Using subprocess.Popen for Process with Large Outp

Question

I have some Python code that executes an external app which works fine when the app has a small amount of output, but hangs when there is a lot. My code looks like:

p = subprocess.Popen(cmd, shell=True, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
errcode = p.wait()
retval = p.stdout.read()
errmess = p.stderr.read()
if errcode:
    log.error('cmd failed <%s>: %s' % (errcode,errmess))

There are comments in the docs that seem to indicate the potential issue. Under wait, there is:

Warning: This will deadlock if the child process generates enough output to a stdout or stderr pipe such that it blocks waiting for the OS pipe buffer to accept more data. Use communicate() to avoid that.

though under communicate, I see:

Note The data read is buffered in memory, so do not use this method if the data size is large or unlimited.

So it is unclear to me that I should use either of these if I have a large amount of data. They don't indicate what method I should use in that case.

I do need the return value from the exec and do parse and use both the stdout and stderr.

So what is an equivalent method in Python to exec an external app that is going to have large output?

Answer 1

You could try communicate and see if that solves your problem. If not, I'd redirect the output to a temporary file.