resolve 'Can't export GPIO problem on Rpi using python script - python-3.x

Question

I am writing a python application which is programming and testing atmel microcontrollers through a SPI port.
The application is running on RaspberryPi model 3B+ and I use the command line application 'avrdude' to do the job. I use subprocess.Popen() from within my python script and in general this runs just fine.
Sometimes, the SPI port gets in a blocked state. The avrdude application then typically reports something like 'Can't export GPIO 8, already exported/busy?: Device or resource busy'
One can observe the exported GPIO's by:
pi#LeptestPost:/ $ ls /sys/class/gpio/
export gpio10 gpio11 gpio8 gpiochip0 gpiochip504 unexport
I get out of this situation by invoking:
pi#LeptestPost:/ $ sudo echo 8 > /sys/class/gpio/unexport
resulting in:
pi#LeptestPost:/ $ ls /sys/class/gpio/
export gpio10 gpio11 gpiochip0 gpiochip504 unexport
So I can unexport them all and move on manually but I would like to have this automated in the application with the following code (after detecting the error in the avrdude output):
args = ['sudo', 'echo', '8', '>', '/sys/class/gpio/unexport']
result, error = self.runCommand(args, wait=True)
def runCommand(self, args, wait = False, outputFileStr = "", errorFileStr = "", workingDir = ""):
# documentation:
#class subprocess.Popen(args,
# bufsize=-1,
# executable=None,
# stdin=None,
# stdout=None,
# stderr=None,
# preexec_fn=None,
# close_fds=True,
# shell=False,
# cwd=None,
# env=None,
# universal_newlines=None,
# startupinfo=None,
# creationflags=0,
# restore_signals=True,
# start_new_session=False,
# pass_fds=(),
# *,
# encoding=None,
# errors=None,
# text=None)
print("Working on executing command " + str(args))
if (outputFileStr != ""):
stdoutFile = open(outputFileStr,'w')
else:
stdoutFile = None
if (errorFileStr != ""):
stderrFile = open(errorFileStr,'w')
else:
stderrFile = None
if (workingDir != ""):
cwdStr = workingDir
else:
cwdStr = None
try:
if (wait):
p = subprocess.Popen(args, stdout = subprocess.PIPE, cwd = cwdStr)
print("started subprocess with PID " + str(p.pid))
p.wait() # Wait for child process to terminate, This will deadlock when using stdout=PIPE or stderr=PIPE
else:
#p = subprocess.Popen(args, stdin = None, stdout = None, stderr = None, close_fds = True)
p = subprocess.Popen(args, stdin = None, stdout = stdoutFile, stderr = stderrFile, close_fds = True, cwd = cwdStr)
print("started subprocess with PID " + str(p.pid))
(result, error) = p.communicate(timeout=15) # Interact with process: Send data to stdin. Read data from stdout and stderr, until end-of-file is reached.
except subprocess.CalledProcessError as e:
sys.stderr.write("common::run_command() : [ERROR]: output = %s, error code = %s\n" % (e.output, e.returncode))
return e.output, e.returncode
except FileNotFoundError as e:
self.master.printERROR("common::run_command() : [ERROR]: output = %s, error code = " + str(e) + "\n")
return "error", str(e)
except subprocess.TimeoutExpired as e:
self.master.printERROR("Process timeout on PID "+ str(p.pid) + ", trying to kill \n")
p.kill()
outs, errs = p.communicate()
return "error", str(e)
if (outputFileStr != ""):
stdoutFile.close()
if (errorFileStr != ""):
stderrFile.close()
return result, error
But that does not do the job (no error, not the wanted result). I can imagine it's related to how the process is started within its shell or environment - but that's beyond my knowledge.
Any idea how to get this working?
Note: the avrdude application is also called using the 'runcommand' method and running fine.