Read a file as text line-by-line from the end

py_read_backwards.py

# %% Imports
from __future__ import annotations
import os

# %%
class FileBackwards:
    """Read a file line by line from the end.
    """
    def __init__(self, fname: str, bufsize: int = 8192):
        file = open(fname, 'rb')
        file.seek(0, os.SEEK_END)
        fsize = fremain = file.tell()
        self._file = file
        self._fsize = fsize
        self._fremain = fremain
        self._segment = None
        self._lines = []
        self._ofst = 0
        self._bufsize = bufsize

    def __iter__(self):
        return self
    
    def __next__(self):
        if self._segment is not None and len(self._segment) == 0 and self._fremain <= 0:
            raise StopIteration
        
        if self._fremain > 0:
            self._ofst = min(self._fsize, self._ofst + self._bufsize)
            self._file.seek(self._fsize - self._ofst)
            buffer = self._file.read(min(self._fremain, self._bufsize))
            # remove file's last "\n" if it exists, only for the first buffer
            if self._fremain == self._fsize and buffer[-1] == ord('\n'):
                buffer = buffer[:-1]
            self._fremain -= self._bufsize
            self._lines = buffer.split('\n'.encode())
            # append last chunk's segment to this chunk's last line
            if self._segment is not None:
                self._lines[-1] += self._segment
            self._segment = self._lines[0]
            self._lines = self._lines[1:]
            # yield lines in this chunk except the segment
            for line in reversed(self._lines):
                # only decode on a parsed line, to avoid utf-8 decode error
                ret = line.decode()
                self._lines.remove(line)
                return ret
        # Don't yield None if the file was empty
        if self._segment is not None:
            ret = self._segment.decode()
            self._segment = ''
            return ret

# %% Test
test = FileBackwards('test.txt')
for line in test:
    print(line)