util/helper: add stderr param to ProcessWrapper

Allow separation of stderr from the output. For example, stderr can be ignored by redirecting it to subprocess.DEVNULL.

Signed-off-by: Hoppy <dev.hoppy@gmail.com>

Author: ImHoppy

labgrid/util/helper.py

@@ -40,7 +40,7 @@ class ProcessWrapper:
     loglevel = logging.INFO
 
     @step(args=['command'], result=True, tag='process')
-    def check_output(self, command, *, print_on_silent_log=False, input=None, stdin=None): # pylint: disable=redefined-builtin
+    def check_output(self, command, *, print_on_silent_log=False, input=None, stdin=None, stderr=None): # pylint: disable=redefined-builtin
         """Run a command and supply the output to callback functions"""
         logger = logging.getLogger("Process")
         res = []
@@ -58,7 +58,10 @@ def check_output(self, command, *, print_on_silent_log=False, input=None, stdin=
         elif stdin is not None:
             kwargs['stdin'] = stdin
 
-        process = subprocess.Popen(command, stderr=sfd,
+        if stderr is None:
+            stderr = sfd
+
+        process = subprocess.Popen(command, stderr=stderr,
                                    stdout=sfd, bufsize=0, **kwargs)
 
         logger.log(ProcessWrapper.loglevel, "[%d] command: %s", process.pid, " ".join(command))