Subprocess error file

Question

I'm using the python module subprocess to call a program and redirect the possible std error to a specific file with the following command:

with open("std.err","w") as err:
    subprocess.call(["exec"],stderr=err)

I want that the "std.err" file is created only if there are errors, but using the command above if there are no errors the code will create an empty file. How i can make python create a file only if it's not empty?

I can check after execution if the file is empty and in case remove it, but i was looking for a "cleaner" way.

Answer 1

You could use Popen, checking stderr:

from subprocess import Popen,PIPE

proc = Popen(["EXEC"], stderr=PIPE,stdout=PIPE,universal_newlines=True)

out, err = proc.communicate()
if err:
    with open("std.err","w") as f:
        f.write(err)

On a side note, if you care about the return code you should use check_call, you could combine it with a NamedTemporaryFile:

from tempfile import NamedTemporaryFile
from os import stat,remove
from shutil import move

try:
    with NamedTemporaryFile(dir=".", delete=False) as err:
        subprocess.check_call(["exec"], stderr=err)
except (subprocess.CalledProcessError,OSError) as e:
    print(e)


if stat(err.name).st_size != 0:
    move(err.name,"std.err")
else:
    remove(err.name)