Develop Reference

Python .readline() reset

In Python 3.4/Asyncio I'm using StreamReader/Writer.
To detect a client disconnect the common method seems to be to read from the client and if there's nothing there then the client disconnected.
while True:
data = (yield from asyncio.wait_for(client_reader.readline(),
timeout=1.0))
if not data: #client disconnected
break
However quickly you run out of lines to read from the client header (it moves to the next line on each loop) and if there are no additional lines sent from the client (in my case the client is not sending just listening) you hit the timeout.
What I would like to do is to only read the first line of the header over and over.. or possibly even just the first character of the first line, or if that's not possible when it gets to the last line loop back around to the first.
What's the best/most elegant way to accomplish this task? 3.4/Asyncio/StreamReader/Writer. (detecting client disconnects)

I had a similar problem. The way that worked for me was to check for EOF first and then raise a ConnectionError exception if true. So for your code I would add the following:
while True:
try:
if client_reader.at_eof():
raise ConnectionError
data = (yield from asyncio.wait_for(client_reader.readline(),
timeout=1.0))
if not data: #client disconnected
break
except ConnectionError:
break
except:
break # This is here to catch things like the asyncio futures timeout exception
Hope that helps. If anyone has a better way I'd be interested.

Optional input() statement

I'm creating an instant messenger program for my school's common drive. I have everything working except for on small detail. In the code below it checks for a new message from a friend and prints the last message they sent. If there are no messages it says so. The problem is when it moves to the next step of the code it waits for the user to put in an input. Until you give an input it won't let you receive any more messages because the program stops reading and searching the while loop and gets caught on the input statement. I want to know if there is anyway to make an input statement optional. To say that it doesn't require an input but if there is an input it will send it and do it's thing. I just can't seem to figure out a way to make the input statement optional. Any ideas or working code would be greatly appreciated. If you need the entire code I don't have a problem with sending it to you or posting it. This is the only bit of code that should really matter for this problem though.
LastMessage = ""
while Message:
Path = "Message"+SendTo+"-"+UserName+".txt"
if path.isfile(Path):
LoadMessage = open(Path, "rb")
NewMessage = pickle.load(LoadMessage)
LoadMessage.close()
else:
NewMessage = "Sorry, No messages found"
if LastMessage != NewMessage:
LastMessage = NewMessage
print(NewMessage)
print("")
SendMessage = raw_input() #--- This is where it gets caught up! ---
Save = open("Message"+UserName+"-"+SendTo+".txt", "wb")
pickle.dump(SendMessage, Save)
Save.close()

You have two main options as I see it:
Simultaneous input and checking (various options, search for e.g. threading or multiprocessing from the standard library); or
Input with timeout and loop (see e.g. How to set time limit on raw_input).

So it sounds like you want to do two separate things at the same time - look for input from a user and poll for new messages from other users. Jonrsharpe gives threading as his first option to solve this and I agree its the most straightforward. What you need to do is something like this:
import threading
class InputMessageThread(threading.Thread):
def run(self):
SendMessage = raw_input() # This thread will hang here for input but thats
# OK as original thread will keep going
Save = open("Message"+UserName+"-"+SendTo+".txt", "wb")
pickle.dump(SendMessage, Save)
Save.close()
inputthread = InputMessageThread()
inputthread.start()
# rest of your code here
While you are at it though you might want to look at some other issues. For example if I understand what you are trying to do correctly you are going to have a file containing a message from a source user to a destination user. But if the source user sends a second message before this file gets processed then the first message will be overwritten. In practice you may never see this but some sort of handshaking to make sure the message has actually been sent before you allow the next to be written would be a more robust approach.

How to avoid blocking using sockets (chat client)

I've been looking around all day, but I haven't been able to fix the problem I've got with my chat client here.
Here's the issue: I recently decided to change the client so that it would allow the user to input any message they wanted without having to wait for a reply first (blocking or something stops my program until a reply is in)
I decided to use the select.select module to do so, but after writing a couple different versions of my client today trying to get it to work, I keep getting stuck at this one point.
Whenever I enter a message, the loop gets stuck somewhere, (probably at .recv data)
how can I fix this? Nothing I try gets it to go by that.
Edit: To be more clear, when I run, I get to the point where I input the message, hit enter and then nothing happens at all. It just stays running like that.
from socket import *
import select
import sys #because why not?
print("New Chat Client Using Select Module")
HOST = input("Host: ")
PORT = int(input("Port: "))
s = socket(AF_INET,SOCK_STREAM)
print("Trying to connect....")
s.connect((HOST,PORT))
s.setblocking(0)
# Not including setblocking(0) because select handles that.
print("You just connected to",HOST,)
# Lets now try to handle the client a different way!
while True:
Incoming_data = [s]
Exportable_data = []
Exceptions = []
User_input = input("Your message: ")
rlist,wlist,xlist = select.select(Incoming_data,Exportable_data,Exceptions)
if User_input == True:
Exportable_data += [User_input]
for i in rlist:
data = i.recv(1024)
if data == "":
continue
for i in wlist:
if Exportable_data is True:
i.send(Exportable_data)
continue
Is there any way I can override the blocking (that I presume is the problem) when it's set to receive data? Won't s.setblocking(0) make it so it won't block(?) (With or without it still gets stuck)
Thanks for taking a look

I think you should have separate thread or process which will interact with your socket and another thread, which will accept user input and print chat messages. Interaction between the threads you can do using 2 queues: for incoming and outgoing messages.
Have a look at threading and queue modules.

why a new process entry then the events of old process stop running when sharing a listening socket for multiple processes?

The problem happened in my proxy program, Considering G10K I use gevent in my program and I use the low-level gevent.core to run all my function.
Before I change my program into multiple processes. everything is OK. But when I changed it, the problem appears.
I find the problem is that when process NO.2 accept the socket, then the events of process NO.1 will stop dispatch. And if I add a sleep(0.1) in my event, then came a surprise. BUT I lower the sleep time, the problem showed again.
The problem have bothered me for a weeks, still nothing to do with that, Could someone help me ?
I use event like that:
core.init()
self.ent_s_send = core.event(core.EV_WRITE,self.conn.fileno(),\
self.ser_send,[self.conn,self.body])
self.ent_s_send.add()
core.dispatch()

I think that the problem is in your code, because this code is working fine, with the same shared socket.
When you accept sa ocket with EV_READ, you must get the client socket and free the control over the main socket; you must not write to it. You should use code similar to the following one:
try:
client_socket, address = sock.accept()
except socket.error, err:
if err[0] == errno.EAGAIN:
sys.exc_clear()
return
raise
core.event(core.EV_READ, client_socket.fileno(), callback)
core.event(core.EV_WRITE, client_socket.fileno(), callback)
core.event(core.EV_READ | core.EV_WRITE, client_socket.fileno(), callback)
After this, set READ and WRITE events for this socket.

Python telnetlib: surprising problem

I am using the Python module telnetlib to create a telnet session (with a chess server), and I'm having an issue I really can't wrap my brain around. The following code works perfectly:
>>> f = login("my_server") #code for login(host) below.
>>> f.read_very_eager()
This spits out everything the server usually prints upon login. However, when I put it inside a function and then call it thus:
>>> def foo():
... f = login("my_server")
... return f.read_very_eager()
...
>>> foo()
I get nothing (the empty string). I can check that the login is performed properly, but for some reason I can't see the text. So where does it get swallowed?
Many thanks.
For completeness, here is login(host):
def login(host, handle="guest", password=""):
try:
f = telnetlib.Telnet(host) #connect to host
except:
raise Error("Could not connect to host")
f.read_until("login: ")
try:
f.write(handle + "\n\r")
except:
raise Error("Could not write username to host")
if handle == "guest":
f.read_until(":\n\r")
else:
f.read_until("password: ")
try:
f.write(password + "\n\r")
except:
raise Error("Could not write password to host")
return f

The reason why this works when you try it out manually but not when in a function is because when you try it out manually, the server has enough time to react upon the login and send data back. When it's all in one function, you send the password to the server and never wait long enough for the server to reply.
If you prefer a (probably more correct) technical answer:
In file telnetlib.py (c:\python26\Lib\telnetlib.py on my Windows computer), function read_very_eager(self) calls self.sock_avail() Now, function sock_avail(self) does the following:
def sock_avail(self):
"""Test whether data is available on the socket."""
return select.select([self], [], [], 0) == ([self], [], [])
What this does is really simple: if there is -anything- to read from our socket (the server has answered), it'll return True, otherwise it'll return False.
So, what read_very_eager(self) does is: check if there is anything available to read. If there is, then read from the socket, otherwise just return an empty string.
If you look at the code of read_some(self) you'll see that it doesn't check if there is any data available to read. It'll try reading till there is something available, which means that if the server takes for instance 100ms before answering you, it'll wait 100ms before returning the answer.

I'm having the same trouble as you, unfortunately the combination of select.select, which I have in a while loop until I am able to read, and then calling read_some() does not work for me, still only reading 1% of the actual output. If I put a time.sleep(10) on before I read and do a read_very_eager() it seems to work...this is a very crude way of doing things but it does work..I wish there was a better answer and I wish I had more reputation points so I could respond to user387821 and see if he has any additional tips.