How to read a file in reverse order?

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

Read the file line by line and then add it on a list in reverse order.

Here is an example of code :

reverse = []
with open("file.txt", "r") as file:
    for line in file:
        line = line.strip()
         reverse[0:0] = line

Answer 2

I don't think this has been mentioned before, but using deque from collections and reverse works for me:

from collections import deque

fs = open("test.txt","rU")
fr = deque(fs)
fr.reverse()  # reverse in-place, returns None

for li in fr:
   print li

fs.close()

Answer 3

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?

Answer 4

Always use with when working with files as it handles everything for you:

with open('filename', 'r') as f:
    for line in reversed(f.readlines()):
        print line

Or in Python 3:

with open('filename', 'r') as f:
    for line in reversed(list(f.readlines())):
        print(line)

Answer 5

I had to do this some time ago and used the below code. It pipes to the shell. I am afraid i do not have the complete script anymore. If you are on a unixish operating system, you can use "tac", however on e.g. Mac OSX tac command does not work, use tail -r. The below code snippet tests for which platform you're on, and adjusts the command accordingly

# We need a command to reverse the line order of the file. On Linux this
# is 'tac', on OSX it is 'tail -r'
# 'tac' is not supported on osx, 'tail -r' is not supported on linux.

if sys.platform == "darwin":
    command += "|tail -r"
elif sys.platform == "linux2":
    command += "|tac"
else:
    raise EnvironmentError('Platform %s not supported' % sys.platform)

Answer 6

A correct, efficient answer written as a generator.

import os

def reverse_readline(filename, buf_size=8192):
    """A generator that returns the lines of a file in reverse order"""
    with open(filename) as fh:
        segment = None
        offset = 0
        fh.seek(0, os.SEEK_END)
        file_size = remaining_size = fh.tell()
        while remaining_size > 0:
            offset = min(file_size, offset + buf_size)
            fh.seek(file_size - offset)
            buffer = fh.read(min(remaining_size, buf_size))
            remaining_size -= buf_size
            lines = buffer.split('\n')
            # The first line of the buffer is probably not a complete line so
            # we'll save it and append it to the last line of the next buffer
            # we read
            if segment is not None:
                # If the previous chunk starts right from the beginning of line
                # do not concat the segment to the last line of new chunk.
                # Instead, yield the segment first 
                if buffer[-1] != '\n':
                    lines[-1] += segment
                else:
                    yield segment
            segment = lines[0]
            for index in range(len(lines) - 1, 0, -1):
                if lines[index]:
                    yield lines[index]
        # Don't yield None if the file was empty
        if segment is not None:
            yield segment