When using subprocess.Popen(), stderr and stdout have no output

Question

I'm using Python to automate an SVN commit, and I want to write the SVN command's output to a log file. The code that I have can make SVN run, but the problem is that on a successful commit, the subprocess invocation does not return any output for my log.

When I run SVN manually, by comparison, I get output that shows the progress of the command and shows which files are being committed. That's what I want in my log file. Is SVN outputting that data to a buffer than stdout or stderr? How can I capture that data for my log?

Here's the code I'm using:

cmd = "svn commit --non-interactive --no-auth-cache -m 'Automatic commit' ./"
process = subprocess.Popen(cmd,
                           shell=True,
                           stdout=subprocess.PIPE,
                           stderr=subprocess.PIPE,
                           universal_newlines=True)
result = process.wait()

# Output
out = process.stdout.read()
err = process.stderr.read()

When I run this code and the commit is successful, the out and err variables are both empty.

Answer 1

Don't use wait() when you are using PIPE. Use communicate()

process = subprocess.Popen(cmd, shell=True, stdout=subprocess.PIPE,
          stderr=subprocess.PIPE, universal_newlines=True)

out, err = process.communicate()

From the subprocess docs:

Warning
Use communicate() rather than .stdin.write, .stdout.read or .stderr.read to avoid deadlocks due to any of the other OS pipe buffers filling up and blocking the child process.