python socket readline without socket.makefile()

SocketStreamReader

Here is a (buffered) line-reader that does not use asyncio. It can be used as a "synchronous" socket-based replacement for asyncio.StreamReader.

import socket
from asyncio import IncompleteReadError  # only import the exception class


class SocketStreamReader:
    def __init__(self, sock: socket.socket):
        self._sock = sock
        self._recv_buffer = bytearray()

    def read(self, num_bytes: int = -1) -> bytes:
        raise NotImplementedError

    def readexactly(self, num_bytes: int) -> bytes:
        buf = bytearray(num_bytes)
        pos = 0
        while pos < num_bytes:
            n = self._recv_into(memoryview(buf)[pos:])
            if n == 0:
                raise IncompleteReadError(bytes(buf[:pos]), num_bytes)
            pos += n
        return bytes(buf)

    def readline(self) -> bytes:
        return self.readuntil(b"\n")

    def readuntil(self, separator: bytes = b"\n") -> bytes:
        if len(separator) != 1:
            raise ValueError("Only separators of length 1 are supported.")

        chunk = bytearray(4096)
        start = 0
        buf = bytearray(len(self._recv_buffer))
        bytes_read = self._recv_into(memoryview(buf))
        assert bytes_read == len(buf)

        while True:
            idx = buf.find(separator, start)
            if idx != -1:
                break

            start = len(self._recv_buffer)
            bytes_read = self._recv_into(memoryview(chunk))
            buf += memoryview(chunk)[:bytes_read]

        result = bytes(buf[: idx + 1])
        self._recv_buffer = b"".join(
            (memoryview(buf)[idx + 1 :], self._recv_buffer)
        )
        return result

    def _recv_into(self, view: memoryview) -> int:
        bytes_read = min(len(view), len(self._recv_buffer))
        view[:bytes_read] = self._recv_buffer[:bytes_read]
        self._recv_buffer = self._recv_buffer[bytes_read:]
        if bytes_read == len(view):
            return bytes_read
        bytes_read += self._sock.recv_into(view[bytes_read:])
        return bytes_read

Usage:

reader = SocketStreamReader(sock)
line = reader.readline()

Four and a half years later, I would suggest asyncio's Streams for this, but here's how you might do it properly using BytesIO

Note that this implementation "shrinks" the in-memory BytesIO object each time a line is detected. If you didn't care about that, this could be a lot fewer lines.

import socket
import time
from io import BytesIO

sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock.connect(('localhost', 1234))
sock.setblocking(False)


def handle_line(line):
    # or, print("Line Received:", line.decode().rstrip())
    print(f"Line Received: {line.decode().rstrip()!r}")


with BytesIO() as buffer:
    while True:
        try:
            resp = sock.recv(100)       # Read in some number of bytes -- balance this
        except BlockingIOError:
            print("sleeping")           # Do whatever you want here, this just
            time.sleep(2)               #   illustrates that it's nonblocking
        else:
            buffer.write(resp)          # Write to the BytesIO object
            buffer.seek(0)              # Set the file pointer to the SoF
            start_index = 0             # Count the number of characters processed
            for line in buffer:
                start_index += len(line)
                handle_line(line)       # Do something with your line

            """ If we received any newline-terminated lines, this will be nonzero.
                In that case, we read the remaining bytes into memory, truncate
                the BytesIO object, reset the file pointer and re-write the
                remaining bytes back into it.  This will advance the file pointer
                appropriately.  If start_index is zero, the buffer doesn't contain
                any newline-terminated lines, so we set the file pointer to the
                end of the file to not overwrite bytes.
            """
            if start_index:
                buffer.seek(start_index)
                remaining = buffer.read()
                buffer.truncate(0)
                buffer.seek(0)
                buffer.write(remaining)
            else:
                buffer.seek(0, 2)

(The original answer was so bad that it wasn't worth keeping (I promise), but should be available in the edit history).