如何使用 Python 子进程捕获子进程的错误？

Question

I have the following Python(2.7) code:

try:    
  FNULL = open(os.devnull,'w')
  subprocess.check_call(["tar", "-czvf", '/folder/archive.tar.gz', '/folder/some_other_folder'], stdout=FNULL, stderr=subprocess.STDOUT)
except Exception as e:
  print str(e)

The problem which I face is that, when there is no more space for the archive, print str(e) prints Command '['tar', '-czvf', '/folder/archive.tar.gz', '/folder/some_other_folder']' returned non-zero exit status 1 , which is true, but I want to catch the real error here, that is gzip: write error: No space left on device (I got the this error when I ran the same tar comand manually). Is that possible somehow? I assume that gzip is another process within tar. Am I wrong? Please keep in mind that upgrading to Python 3 is not possible.

EDIT: I also tried to use subprocess.check_output() and print the contents of e.output but that also didn't work

Answer 1

Python 3 solution for sane people

On Python 3, the solution is simple, and you should be using Python 3 for new code anyway (Python 2.7 ended all support nearly a year ago):

The problem is that the program is echoing the error to stderr , so check_output doesn't capture it (either normally, or in the CalledProcessError ). The best solution is to use subprocess.run (which check_call / check_output are just a thin wrapper over) and ensure you capture both stdout and stderr . The simplest approach is:

try:
    subprocess.run(["tar", "-czvf", '/folder/archive.tar.gz', '/folder/some_other_folder'],
                   check=True, stdout=subprocess.DEVNULL, stderr=subprocess.PIPE)
                             # ^ Ignores stdout           ^ Captures stderr so e.stderr is populated if needed
except CalledProcessError as e:
    print("tar exited with exit status {}:".format(e.returncode), e.stderr, file=sys.stderr)

---

Python 2 solution for people who like unsupported software

If you must do this on Python 2, you have to handle it all yourself by manually invoking Popen , as none of the high level functions available there will cover you ( CalledProcessError didn't spawn a stderr attribute until 3.5, because no high-level API that raised it was designed to handle stderr at all):

with open(os.devnull, 'wb') as f:
    proc = subprocess.Popen(["tar", "-czvf", '/folder/archive.tar.gz', '/folder/some_other_folder'],
                 stdout=f, stderr=subprocess.PIPE)
    _, stderr = proc.communicate()
if proc.returncode != 0:
    # Assumes from __future__ import print_function at top of file
    # because Python 2 print statements are terrible
    print("tar exited with exit status {}:".format(proc.returncode), stderr, file=sys.stderr)