help with popen, one-way IPC.

[Roeland Rengelink]

Brian Alexander wrote:
> 
> Hello;
> 
> I have a program that might possibly be writing to stdout (not a python
> program) and would like to use redirection in Python to collected the
> output. I'm having a little trouble with the call to popen(), though.
> 
> I thought it was this:
> 
> otherapp_stdout = popen( "appname", "r")
> 
> while otherapp_stdout.readline != "\n"
>     # do stuff with each line.
> 
> Is this the right idea. Communication only needs to be one way.
> 
> Many thanks in advance,
> 
> Brian.

Hi Brian,

You're on the right track.
There are basically two ways to do this.

import os
otherapp_stdout = os.popen( "appname", "r")
lines = otherapp_stdout.readlines()      # get all output at once
for line in lines:
    print line

or:

import os
otherapp_stdout = os.popen( "appname", "r")

while 1:
    line = otherapp_stdout.readline()   # get them one at a time
    if line == '':        # an empty line means EOF
        break
    print line

Because assignment is a statement, there is no result that can be used
as a condition for while or if

Note the difference between readlines() (returning a list of lines)
and readline() (returning one line at a time). 

Also note that an empty line denotes end-of-file. Don't confuse a blank
line (i.e. containing only the newline character '\n') for the empty
line marking EOF

Hope this helps.

Roeland