Using 'gdalwarp' in Python script for Grass

Question

I'm writing a Python script that aims to re-project various LANDSAT tiles into a common projection and then import the tiles into Grass. The script at present is shown below:

#!/usr/bin/python
import os
import sys
import glob
import grass.script as grass
def import_tifs(dirpath):
 for dirpath, dirname, filenames in os.walk(dirpath):
 for tile in filenames:
 if tile.upper().endswith('.TIF'):
 full_path = os.path.join(dirpath, tile) 
 name = os.path.splitext(tile)[0] 
 rename = name[:-4] + '.' + name[-2]
 new_path = os.path.join(dirpath, rename)
 re_project = 'EPSG:32643'
 grass.message('Importing %s -> %s@%s...' % (tile, name, dirpath)) 
 grass.run_command('gdalwarp',
 '-t_srs',
 re_project,
 full_path,
 new_path)
 grass.run_command('r.in.gdal',
 flags = 'e',
 input = full_path,
 output = name,
 quiet = True,
 overwrite = True)
def main(): 
 if len(sys.argv) == 1: 
 for directory in filter(os.path.isdir, os.listdir(os.getcwd())):
 import_tifs(directory)
 else:
 import_tifs(sys.argv[1]) 
if __name__ == "__main__":
 main()

I'm having real trouble using the 'gdalwarp' command to reproject the LANDSAT tiles. I keep getting the following error:

Traceback (most recent call last):
 File "C:/Users/Simon/Documents/import_landsat3.py", line
76, in <module>
 main()
 File "C:/Users/Simon/Documents/import_landsat3.py", line
73, in main
 import_tifs(sys.argv[1])
 File "C:/Users/Simon/Documents/import_landsat3.py", line
31, in import_tifs
 new_path)
 File "C:\GRASS64\etc\python\grass\script\core.py", line
189, in run_command
 ps = start_command(*args, **kwargs)
 File "C:\GRASS64\etc\python\grass\script\core.py", line
167, in start_command
 args = make_command(prog, flags, overwrite, quiet,
verbose, **options)
 File "C:\GRASS64\etc\python\grass\script\core.py", line
124, in make_command
 raise ScriptError("'-' is not a valid flag")
grass.script.core.ScriptError: "'-' is not a valid flag"

I'm assuming this is related to '-t_srs' but I haven't been able to work out a way around the problem - can anyone suggest how to do this?

Many thanks.

Answer 1

Have you tried calling gdalwarp using os.sys? Check this post for more details on the topic.

• si_2012: Thanks - I've given this a try, but have got the error: TypeError: 'module' object is not callable What module do I need to import to use gdalwarp with os.sys in a Python script?
• dmci: what about trying os.system('gdalwarp ... ')?
• si_2012: Thanks - this has worked! Could you tell me why os.system worked and os.sys didn't?

Answer 2

Try the subprocess module:

import subprocess
cmd = ['gdalwarp','-t_srs',re_project,full_path,new_path]
proc = subprocess.Popen(cmd,stdout=subprocess.PIPE,stderr=subprocess.PIPE)
stdout,stderr=proc.communicate()
exit_code=proc.wait()
if exit_code: #Oops, something went wrong!
 raise RuntimeError(stderr)
else:print stdout

• si_2012: Thanks for this - I've managed to get the code to work using os.system as suggested by @dmci but I'm going to try using the subprocess module as I haven't come across it before.
• user2856: From the python docs for os.system: "The subprocess module provides more powerful facilities for spawning new processes and retrieving their results; using that module is preferable to using this function."