[::-1]:
    print line

Answer 4

for line in reversed(open("file").readlines()):
    print line.rstrip()

If you are on linux, you can use tac command.

$ tac file

2 recipes you can find in ActiveState here and here

Answer 5

for line in reversed(open("filename").readlines()):
    print line.rstrip()

And in Python 3:

for line in reversed(list(open("filename"))):
    print(line.rstrip())

Answer 6

If you are concerned about file size / memory usage, memory-mapping the file and scanning backwards for newlines is a solution:

How to search for a string in text files?