We Keep Coding

python socket programming for transferring a photo

I'm new to socket programming in python. Here is an example of opening a TCP socket in a Mininet host and sending a photo from one host to another. In fact I changed the code that I had used to send a simple message to another host (writing the received data to a text file) in order to meet my requirements. Although when I implement this revised code, there is no error and it seems to transfer correctly, I am not sure whether this is a correct way to do this transmission or not. Since I'm running both hosts on the same machine, I thought it may have an influence on the result. I wanted to ask you to check whether this is a correct way to transfer or I should add or remove something.
mininetSocketTest.py
#!/usr/bin/python
from mininet.topo import Topo, SingleSwitchTopo
from mininet.net import Mininet
from mininet.log import lg, info
from mininet.cli import CLI
def main():
lg.setLogLevel('info')
net = Mininet(SingleSwitchTopo(k=2))
net.start()
h1 = net.get('h1')
p1 = h1.popen('python myClient2.py')
h2 = net.get('h2')
h2.cmd('python myServer2.py')
CLI( net )
#p1.terminate()
net.stop()
if __name__ == '__main__':
main()
myServer2.py
import socket
import sys
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
s.bind(('10.0.0.1', 12345))
buf = 1024
f = open("2.jpg",'wb')
s.listen(1)
conn , addr = s.accept()
while 1:
data = conn.recv(buf)
print(data[:10])
#print "PACKAGE RECEIVED..."
f.write(data)
if not data: break
#conn.send(data)
conn.close()
s.close()
myClient2.py:
import socket
import sys
f=open ("1.jpg", "rb")
print sys.getsizeof(f)
buf = 1024
data = f.read(buf)
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect(('10.0.0.1',12345))
while (data):
if(s.sendall(data)):
#print "sending ..."
data = f.read(buf)
print(f.tell(), data[:10])
else:
s.close()
s.close()

This loop in client2 is wrong:
while (data):
if(s.send(data)):
print "sending ..."
data = f.read(buf)
As the send
docs say:
Returns the number of bytes sent. Applications are responsible for checking that all data has been sent; if only some of the data was transmitted, the application needs to attempt delivery of the remaining data. For further information on this topic, consult the Socket Programming HOWTO.
You're not even attempting to do this. So, while it probably works on localhost, on a lightly-loaded machine, with smallish files, it's going to break as soon as you try to use it for real.
As the help says, you need to do something to deliver the rest of the buffer. Since there's probably no good reason you can't just block until it's all sent, the simplest thing to do is to call sendall:
Unlike send(), this method continues to send data from bytes until either all data has been sent or an error occurs. None is returned on success. On error, an exception is raised…
And this brings up the next problem: You're not doing any exception handling anywhere. Maybe that's OK, but usually it isn't. For example, if one of your sockets goes down, but the other one is still up, do you want to abort the whole program and hard-drop your connection, or do you maybe want to finish sending whatever you have first?
You should at least probably use a with clause of a finally, to make sure you close your sockets cleanly, so the other side will get a nice EOF instead of an exception.
Also, your server code just serves a single client and then quits. Is that actually what you wanted? Usually, even if you don't need concurrent clients, you at least want to loop around accepting and servicing them one by one.
Finally, a server almost always wants to do this:
s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
Without this, if you try to run the server again within a few seconds after it finished (a platform-specific number of seconds, which may even depend whether it finished with an exception instead of a clean shutdown), the bind will fail, in the same way as if you tried to bind a socket that's actually in use by another program.

First of all, you should use TCP and not UDP. TCP will ensure that your client/server has received the whole photo properly. UDP is more used for content streaming.
Absolutely not your use case.

Python Sockets, requesting file from server then waiting to receive it

I am attempting to send a string to my server from my client with a specific filename and then send that file to the client. For some reason it hangs even after it's received all of the file. It hangs on the:
m = s.recv(1024)
client.py
import socket
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect(("192.168.1.2", 54321))
s.send(b"File:test.txt")
f = open("newfile.txt", "wb")
data = None
while True:
m = s.recv(1024)
data = m
if m:
while m:
m = s.recv(1024)
data += m
else:
break
f.write(data)
f.close()
print("Done receiving")
server.py
import socket
import os
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.bind(("", 54321))
while True:
client_input = c.recv(1024)
command = client_input.split(":")[0]
if command == "File":
command_parameter = client_input.split(":")[1]
f = open(command_parameter, "rb")
l = os.path.getsize(command_parameter)
m = f.read(l)
c.sendall(m)
f.close()

TLDR
The reason recv blocks is because the socket connection is not shutdown after the file data was sent. The implementation currently has no way to know when the communication is over, which results in a deadlock between the two, remote processes. To avoid this, close the socket connection in the server, which will generate an end-of-file event in the client (i.e. recv returns a zero-length string).
More insight
Whenever you design any software where two processes communicate with each other, you have to define a protocol that disambiguates the communication such that both peers know exactly which state they are in at all times. Typically this involves using the syntax of the communication to help guide the interpretation of the data.
Currently, there are some problems with your implementation: it doesn't define an adequate protocol to resolve potential ambiguity. This becomes apparent when you consider the fact that each call to send in one peer doesn't necessarily correspond to exactly one call to recv in the other. That is, the calls to send and recv are not necessarily one-to-one. Consider sending the file name to the server on a heavily congested network: perhaps only half of the file name makes it to the server when the first call to recv returns. The server has no way (currently) to know if it has finished receiving the file name. The same is true in the client: how does the client know when the file has finished?
To work around this, we can introduce some syntax into the protocol and some logic into the server to ensure we get the complete file name before continuing. A simple solution would be to use an EOL character, i.e. \n to denote the end of the client's message. Now, 99.99% of the time in your testing this will take a single call to recv to read in. However you have to anticipate the cases in which it might take more than one call to recv. This can be implemented using a loop, obviously.
The client end is simpler for this demo. If the communication is over after the sending of the file, then that event can be used to denote the end of the data stream. This happens when the server closes the connection on its end.
If we were to expand the implementation to, say, allow for requests for multiple, back-to-back files, then we'd have to introduce some mechanism in the protocol for distinguishing the end of one file and the beginning of the next. Note that this also means the server would need to potentially buffer extra bytes that it reads in on previous iterations in case there is overlap. A stream implementation is generally useful for these sorts of things.

What is the best way to handle multiple socket connections

What I need to do is read x amount of accounts from a file based on the amount of lines and make x amount of individual sockets that I can manipulate as much as I like (send messages to IRC and anything else)
How I'm going about it as of now:
lines=tuple(open('accts.txt', 'r'))
for line in lines:
data=line.split(' ',1)
a=threading.Thread(target=Spawn,args=(data[0].replace('\n',''),data[1].replace('\n','')))
a.start()
#s.send wont work here because it doesn't exist in this context
I tried to use threads but it seems threads don't allow you to access them from outside of the thread itself from what I understand
Must support a while True: in a thread but I can live w/o it if its not posible
Here is the Spawn function that was being created by the thread:
def Spawn(nick,password):
Active=True
s=socket.socket(socket.AF_INET,socket.SOCK_STREAM)
s.connect(('irc.boats.gov',6667))
s.send('PASS '+password+'\r\n')
s.send('NICK '+nick+'\r\n')
s.send('JOIN '+channel+'\r\n')
while True:
buf=s.recv(1024)
if('PRIVMSG' in buf):
sender=buf.split('!',1)[0].split(':')
message=buf.split(':',2)[2].replace('\n','')
if(sender[1]==owner):
if(sender[1]==owner):
if(message.strip()=='!stop'):
Active=False
print '('+nick+')'+' has been disabled'
else:
if(message.strip()=='!start'):
Active=True
print '('+nick+')'+' has been enabled'
else:
if(Active):
print 'sent'

If you want to create multiple connections you can do it like this:
from socket import *
SERVER = ('irc.boats.gov',6667) # Server address
# Open up connections
connections = []
with open('accts.txt', 'r') as f:
for line in f:
s = socket(AF_INET,SOCK_STREAM)
s.connect(SERVER)
connections.append(s)
s.send('PASS '+password+'\r\n')
s.send('NICK '+nick+'\r\n')
s.send('JOIN '+channel+'\r\n')
Then you can do whatever you want with them with select module for example. Threads won't help much here and can even degrade performance. You could also try Twisted, as suggested or use multiple processes.
Here is a nice related read from David Beazley on concurrency, I adapted the code from it.

Can select() be used with files in Python under Windows?

I am trying to run the following python server under windows:
"""
An echo server that uses select to handle multiple clients at a time.
Entering any line of input at the terminal will exit the server.
"""
import select
import socket
import sys
host = ''
port = 50000
backlog = 5
size = 1024
server = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
server.bind((host,port))
server.listen(backlog)
input = [server,sys.stdin]
running = 1
while running:
inputready,outputready,exceptready = select.select(input,[],[])
for s in inputready:
if s == server:
# handle the server socket
client, address = server.accept()
input.append(client)
elif s == sys.stdin:
# handle standard input
junk = sys.stdin.readline()
running = 0
else:
# handle all other sockets
data = s.recv(size)
if data:
s.send(data)
else:
s.close()
input.remove(s)
server.close()
I get the error message (10038, 'An operation was attempted on something that is not a socket'). This probably relates back to the remark in the python documentation that "File objects on Windows are not acceptable, but sockets are. On Windows, the underlying select() function is provided by the WinSock library, and does not handle file descriptors that don’t originate from WinSock.". On internet there are quite some posts on this topic, but they are either too technical for me or simply not clear. So my question is: is there any way the select() statement in python can be used under windows? Please add a little example or modify my code above. Thanks!

Look like it does not like sys.stdin
If you change input to this
input = [server]
the exception will go away.
This is from the doc
Note:
File objects on Windows are not acceptable, but sockets are. On Windows, the
underlying select() function is provided by the WinSock library, and does not
handle file descriptors that don’t originate from WinSock.

I don't know if your code has other problems, but the error you're getting is because of passing input to select.select(), the problem is that it contains sys.stdin which is not a socket. Under Windows, select only works with sockets.
As a side note, input is a python function, it's not a good idea to use it as a variable.

Of course and the answers given are right...
you just have to remove the sys.stdin from the input but still use it in the iteration:
for s in inputready+[sys.stdin]:

non-blocking read/log from an http stream

I have a client that connects to an HTTP stream and logs the text data it consumes.
I send the streaming server an HTTP GET request... The server replies and continuously publishes data... It will either publish text or send a ping (text) message regularly... and will never close the connection.
I need to read and log the data it consumes in a non-blocking manner.
I am doing something like this:
import urllib2
req = urllib2.urlopen(url)
for dat in req:
with open('out.txt', 'a') as f:
f.write(dat)
My questions are:
will this ever block when the stream is continuous?
how much data is read in each chunk and can it be specified/tuned?
is this the best way to read/log an http stream?

Hey, that's three questions in one! ;-)
It could block sometimes - even if your server is generating data quite quickly, network bottlenecks could in theory cause your reads to block.
Reading the URL data using "for dat in req" will mean reading a line at a time - not really useful if you're reading binary data such as an image. You get better control if you use
chunk = req.read(size)
which can of course block.
Whether it's the best way depends on specifics not available in your question. For example, if you need to run with no blocking calls whatever, you'll need to consider a framework like Twisted. If you don't want blocking to hold you up and don't want to use Twisted (which is a whole new paradigm compared to the blocking way of doing things), then you can spin up a thread to do the reading and writing to file, while your main thread goes on its merry way:
def func(req):
#code the read from URL stream and write to file here
...
t = threading.Thread(target=func)
t.start() # will execute func in a separate thread
...
t.join() # will wait for spawned thread to die
Obviously, I've omitted error checking/exception handling etc. but hopefully it's enough to give you the picture.

You're using too high-level an interface to have good control about such issues as blocking and buffering block sizes. If you're not willing to go all the way to an async interface (in which case twisted, already suggested, is hard to beat!), why not httplib, which is after all in the standard library? HTTPResponse instance .read(amount) method is more likely to block for no longer than needed to read amount bytes, than the similar method on the object returned by urlopen (although admittedly there are no documented specs about that on either module, hmmm...).

Another option is to use the socket module directly. Establish a connection, send the HTTP request, set the socket to non-blocking mode, and then read the data with socket.recv() handling 'Resource temporarily unavailable' exceptions (which means that there is nothing to read). A very rough example is this:
import socket, time
BUFSIZE = 1024
s = socket.socket()
s.connect(('localhost', 1234))
s.send('GET /path HTTP/1.0\n\n')
s.setblocking(False)
running = True
while running:
try:
print "Attempting to read from socket..."
while True:
data = s.recv(BUFSIZE)
if len(data) == 0: # remote end closed
print "Remote end closed"
running = False
break
print "Received %d bytes: %r" % (len(data), data)
except socket.error, e:
if e[0] != 11: # Resource temporarily unavailable
print e
raise
# perform other program tasks
print "Sleeping..."
time.sleep(1)
However, urllib.urlopen() has some benefits if the web server redirects, you need URL based basic authentication etc. You could make use of the select module which will tell you when there is data to read.

Yes when you catch up with the server it will block until the server produces more data
Each dat will be one line including the newline on the end
twisted is a good option
I would swap the with and for around in your example, do you really want to open and close the file for every line that arrives?