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

Thanks for the answer @srohde. It has a small bug checking for newline character with 'is' operator, and I could not comment on the answer with 1 reputation. Also I'd like to manage file open outside because that enables me to embed my ramblings for luigi tasks.

What I needed to change has the form:

with open(filename) as fp:
    for line in fp:
        #print line,  # contains new line
        print '>{}<'.format(line)

I'd love to change to:

with open(filename) as fp:
    for line in reversed_fp_iter(fp, 4):
        #print line,  # contains new line
        print '>{}<'.format(line)

Here is a modified answer that wants a file handle and keeps newlines:

def reversed_fp_iter(fp, buf_size=8192):
    """a generator that returns the lines of a file in reverse order
    ref: https://stackoverflow.com/a/23646049/8776239
    """
    segment = None  # holds possible incomplete segment at the beginning of the buffer
    offset = 0
    fp.seek(0, os.SEEK_END)
    file_size = remaining_size = fp.tell()
    while remaining_size > 0:
        offset = min(file_size, offset + buf_size)
        fp.seek(file_size - offset)
        buffer = fp.read(min(remaining_size, buf_size))
        remaining_size -= buf_size
        lines = buffer.splitlines(True)
        # 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':
                #print 'buffer ends with newline'
                yield segment
            else:
                lines[-1] += segment
                #print 'enlarged last line to >{}<, len {}'.format(lines[-1], len(lines))
        segment = lines[0]
        for index in range(len(lines) - 1, 0, -1):
            if len(lines[index]):
                yield lines[index]
    # Don't yield None if the file was empty
    if segment is not None:
        yield segment