readline_from_read.py

import io


def gen_lines(readable, chunk_size=8192, sep=b'\n'):
    buf = io.BytesIO()

    while 1:
        buf.seek(0)
        buffered = buf.read()

        idx = buffered.find(sep)
        if idx >= 0:
            idx += len(sep)
            buf.seek(0)
            buf.truncate()
            buf.write(buffered[idx:])
            yield buffered[:idx]
            continue

        data = readable.read(chunk_size)
        if data == b'': # EOF
            break
        buf.write(data)

    # only broke out of the loop after hitting EOF with no sep,
    # so if there's anything dangling pass that out as well.
    if buffered:
        yield buffered


class LineReader(object):
    def __init__(self, readable, chunk_size=8192, sep=b'\n'):
        self._gen = gen_lines(readable, chunk_size, sep)

    def __iter__(self):
        return self._gen

    def readline(self):
        return next(self._gen)