what happens to a python object when you throw an exception from it

Question

My class contains a socket that connects to a server. Some of the methods of the class can throw an exception. The script I'm running contains an outer loop that catches the exception, logs an error, and creates a new class instance that tries to reconnect to the server.

Problem is that the server only handles one connection at a time (by design) and the "old" socket is still connected. So the new connection attempt hangs the script. I can work around this by forcing the old socket closed, but I wonder: why doesn't the socket automatically close?

When it is "stuck", netstat shows two sockets connected to the port. The server is waiting for input from the first socket though, it isn't handling the new one yet.

I run this against a dummy server that replies "error\n" to every incoming line.

EDIT: see my comment on Mark Rushakoff's answer below. An assert(False) [that I subsequently catch] from within the exception handler seems to force the socket closed.

import socket

class MyException(Exception):
    pass

class MyClient(object):

    def __init__(self, port):
        self.sock = socket.create_connection(('localhost', port))
        self.sockfile = self.sock.makefile()

    def do_stuff(self):
        self._send("do_stuff\n")
        response = self._receive()
        if response != "ok\n":
            raise MyException()
        return response

    def _send(self, cmd):
        self.sockfile.write(cmd)
        self.sockfile.flush()

    def _receive(self):
        return self.sockfile.readline()

def connect():
    c = MyClient(9989)
    # On the second iteration, do_stuff() tries to send data and
    # hangs indefinitely.
    print c.do_stuff()

if __name__ == '__main__':
    for _ in xrange(3):
        try:
            connect()
        except MyException, e:
            print 'Caught:', e
            # This would be the workaround if I had access to the
            # MyClient object:
            #c.sock.close()
            #c.sockfile.close()

EDIT: Here's the (ugly) server code:

import socket
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM, 0)
s.bind(('localhost', 9989))
s.listen(5)
(c,a) = s.accept()
f = c.makefile()
print f.readline()
f.write('error\n')
f.flush()
(c2,a) = s.accept()
f = c.makefile()
print f.readline()
s.close()

Answer 1

This is an artifact of garbage collection. Even though the object is out of scope, it is not necessarily collected and therefore destroyed until a garbage collection run occurs -- this is not like C++ where a destructor is called as soon as an object loses scope.

You can probably work around this particular issue by changing connect to

def connect():
    try:
        c = MyClient(9989)
        # On the second iteration, do_stuff() tries to send data and
        # hangs indefinitely.
        print c.do_stuff()
    finally:
        c.sock.close()
        c.sockfile.close()

Alternatively, you could define __enter__ and __exit__ for MyClient, and do a with statement:

def connect():
    with MyClient(9989) as c:
        print c.do_stuff()

Which is effectively the same as a try-finally.

Answer 2

Ok, here's the final version. Explicitly close the socket objects when something gets borked.

import socket

class MyException(Exception):
    pass

class MyClient(object):

    def __init__(self, port):
        self.sock = socket.create_connection(('localhost', port))
        self.sockfile = self.sock.makefile()

    def close(self):
        self.sock.close()
        self.sockfile.close()

    def do_stuff(self):
        self._send("do_stuff\n")
        response = self._receive()
        if response != "ok\n":
            raise MyException()
        return response

    def _send(self, cmd):
        self.sockfile.write(cmd)
        self.sockfile.flush()

    def _receive(self):
        return self.sockfile.readline()

def connect():
    try:
        c = MyClient(9989)
        print c.do_stuff()
    except MyException:
        print 'Caught MyException'
    finally:
        c.close()

if __name__ == '__main__':
    for _ in xrange(2):
        connect()

Answer 3

The garbage collector can be flagged to clean up by setting the relevant object to None.

Answer 4

Can you really handle the Exception in connect()?

I think you should provide a MyClient.close() method, and write connect() like this:

def connect():
    try:
        c = MyClient(9989)
        print c.do_stuff()
    finally:
        c.close()

This is in complete analogy with file-like objects (and the with statement)