python - real time subprocess.Popen via stdout and PIPE

Question

I am trying to grab stdout from a subprocess.Popen call and although I am achieving this easily by doing:

cmd = subprocess.Popen('ls -l', shell=True, stdout=PIPE)
for line in cmd.stdout.readlines():
    print line

I would like to grab stdout in "real time". With the above method, PIPE is waiting to grab all the stdout and then it returns.

So for logging purposes, this doesn't meet my requirements (e.g. "see" what is going on while it happens).

Is there a way to get line by line, stdout while is running? Or is this a limitation of subprocess(having to wait until the PIPE closes).

EDIT If I switch readlines() for readline() I only get the last line of the stdout (not ideal):

In [75]: cmd = Popen('ls -l', shell=True, stdout=PIPE)
In [76]: for i in cmd.stdout.readline(): print i
....: 
t
o
t
a
l

1
0
4

Answer 1

Your interpreter is buffering. Add a call to sys.stdout.flush() after your print statement.