What does Python's socket.recv() return for non-blocking sockets if no data is received until a timeout occurs?

In the case of a non blocking socket that has no data available, recv will throw the socket.error exception and the value of the exception will have the errno of either EAGAIN or EWOULDBLOCK. Example:

import sysimport socketimport fcntl, osimport errnofrom time import sleeps = socket.socket(socket.AF_INET, socket.SOCK_STREAM)s.connect(('127.0.0.1',9999))fcntl.fcntl(s, fcntl.F_SETFL, os.O_NONBLOCK)while True:    try:        msg = s.recv(4096)    except socket.error, e:        err = e.args[0]        if err == errno.EAGAIN or err == errno.EWOULDBLOCK:            sleep(1)            print 'No data available'            continue        else:            # a "real" error occurred            print e            sys.exit(1)    else:        # got a message, do something :)

The situation is a little different in the case where you've enabled non-blocking behavior via a time out with socket.settimeout(n) or socket.setblocking(False). In this case a socket.error is stil raised, but in the case of a time out, the accompanying value of the exception is always a string set to 'timed out'. So, to handle this case you can do:

import sysimport socketfrom time import sleeps = socket.socket(socket.AF_INET, socket.SOCK_STREAM)s.connect(('127.0.0.1',9999))s.settimeout(2)while True:    try:        msg = s.recv(4096)    except socket.timeout, e:        err = e.args[0]        # this next if/else is a bit redundant, but illustrates how the        # timeout exception is setup        if err == 'timed out':            sleep(1)            print 'recv timed out, retry later'            continue        else:            print e            sys.exit(1)    except socket.error, e:        # Something else happened, handle error, exit, etc.        print e        sys.exit(1)    else:        if len(msg) == 0:            print 'orderly shutdown on server end'            sys.exit(0)        else:            # got a message do something :)

As indicated in the comments, this is also a more portable solution since it doesn't depend on OS specific functionality to put the socket into non-blockng mode.

See recv(2) and python socket for more details.

It is simple: if recv() returns 0 bytes; you will not receive any more data on this connection. Ever. You still might be able to send.

It means that your non-blocking socket have to raise an exception (it might be system-dependent) if no data is available but the connection is still alive (the other end may send).

When you use recv in connection with select if the socket is ready to be read from but there is no data to read that means the client has closed the connection.

Here is some code that handles this, also note the exception that is thrown when recv is called a second time in the while loop. If there is nothing left to read this exception will be thrown it doesn't mean the client has closed the connection :

def listenToSockets(self):    while True:        changed_sockets = self.currentSockets        ready_to_read, ready_to_write, in_error = select.select(changed_sockets, [], [], 0.1)        for s in ready_to_read:            if s == self.serverSocket:                self.acceptNewConnection(s)            else:                self.readDataFromSocket(s)

And the function that receives the data :

def readDataFromSocket(self, socket):    data = ''    buffer = ''    try:        while True:            data = socket.recv(4096)            if not data:                 break            buffer += data    except error, (errorCode,message):         # error 10035 is no data available, it is non-fatal        if errorCode != 10035:            print 'socket.error - ('+str(errorCode)+') ' + message    if data:        print 'received '+ buffer    else:        print 'disconnected'