[GRASS-dev] Python Scripting

[Glynn Clements]

W. Chris Carleton wrote:

> I'm trying to automate some modules in GRASS 6.3.2 using Python scripts
> and g.parser. I've had a look at the examples in /Scripts, but I'm
> having some trouble. I want to pass arguments to a GRASS module by
> iterating over a list of values. Here's what I have;

> 	v.extract input="$GIS_OPT_INPUT" output="$GIS_OPT_OUTPUT""_"i \
> type="point" layer=1 new=-1 list=i

The above is shell syntax.

In Python, it would look something like:

	import subprocess

	...

	subprocess.call([
		"v.extract",
		"input=%s" % os.getenv("GIS_OPT_INPUT"),
		"output=%s_%s" % (os.getenv("GIS_OPT_OUTPUT"), i),
		"type=point", "layer=1", "new=-1", "list=%s" % i])

The main differences between Python and the shell:

1. All strings must be quoted.

2. Environment variables have to be accessed via os.getenv().

3. String manipulation requires explicit operators, e.g. +
(concatenation) or % (substitution).

4. Executing external programs requires the functions in the
subprocess module.

-- 
Glynn Clements <glynn at gclements.plus.com>