How to use subprocess module to run QGIS toolbox modules

Question

I've been trying to use subprocess for a while now, but can't get it to work when calling QGIS modules. Here's my script:

import subprocess

call("processing.runalg('gdalogr:merge', 
 [rasterin1,rasterin2],'false','false',5,rasterout)")

No matter the input rasters I use (or how I write their paths: I assume they should contain / , not \ to separate folders), I always get this error :

Traceback (most recent call last):
   File "<input>", line 1, in <module>
   File "C:\OSGEO4~1\apps\Python27\lib\subprocess.py", line 711, in __init__
     errread, errwrite)
   File "C:\OSGEO4~1\apps\Python27\lib\subprocess.py", line 948, in _execute_child
     startupinfo)
WindowsError: [Error 2] The system cannot find the file specified

I used gdalogr here (and it doesn't work), but could have used saga:module or grass:module.

The script works perfectly in the console (i.e. when not wrapped by subprocess.call).

I also note people use subprocess successfully to run gdal (Use subprocess.call with gdalwrap), so why subprocessing the processing module would not work?

Any clues?

Answer 1

Subprocess can be used to start new processes - i.e. you use it to execute command line tools from within python. Gdalwarp is a command line tool - processing.runalg is not - hence your Windows error that it can't find it.

What you are trying to launch is not a command line tool but a method of the QGIS python module.

If you want to use that inside a Python script independent of the QGIS command line have a look at pyQgis. You can import QGIS functions into any Python script and execute them from there. No need to use subprocess.