Asynchronously read stdout from subprocess.Popen

Question

I am running a sub-program using subprocess.popen. When I start my Python program from the command window (cmd.exe), the program writes some info and dates in the window as the program evolves.

When I run my Python code not in a command window, it opens a new command window for this sub-program's output, and I want to avoid that. When I used the following code, it doesn't show the cmd window, but it also doesn't print the status:

p = subprocess.Popen("c:/flow/flow.exe", shell=True, stdout=subprocess.PIPE)
print p.stdout.read()

How can I show the sub-program's output in my program's output as it occurs?

Answer 1

Use this:

cmd = subprocess.Popen(["c:/flow/flow.exe"], stdout=subprocess.PIPE)
for line in cmd.stdout:
    print line.rstrip("\n")
cmd.wait()  # you may already be handling this in your current code

Note that you will still have to wait for the sub-program to flush its stdout buffer (which is commonly buffered differently when not writing to a terminal window), so you may not see each line instantaneously as the sub-program prints it (this depends on various OS details and details of the sub-program).

Also notice how I've removed the shell=True and replaced the string argument with a list, which is generally recommended.