[GRASS-dev] Python Scripting

[Michael Barton]

These sound promising, though I admit to only partly following it.

Michael
____________________
C. Michael Barton, Professor of Anthropology
Director of Graduate Studies
School of Human Evolution & Social Change
Center for Social Dynamics & Complexity
Arizona State University

Phone: 480-965-6262
Fax: 480-965-7671
www: <www.public.asu.edu/~cmbarton>



On Jul 16, 2008, at 11:53 PM, Glynn Clements wrote:

>
> Michael Barton wrote:
>
>> This seems very handy for a lot of basic GRASS scipting, including
>> replacing bash scripts with Python ones--something that I'd like to
>> start on soon
>
> Before we start, we need to figure out a suitable template for Python
> scripts. I've recently fixed some bugs in the g.parser/test.py
> example, but I've just discovered another issue:
>
> g.parser needs to both open the script as a text file (to parse the
> option definitions), and to execute it. For the former, the script
> needs to be able to obtain its own absolute path.
>
> However; the Python documentation for sys.argv says:
>
> 	The list of command line arguments passed to a Python script.
> 	argv[0] is the script name (it is operating system dependent
> 	whether this is a full pathname or not).
>
> If it isn't a full path, sys.path[0] should be the directory
> containing the script. So, the code which invokes g.parser needs to
> force argv[0] to an absolute path before passing it to g.parser.
>
> Even more fundamentally, we should question whether the existing
> g.parser interface is the right way to go for Python. I'm thinking
> that it would be better to give g.parser an option to write the
> option/value pairs to stdout as a Python dictionary, e.g.:
>
> 	output = Popen(["g.parser"] + sys.argv, stdout=PIPE).communicate()[0]
> 	options = eval(output)
>
> [The Popen(..., stdout=PIPE).communicate()[0] idiom is equivalent to
> shell backticks; it returns a string containing the data written to
> stdout.]
>
> Actually; we probably want some error handling, e.g.:
>
> 	child = Popen(["g.parser"] + sys.argv, stdout=PIPE)
> 	output = child.communicate()[0]
> 	if child.returncode != 0:
> 	    raise OSError("Error executing g.parser")
> 	options = eval(output)
>
> Rather than having to put this in every script, we should provide a
> module for it, and have the GRASS startup set PYTHONPATH.
>
> -- 
> Glynn Clements <glynn at gclements.plus.com>
>