How Can I Read a File in Reverse Order Using Python?

Reading a File in Reverse Order in Python

When the need arises to process data from the end of a file, it becomes necessary to read a file in reverse order. In Python, this can be achieved effectively using a generator-based approach.

Generator for Reverse Line Reading

The following Python generator, reverse_readline, allows for iterating over the lines of a file in reverse order:

import os

def reverse_readline(filename, buf_size=8192):
    """A generator that returns the lines of a file in reverse order"""
    with open(filename, 'rb') 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))
            # remove file's last "\n" if it exists, only for the first buffer
            if remaining_size == file_size and buffer[-1] == ord('\n'):
                buffer = buffer[:-1]
            remaining_size -= buf_size
            lines = buffer.split('\n'.encode())
            # append last chunk's segment to this chunk's last line
            if segment is not None:
                lines[-1] += segment
            segment = lines[0]
            lines = lines[1:]
            # yield lines in this chunk except the segment
            for line in reversed(lines):
                # only decode on a parsed line, to avoid utf-8 decode error
                yield line.decode()
        # Don't yield None if the file was empty
        if segment is not None:
            yield segment.decode()

Usage

To use the reverse_readline generator, open a file for reading in binary mode and iterate over the lines in reverse order:

with open('input.txt', 'rb') as input_file:
    for line in reverse_readline(input_file):
        # Process line in reverse order

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