Python sockets, how to escape from infinite loop and handle exceptions

Question

I have a script that connects to a remote server. The code is below

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect((remote_host,remote_port))
s.setblocking(False)


while True:
    try:
        data = s.recv(1024)

        if not data:
           break

        pkt_type = ord(data[2]) # get pkt type

        if pkt_type == Reset:
           s.send(data)

        if pkt_type == Authenticate:
           processAuthenticate(s,data)
           break

    except:
        pass

while(True)
 .
 .
 .

I wait for reset and echo back to the server, then wait for an Authenticate packet, twiddle a few bit and echo that back to the server. Once this is done successfully I can now request data from the the server. This is done in the next while(true) loop.

Is this the best way of doing this. Sometimes when I run the script I get an error, what is the simplest way of handling the exception and preventing execuation of the next wile loop?

Thanks

Could you please paste complete code

Kimvais
– Kimvais

2010-01-20 10:09:01 +00:00
Commented Jan 20, 2010 at 10:09 — Kimvais
– Kimvais, Commented Jan 20, 2010 at 10:09

Andrew McGregor · Accepted Answer · 2010-01-20 09:48:33Z

1

A Finite State Machine (FSM) is pretty much the canonical way to do this sort of thing. A good reference for doing FSMs in Python is this: http://wiki.python.org/moin/FiniteStateMachine

answered Jan 20, 2010 at 9:48

Andrew McGregor

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Kimvais · Accepted Answer · 2010-01-20 10:49:08Z

0

EDIT: ~~Looks like a FSM should be handy here.~~

Actually, I suggest you take a look at Twisted Reactor - haven't used it myself (yet), but it does all the bulk work and nasty stuff that you would have to implement yourself if using a FSM and an event loop (which your while -loop essentially is)

EDIT 2:

A few notes (while waiting for your complete code)

the fact that you have 2 consecutive while(true) is, well, odd
you probably want to move the except: statement up, before the if not data statement and replace pass with continue
ord(data[2]) suggests that you are using a binary protocol, you really should consider using the struct modules unpack() and pack() instead.

edited Jan 20, 2010 at 10:49

answered Jan 20, 2010 at 9:32

Kimvais

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5 Comments

mikip Over a year ago

Thanks, Do you have an example of an FSM using sockets or could you point me in the right direction

mikip Over a year ago

Thanks but I am a python beginner and would like to use the standard library and the socket library to do this. What I have got is sort of working but not very elegant. I am sort of on a steep learning curve as I have not used sockets before and only just starting to learn python

mikip Over a year ago

Thanks for the advice. This is essentially the complete code, the rest of it is not really relevant. When you say move the except statement 'up' , do you mean just before the 'if not data:' statement. The reason I used 'pass' is that I know something should go here, but I dont know what?

Kimvais Over a year ago

Yes, that's where it should go - and continue just means that "skip to the beginning of the loop" - i.e. skip the packet processing.

mikip Over a year ago

Thanks for your patience and advice Kimvais

Kylotan · Accepted Answer · 2010-01-20 11:45:13Z

0

In addition to the above tips, you need to buffer the data - when using a stream protocol you can't just assume that you will get all the data you want in one call to recv. Instead you must take what you read from recv and add it to a buffer, then examine the buffer to see if it contains all the data for a message yet. If it does, extract the message, handle it, then repeat with the rest of the buffer. When you can't extract any further messages, you go back and read more from the socket.

answered Jan 20, 2010 at 11:45

Kylotan

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