Executing a subprocess fails

Question

I tried to call a process via Python with several arguments. Executing the batch file itself works fine for me but translating it into Python makes me scream. Here the contents of the batch file:

"C:\Program Files\bin\cspybat" "C:\Program Files\bin\armproc.dll" "C:\Program Files\bin\armjlink.dll" "C:\Documents and Settings\USER\Desktop\CAL\testing\Verification\FRT\Code\TC1\Output\Genericb\Debug\Exe\Gen.out" --download_only --backend -B "--endian=little" "--cpu=Cortex-M3" "--fpu=None" "-p" "C:\Program Files\CONFIG\debugger\ST\iostm32f10xxb.ddf" "--drv_verify_download" "--semihosting" "--device=STM32F10xxB" "-d" "jlink" "--drv_communication=USB0" "--jlink_speed=auto" "--jlink_initial_speed=32" "--jlink_reset_strategy=0,0" 

The executable that is run by the batch file is named cspybat. The output of the executable provides the information: All parameters after--backendare passed to the back end.

Also note that some of the the parameters are strings and some not.

Solution

That works for me now:

    """ MCU flashing function""" 
params = [r"C:\Program Files\bin\cspy",
          r"C:\Program Files\bin\arpro.dll",
          r"C:\Program Files\bin\arjink.dll",
          r"C:\Documents and Settings\USER\Desktop\Exe\GenerV530b.out",
          "--download_only", "--backend", "-B", "--endian=little", "--cpu=Cort3", "--fpu=None", "-p", 
          r"C:\Program Files\CONFIG\debugger\ST\iostm32f10xxb.ddf",
           "--drv_verify_download", "--semihosting", "--device=STM32F10xxB", "-d", "jlink", "--drv_communication=USB0",
            "--jlink_speed=auto", "--jlink_initial_speed=32", "--jlink_reset_strategy=0,0" ]
print(subprocess.list2cmdline(params))
p = subprocess.Popen(subprocess.list2cmdline(params))

Answer 1

To execute a batch file in Windows:

from subprocess import Popen
p = Popen("batchfile.bat", cwd=r"c:\directory\containing\batchfile")
stdout, stderr = p.communicate()

If you don't want to execute the batch file, but rather execute the command in your question directly from Python, you need to experiment a bit with the first argument to Popen.

First of all, the first argument can either be a string or a sequence.

So you either write:

p = Popen(r'"C:\Program Files\Systems\Emb Work 5.4\common\bin\run" "C:\Program Files\Systems\Emb Work 5.4\arm\bin\mpr.dll" ... ...', cwd=r"...")

or

p = Popen([r"C:\Program Files\Systems\Emb Work 5.4\common\bin\run", r"C:\Program Files\Systems\Emb Work 5.4\arm\bin\mpr.dll", ...], cwd=r"...")
# ... notice how you don't need to quote the elements containing spaces

According to the documentation:

On Windows: the Popen class uses CreateProcess() to execute the child program, which operates on strings. If args is a sequence, it will be converted to a string using the list2cmdline() method. Please note that not all MS Windows applications interpret the command line the same way: list2cmdline() is designed for applications using the same rules as the MS C runtime.

So if you use a sequence, it will be converted to a string. I would probably try with a sequence first, since then you won't have to quote all the elements that contain spaces (list2cmdline() does that for you).

For troubleshooting, I recommend you pass your sequence to subprocess.list2cmdline() and check the output.

Edit:

Here's what I'd do if I were you:

a) Create a simple Python script (testparams.py) like this:

import subprocess
params = [r"C:\Program Files\Systems\Emb Work 5.4\common\bin\run.exe", ...]
print subprocess.list2cmdline(params)

b) Run the script from the command line (python testparams.py), copy and paste the output to another command line, press enter and see what happens.

c) If it does not work, edit the python file and repeat until it works.