GRASS and Python

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This page discusses usage GRASS GIS and Python in a general way if you just want to write a script to run in GRASS GIS go to GRASS Python Scripting Library wiki page or Python API for GRASS GIS 7 documentation.

Writing Python scripts in GRASS

Python is a programming language which is more powerful than shell scripting but easier and more forgiving than C. The Python script can contain simple module description definitions which will be processed with g.parser, as shown in the example below. In this way with no extra coding a GUI can be built, inputs checked, and a skeleton help page can be generated automatically. In addition it adds links to the GRASS message translation system. The library for "scripting" is "grass.script", typically used as: 

import grass.script as gscript

The related files are at $GISBASE/etc/python/grass/script/*.py. See below for more details.

Note: For code which needs access to the power of C, you can access the GRASS C library functions via the Python "ctypes" interface.

Integrated Python shell

The wxGUI Layer Manager in GRASS GIS 6.4.3+ comes with a "Python Shell" which enables users to type and execute Python commands directly in wxGUI environment.

Embedded interactive Python Shell in wxGUI Layer Manager
Embedded interactive Python Shell in wxGUI Layer Manager

Integrated Python editor

The wxGUI Layer Manager in GRASS GIS 7.2.0+ comes with a "Simple Python Editor" which enables users to author Python scripts directly in GRASS GIS GUI. User can also run the script easily in GRASS GIS environment with all the dependencies loaded. The editor comes with several examples, templates, and links to documentation.

Python interactive shell (console) and a simple Python editor are a powerful option how to interact with GRASS GIS as well as extent its functionality

External Python editors (IDE)

Besides the wxGUI Python shell and script editor, also advanced editors like Spyder (The Scientific PYthon Development EnviRonment) can be used in connection with GRASS GIS. For details, see Tools for Python programming.

The correct editor settings for Python indentation

Be sure to use only white spaces and no tabs to indent Python code!

Settings

  • Geany editor:
    • Edit > Preferences > Editor > Intentation tab > Type: Spaces

Using the GRASS Python Scripting Library

You can run Python scripts easily in a GRASS session.

To write these scripts,

  • check the code in lib/python/ which provides grass.script in order to support GRASS scripts written in Python.
 See the GRASS Python Scripting Library for notes and examples.
  • The scripts/ directory of GRASS contains a series of examples actually provided to the end users.

For the desired Python code style, have a look at Submitting Python.

Creating Python scripts that call GRASS functionality from outside

For calling GRASS functionality from outside, see the documentation and also Working with GRASS without starting it explicitly.

Note: This is a more advanced use case of using GRASS' functionality from outside via Python. Commonly, a user will run GRASS Python script from inside a GRASS session, i.e. either from the command line or from the Python shell embedded in the wxGUI (screenshot).

Running external commands from Python

For information on running external commands from Python, see: http://docs.python.org/lib/module-subprocess.html

Avoid using the older os.* functions. Section 17.1.3 lists equivalents using the Popen() interface, which is more robust (particularly on Windows).

Testing and installing Python extensions

Debugging

Make sure the script is executable: 

   chmod +x /path/to/my.extension.py

During development, a Python script can be debugged using the Python Debugger (pdb): 

   python -m pdb /path/to/my.extension.py input=my_input_layer output=my_output_layer option=value -f

Installation

Once you're happy with your script, you can put it in the scripts/ folder of your GRASS install. To do so, first create a directory named after your extension, then create a Makefile for it, and a HTML man page: 

   cd /path/to/grass_src/
   cd scripts
   ls # It is useful to check out the existing scripts and their structure
   mkdir my.extension
   cd my.extension
   cp path/to/my.extension.py .
   touch my.extension.html
   touch Makefile

Next step is to edit the Makefile. It is a very simple text file, the only thing to check is to put the right extension name (WITHOUT the .py file extension) after PGM: 

   MODULE_TOPDIR = ../..
   
   PGM = my.extension
   
   include $(MODULE_TOPDIR)/include/Make/Script.make
   
   default: script

The HTML file would be generated automatically. If you want to add more precisions in it, you can do it (just make sure you start at DESCRIPTION. See existing scripts.)

In the /path/to/grass_src/scripts folder there is also a Makefile. Before building your new folder name should be added to the SUBDIRS list in this file to be built correctly.

You can then run "make" within the my.extension folder. Running "make" in the extension directory places the resulting files in the staging directory (path/to/grass_src/dist.<YOUR_ARCH>/). If you're running GRASS from the staging directory (/path/to/grass_src/bin.<YOUR_ARCH>/grass7), subsequent commands will used the updated files. 

   # in your extension directory (/path/to/grass_src/scripts/my.extension/)
   make
   # Starting GRASS from the staging directory
   /path/to/grass_src/bin.<YOUR_ARCH>/grass7
   my.extension help

You can also run "make install" from the top level directory of your GRASS install (say /usr/local/src/grass_trunk/). Running "make install" from the top level just copies the whole of the dist.<YOUR_ARCH>/ directory to the installation directory (e.g. /usr/local/grass70) and the bin.<YOUR_ARCH>/grass70 bin file to the bin directory (e.g. /usr/local/bin), and fixes any embedded paths in scripts and configuration files. 

   cd /path/to/grass_src
   make install
   # Starting GRASS as usual would work and show your extension available
   grass7
   my.extension help

Using the parser to get GUI and standardized CLI

Read the documentation for GRASS GIS Python scripting with script package.

For more details, see

Be sure to use the standardized options such as G_OPT_R_INPUT or G_OPT_V_OUTPUT if possible. See the overview table of parser standard options in the documentation.

Using a version of Python different from the default installation

Sometimes you don't want to use the default Python version installed on your system. You can control the interpreter used for building by overriding the PYTHON make variable, e.g. 

 make PYTHON=/path/to/python ...

For controlling the interpreter used at run time:

  • On Windows, Python scripts are invoked via %GRASS_PYTHON%, so changing that environment variable will change the interpreter.
  • On Unix, GRASS_PYTHON is only used for components which use wxPython, e.g. wxGUI or the parameter dialogs which are displayed when modules are run without arguments.

Scripts rely upon the "#!/usr/bin/env python" line. You can control the interpreter used by scripts by creating a directory containing a symlink named "python" which refers to your preferred version of the interpreter, and placing that directory at the front of $PATH.

Python extensions in GRASS GIS

Python Scripting Library

pygrass Library

Pygrass is an object oriented Python Application Programming Interface (API) for GRASS GIS. pygrass wants to improve the integration between GRASS and Python, render the use of Python under GRASS more consistent with the language itself and make the GRASS scripting and programming activity easier and more natural to the final users. Some simple examples below.

For more details, see pygrass

Example 1: 

from grass.pygrass.modules import Module

slope_aspect = Module("r.slope.aspect")
slope_aspect(elevation='elevation', slope='slp',  aspect='asp', 
               format='degrees', overwrite=True)

Example 2, using "shortcuts": 

#!/usr/bin/env python

# simple example for pyGRASS usage: raster processing via modules approach

from grass.pygrass.modules.shortcuts import general as g
from grass.pygrass.modules.shortcuts import raster as r

g.message("Filter elevation map by a threshold...")

# set computational region
input = 'elevation'
g.region(raster=input)

# hardcoded:
# r.mapcalc('elev_100m = if(elevation > 100, elevation, null())', overwrite = True)
# with variables

output = 'elev_100m'
thresh = 100.0
r.mapcalc("%s = if(%s > %d, %s, null())" % (output, input, thresh, input), overwrite = True)
r.colors(map=output, color="elevation")


Example 3, using "shortcuts" and external raster format support: 

#!/usr/bin/env python

# simple example for pyGRASS usage: raster processing via Modules approach
# Read GeoTIFF directly, write out GeoTIFF directly
import os
import tempfile
from grass.pygrass.modules.shortcuts import general as g
from grass.pygrass.modules.shortcuts import raster as r
from grass.pygrass.modules import Module

# use alias name for GRASS GIS commands with more than one dot in name
r.external_out = Module('r.external.out')

# get the raster elevation map into GRASS without import
geotiff = 'elevation.tif' # with path
curr_raster='myraster'
r.external(input=geotiff, output=curr_raster)

# define output directory for files resulting from GRASS calculation:
gisdb = os.path.join(tempfile.gettempdir(), 'gisoutput')
try:
    os.stat(gisdb)
except:
    os.mkdir(gisdb)
g.message("Output will be written to %s" % gisdb)

# define where to store the resulting files and format
r.external_out(directory=gisdb, format='GTiff')

# do the GRASS GIS job
g.message("Filter elevation map by a threshold...")
# set computational region
g.region(raster=curr_raster)

# generate GeoTIFF
output = 'elev_100m.tif'
thresh = 100.0
r.mapcalc("%s = if(%s > %d, %s, null())" % (output, curr_raster, thresh, curr_raster), overwrite = True)
r.colors(map=output, color="elevation")

# cease GDAL mode, back to GRASS data writing mode
r.external_out(flags = 'r')
# the resulting GeoTIFF file is in the gisdb directory

Example 4: 

#!/usr/bin/env python
 
# Example for pyGRASS usage - vector API

from grass.pygrass.modules.shortcuts import general as g
from grass.pygrass.vector import VectorTopo
from grass.pygrass.modules import Module

vectmap = 'myzipcodes_wake'

g.message("Importing SHAPE file ...")
ogrimport = Module('v.in.ogr')
ogrimport('/home/neteler/gis_data/zipcodes_wake.shp', output=vectmap)

g.message("Assessing vector topology...")
zipcodes = VectorTopo(vectmap)

# Open the map with topology:
zipcodes.open()

# query number of topological features
areas   = zipcodes.number_of("areas")
islands = zipcodes.number_of("islands")
print 'Map: <' + vectmap + '> with %d areas and %d islands' % (areas, islands)

dblink = zipcodes.dblinks[0]
print 'DB name:'
print dblink.database
table = dblink.table()
print 'Column names:'
print table.columns.names()
print 'Column types:'
print table.columns.types()

zipcodes.close()

Python Ctypes Interface

This interface allows calling GRASS library functions from Python scripts. See Python Ctypes Examples for details.

Examples:

  • Latest and greatest: GRASS 7 Python scripts

Sample script for GRASS 6 raster access (use within GRASS, Spearfish session): 

#!/usr/bin/env python

## TODO: update example to Ctypes

import os, sys
from grass.lib import grass

if "GISBASE" not in os.environ:
    print "You must be in GRASS GIS to run this program."
    sys.exit(1)

if len(sys.argv)==2:
  input = sys.argv[1]
else:
  input = raw_input("Raster Map Name? ")

# initialize
grass.G_gisinit('')

# find map in search path
mapset = grass.G_find_cell2(input, '')

# determine the inputmap type (CELL/FCELL/DCELL) */
data_type = grass.G_raster_map_type(input, mapset)

infd = grass.G_open_cell_old(input, mapset)
inrast = grass.G_allocate_raster_buf(data_type)

rown = 0
while True:
    myrow = grass.G_get_raster_row(infd, inrast, rown, data_type)
    print rown, myrow[0:10]
    rown += 1
    if rown == 476:
        break

grass.G_close_cell(inrast)
grass.G_free(cell)

Sample script for vector access (use within GRASS, Spearfish session): 

#!/usr/bin/python

# run within GRASS Spearfish session
# run this before starting python to append module search path:
#   export PYTHONPATH=/usr/src/grass70/swig/python
#   check with "import sys; sys.path"
# or:
#   sys.path.append("/usr/src/grass70/swig/python")
# FIXME: install the grass bindings in $GISBASE/lib/ ?

import os, sys
from grass.lib import grass
from grass.lib import vector as grassvect

if "GISBASE" not in os.environ:
    print "You must be in GRASS GIS to run this program."
    sys.exit(1)

if len(sys.argv)==2:
  input = sys.argv[1]
else:
  input = raw_input("Vector Map Name? ")

# initialize
grass.G_gisinit('')

# find map in search path
mapset = grass.G_find_vector2(input,'')

# define map structure
map = grassvect.Map_info()

# define open level (level 2: topology)
grassvect.Vect_set_open_level (2)

# open existing map
grassvect.Vect_open_old(map, input, mapset)

# query
print 'Vect map: ', input
print 'Vect is 3D: ', grassvect.Vect_is_3d (map)
print 'Vect DB links: ', grassvect.Vect_get_num_dblinks(map)
print 'Map Scale:  1:', grassvect.Vect_get_scale(map)
print 'Number of areas:', grassvect.Vect_get_num_areas(map)

# close map
grassvect.Vect_close(map)

Using ipython

Using ipython is great due to the TAB completion. Don't remember the function name or curious about what's offered? Hit TAB (maybe twice for a list).

Here an example how to connect to C functions directly from Python: 

grass70
# ... in the GRASS GIS terminal
ipython
In [1]: from grass.lib import gis

In [2]: gis.<TAB>
Display all 729 possibilities? (y or n)
gis.ARRAY                                 gis.G_fatal_longjmp                       gis.G_set_key_value
gis.ArgumentError                         gis.G_file_name                           gis.G_set_keywords
gis.Array                                 gis.G_file_name_misc                      gis.G_set_ls_exclude_filter
...
gis.G_fatal_error                         gis.G_set_gisrc_mode                      gis.wstring_at

In [2]: gis.G_message("Hello world")
Hello world

A full ipython workshop is available here: https://github.com/wenzeslaus/python-grass-addon

wxPython GUI development

3rd party Python software connected to GRASS GIS

FAQ

  • Q: Error message "execl() failed: Permission denied" - what to do?
A: Be sure that the execute bit of the script is set.

Links

General guides

Programming

  • Python and Statistics:
    • RPy - Python interface to the R-statistics programming language

Presentations

From FOSS4G2006:

References

  • Zambelli, P., Gebbert, S., Ciolli, M., 2013. Pygrass: An Object Oriented Python Application Programming Interface (API) for Geographic Resources Analysis Support System (GRASS) Geographic Information System (GIS). ISPRS International Journal of Geo-Information 2, 201–219. (DOI | PDF)
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