Getting progress message from a subprocess

Question

I want to start a program which needs several minutes to complete. During this time I want to read the progress message of the program (which are printed on the stdout). The pro

Answer 1

It's certainly possible: my package python-gnupg does exactly this, spawning gpg (Gnu Privacy Guard) under a subprocess. In the general case you need to specify subprocess.PIPE for the subprocess stdout and stderr; then create two separate threads which read the subprocess stdout and stderr to wherever you like.

In the case of python-gnupg, status messages from gpg are read and acted upon while the gpg process is running (not waiting until it's finished).

Basically, pseudocode is

process = subprocess.Popen(..., stdout=subprocess.PIPE, stderr=subprocess.PIPE)
stderr = process.stderr
rr = threading.Thread(target=response_reader_func, args=(process.stderr,))
rr.setDaemon(True)
rr.start()

dr = threading.Thread(target=data_reader_func, args=(process.stdout,))
dr.setDaemon(True)
dr.start()

dr.join()
rr.join()
process.wait()

The reader functions are typically methods of an enclosing class which do the right thing based on what they're reading (in your case, updating progress info in some way).