Python scripting

Syntax

First, try a standard command in Command console in Layer Manager: 
r.info map=elevation -g
We are running r.info with an option map set to elevation Now, switch to Python shell and type the same command but in Python syntax: 
grass.read_command('r.info', map='elevation', flags='g')
We used function read_command() from the grass.script package which is imported under the name grass in the Python shell in GRASS GUI. There are also other functions besides read_command() most notably run_command(), write_command() and parse_command(). The first parameter for functions from this group is the name of the GRASS module as string. Other parameters are options of the module. Python keyword arguments syntax is used for the options. Flags can be passed in a parameter flags where value of the parameter is a string containing all the flags we want to set. The general syntax is the following: 
function_name('module.name', option1=value1, option2=..., flags='flagletters')
The function parameters are the same as module options, so you can just use standard module manual page to learn about the interface.

Most of the GRASS functionality is available through modules and all of them can be called using the functions above. However, in some cases, it is more advantageous to use specialized Python functions. This is the case for mapcalc() function (wrapper for r.mapcalc module) and list_strings() function (wrapper for g.list module).

Combining multiple modules

To launch a Python script from GUI, use File -> Launch Python script. 
import grass.script as gscript

def main():
    input_raster = 'elevation'
    output_raster = 'high_areas'
    stats = gscript.parse_command('r.univar', map='elevation', flags='g')
    raster_mean = float(stats['mean'])
    raster_stddev = float(stats['stddev'])
    raster_high = raster_mean + raster_stddev
    gscript.mapcalc('{r} = {a} > {m}'.format(r=output_raster, a=input_raster,
                                             m=raster_high))

if __name__ == "__main__":
    main()

Processing many maps

import grass.script as gscript

def main():
    rasters = ['lsat7_2002_10', 'lsat7_2002_20', 'lsat7_2002_30', 'lsat7_2002_40']
    max_min = None
    for raster in rasters:
        stats = gscript.parse_command('r.univar', map=raster, flags='g')
        if max_min is None or max_min < stats['min']:
            max_min = stats['min']
    print max_min

if __name__ == "__main__":
    main()

Providing GRASS module interface to a script

#!/usr/bin/env python

#%module
#% description: Adds the values of two rasters (A + B)
#% keywords: raster
#% keywords: algebra
#% keywords: sum
#%end
#%option G_OPT_R_INPUT
#% key: araster
#% description: Name of input raster A in an expression A + B
#%end
#%option G_OPT_R_INPUT
#% key: braster
#% description: Name of input raster B in an expression A + B
#%end
#%option G_OPT_R_OUTPUT
#%end


import grass.script as gscript


def main():
    options, flags = gscript.parser()
    araster = options['araster']
    braster = options['braster']
    output = options['output']

    gscript.mapcalc('{r} = {a} + {b}'.format(r=output, a=araster, b=braster))

    return 0


if __name__ == "__main__":
    main()
Note that the previous code samples were missing some whitespace which Python PEP8 style guide requires but this last sample fulfills all the requirements. You should always use pep8 to check your syntax and style or set your editor to do it for you.

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