Python - How to check if socket is still connected

Question

I have the following code, which is self explanatory:

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect(host, port)
s.send("some data")
# don't close socket just yet... 
# do some other stuff with the data (normal string operations)
if s.stillconnected() is true:
    s.send("some more data")
if s.stillconnected() is false:
    # recreate the socket and reconnect
    s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    s.connect(host, port)
    s.send("some more data")
s.close()

How do I implement s.stillconnected() I do not wish to recreate the socket blindly.

Answer 1

If the server connection is no longer alive, calling the send method will throw an exception, so you can use a try-exception block to attempt to send data, catch the exception if it's thrown, and reestablish the connection:

try:
    s.send("some more data")
except:
    # recreate the socket and reconnect
    s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    s.connect(host, port)
    s.send("some more data")

EDIT: As per @Jean-Paul Calderone's comments, please consider using the sendall method, which is a higher level method that sends all the data or throws an error, instead of send, which is a lower level method that does not guarantee the transmission of all the data, OR use higher level modules like an HTTP library that can handle socket lifecycles.

Answer 2

I've had good results with this variant to check if a socket is closed (negate the result if you want to check if it's still connected):

import logging
import socket


logger = logging.getLogger(__name__)


def is_socket_closed(sock: socket.socket) -> bool:
    try:
        # this will try to read bytes without blocking and also without removing them from buffer (peek only)
        data = sock.recv(16, socket.MSG_DONTWAIT | socket.MSG_PEEK)
        if len(data) == 0:
            return True
    except BlockingIOError:
        return False  # socket is open and reading from it would block
    except ConnectionResetError:
        return True  # socket was closed for some other reason
    except Exception as e:
        logger.exception("unexpected exception when checking if a socket is closed")
        return False
    return False