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I know a very similar question has already been asked but since none of the solutions posted there worked for my problem I try to make it replicable:

So I'm calling this script to merge some shapefiles (all files in one folder) like this:

shpfiles = 'shapefile_a.shp shapefile_b.shp'
subprocess.call(['python', 'shapemerger.py', '%s' % shpfiles])

I only get the Usage Instructions from the script so I cant determine what goes wrong. If i call the script directly in the terminal it works. Any help is appreciated.

maxwhere
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    use subprocess.run , if you are using python 3+ or use subprocess.Popen for both python 3 and python2. Try this : `subprocess.Popen("python shapemerger.py {}".format(shpfiles), shell = True)` – Stack Jul 29 '17 at 14:21
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    split the files to individual names `['python', 'shapemerger.py'] + shpfiles.split()` – VPfB Jul 29 '17 at 14:23
  • @Stack why `shell=True` in that case? – Jean-François Fabre Jul 29 '17 at 14:26
  • your shape files are passed as one sole argument with space in between. VPfB comment solves it. Use list of parameters and let subprocess handle the quoting, you'll be fine. – Jean-François Fabre Jul 29 '17 at 14:27
  • read here https://stackoverflow.com/a/30831028/8150371 – Stack Jul 29 '17 at 14:29
  • Thank you all so much for your help. Splitting the string as suggested by VPfB did the trick. Feel free to post that answer. @Stack python 2.7 i should have added. – maxwhere Jul 29 '17 at 15:16
  • Use `sys.executable` for your first argument - it will be the full path to the python interpreter you are currently running, which might be different from "python" – user2722968 Jul 29 '17 at 15:50

1 Answers1

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Every time a program is started, it receives a list of arguments it was invoked with. This is often called argv (v stands for vector, i.e. one-dimensional array). The program parses this list, extracts options, parameters, filenames, etc. depending on its own invocation syntax.

When working at the command line, the shell takes care of parsing the input line, starting new program or programs and passing them their argument list.

When a program is called from another program, the caller is responsible to provide the correct arguments. It could delegate this work to shell. The price for it is very high. There is substantial overhead and possibly a security risk! Avoid this approach whenever possible.

Finally to the question itself:

shpfiles = 'shapefile_a.shp shapefile_b.shp'
subprocess.call(['python', 'shapemerger.py', '%s' % shpfiles])

This will invoke python to run the script shapemerger.py with one argument shapefile_a.shp shapefile_b.shp. The script expects filenames and receives this one name. The file "shapefile_a.shp shapefile_b.shp" does not exist, but the script probably stops before attempting to access that file, because it expect 2 or more files to process.

The correct way is to pass every filename as one argument. Assuming shpfiles is a whitespace separated list:

subprocess.call(['python', 'shapemerger.py'] + shpfiles.split())

will generate a list with 4 items. It is important to understand that this approach will fail if there is a space in a filename.

VPfB
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