Processing messages from a child process thorough stderr and stdout with Python

Question

My python code spawns the child process, and it prints out messages both stdout and stderr. I need to print them differently.

I have the following code to spawn child process and get the stdout result from it.

cmd = ["vsmake.exe", "-f"]
p = subprocess.Popen(cmd, stdout=subprocess.PIPE)
for line in iter(p.stdout.readline, ''):
    print line,
    sys.stdout.flush()
    pass
p.wait()

How can I modify the code to check if the child process prints out message through stderr also?

ADDED

I need to print out the stderr and stdout as soon as the child process prints out something. And it is cross platform implementation, so it should run on Mac/Linux/PC.

Answer 1

p = Popen(cmd, bufsize=1024,
stdin=PIPE, stdout=PIPE, stderr=PIPE, close_fds=True)
p.stdin.close()
print p.stdout.read() #This will print the standard output from the spawned process
print p.stderr.read() #This is what you need, error output <-----

So basically the error output gets redirected to the stderr Pipe.

If you need something more in real in time. I mean lines printed as soon as the spawned process prints something to stdout orstderr` then you can do something like:

def print_pipe(type_pipe,pipe):
    for line in iter(pipe.readline, ''):
         print "[%s] %s"%(type_pipe,line),

p = Popen(cmd, bufsize=1024,
stdin=PIPE, stdout=PIPE, stderr=PIPE, close_fds=True)

t1 = Thread(target=print_pipe, args=("stdout",p.stdout,))
t1.start()
t2 = Thread(target=print_pipe, args=("stderr",p.stderr,))
t2.start()

#optionally you can join the threads to wait till p is done. This is avoidable but it 
# really depends on the application.
t1.join()
t2.join()

In this case two threads will print every time that a line is written either to stdout or stderr. The parameter type_pipe just makes the distinction when the lines are printed to know if they are coming from stderr or stdout.

Answer 2

The easiest way to do this platform-independently is using threads (unfortunately). Here is some example code:

def redirect_to_stdout(stream):
    for line in stream:
        sys.stdout.write(line)
        sys.stdout.flush()

cmd = ["vsmake.exe", "-f"]
p = subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
stderr_thread = threading.Thread(target=redirect_to_stdout, args=(p.stderr,))
stderr_thread.start()
redirect_to_stdout(p.stdout)
p.wait()
stderr_thread.join()