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IPC

brianc at temple.edu brianc at temple.edu
Tue Jul 27 15:08:52 EDT 2004 
Yannick,
There's two modules that I like to use for this kind of stuff:
http://starship.python.net/crew/tmick/ (process.py)
http://www.lysator.liu.se/~astrand/popen5/ (also process.py
just to be confusing)

Hopefully one of these will make it into the standard library.
I honestly like them both and combine parts from both when I
need to create my own classes. 

The reason your program hangs is because .readlines() will
keep reading untill an EOF(ie: program exits) is reached and
since the pipe is still open it never will be. (The example
given in the popen2 module,
http://docs.python.org/lib/popen2-flow-control.html, eludes to
this but never actually says it.)

I recently did a quick hack using select and popen2, but I
doubt it's fast or even safe, but it works.

#yadda yadda
r, w, e = popen2.popen3(cmd)
cmd = "?\n"
w.write(cmd)
w.flush()
#continuely test for output to read
while select.select([e],[],[],1)[0]:#timeout guessing game
    line=e.readline()
    #do something with line
while select.select([r],[],[],1)[0]:#timeout guessing game
    line=r.readline()
    #do something with line

If you're output is line buffered you probably shouldn't have
many problems with this approach if you can be assured your
program will respond within the 1 second alloted to it. If
you're positive you'll get output, just remove the timeout.

Hope this helps.

-Brian



---- Original message ----
>Date: Tue, 27 Jul 2004 14:20:19 -0400
>From: "Yannick Turgeon" <nobody at nowhere.com>  
>Subject: Re: IPC  
>To: python-list at python.org
>
>Larry,
>
>As I said, I use FTP only to test the IPC and give here a
known example.
>It's in fact with a custom program that I have to communicate.
>
>Any help in this regard?
>
>Yannick
>
>"Larry Bates" <lbates at swamisoft.com> wrote in message
>news:e9OdnbY0ddteC5vcRVn-sw at comcast.com...
>> Python has built in support for ftp (see ftplib)
>> use it instead of trying to "communicate" with
>> external FTP program.
>>
>> http://www.python.org/doc/current/lib/module-ftplib.html
>>
>> You can catch any exceptions (like failure to connect)
>> by using python try:/except: blocks.
>>
>> HTH,
>> Larry Bates
>> Syscon, Inc.
>>
>> "Yannick Turgeon" <nobody at nowhere.com> wrote in message
>> news:Z6wNc.21572$i_2.899460 at news20.bellglobal.com...
>> > Hi,
>> >
>> > I'm relatively new to Python. I'm using 2.3.4 on W2K.
>> >
>> > What I want to do is to start a program and interact with
it. Say my
>> program
>> > is FTP, I want to start FTP then send the commande "open
x.x.x.x" then
>> look
>> > for the answer (if the connection is opened or not), then
do something
>> > dependant of the success or error.
>> >
>> > I tried with popen3. The problem I got with this: it
seems that I have
>to
>> > end the program before being able to read the output. Or
maybe I'm not
>> using
>> > it correctly. I do test the communication with FTP exec.
but it will be
>a
>> > custom program in real. Here is my code:
>> >
>> >
>> >     def test(self):
>> >         cmd = "ftp"
>> >         r, w, e = popen2.popen3(cmd)
>> >
>> >         cmd = "?\n"            # A simple FTP commande
>> >         w.write(cmd)
>> >         w.flush()
>> >
>> >         # That is what I would like but it's hanging
here. I have to
>> remove
>> > this group and read at the end.
>> >         for line in e.readlines():
>> >             # Do something conditionnal to the result of
"line"
>> >             pass
>> >         for line in r.readlines():
>> >             # Do something conditionnal to the result of
"line"
>> >             pass
>> >
>> >
>> >         cmd = "quit\n"
>> >         w.write(cmd)
>> >         w.flush()
>> >
>> >         for line in e.readlines():
>> >             print line
>> >         for line in r.readlines():
>> >             print line
>> >
>> >         w.close()
>> >         r.close()
>> >         e.close()
>> > ----------------------
>> >
>> > Anybody can help? Thanks for your time.
>> >
>> > Yannick
>> >
>> >
>>
>>
>
>
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