Submitting Python Code

When submitting Python code to GRASS GIS GitHub repository (, please take care of following rules:

File structure

GRASS module

Instructions for the GRASS script parser can be found in the g.parser module's help page.

Use the directory structure to place your script appropriately into the source tree: scripts go into scripts directory.

Also add a Makefile and a <module>.html file into this directory. See existing Python scripts for examples.

Add a header section to the script you submit and make sure you include the copyright. The purpose section is meant to contain a general over view of the code in the file to assist other programmers that will need to make changes to your code. For this purpose use Python Docstring.

Example (fictitious header for a script called g.myscript):

MODULE:    g.myscript

AUTHOR(S): John Doe <email AT some domain>

PURPOSE:   Describe your script here...

COPYRIGHT: (C) 2007 John Doe, and by the GRASS Development Team

           This program is free software under the GNU General Public
           License (>=v2). Read the file COPYING that comes with GRASS
           for details.

The copyright protects your rights according to GNU General Public License (

You can easily autogenerate the header and parameters from an existing module using the --script flag. Example:

d.rast --script

Just select an existing module which is close to your application to save efforts.

Python library

Files are placed in lib/python. This directory becomes a package grass after compilation. Each subdirectory is a subpackage.



Use 4-space indentation (GNU Emacs python-mode default). Do not use tabs (tabulators) at all. Note that Python determines nesting based upon indentation, so it is quite crucial to be consistent, i.e. use given rules.


Use Black to format new files:


Use Flake8 to check formatting of all files:

flake8 --extend-ignore=E203,E266,E501 --max-line-length=88

If the file you changed gives too many error in lines you did not change, see if the directory or any parent directory contains a file called .flake8 which contains a less strict configuration for this legacy code. Use it with the --config pararameter:

flake8 --config {path_to_flake8_file} {path_to_python_file}

For example like this:

flake8 --config lib/python/.flake8 lib/python/temporal/

Editor settings for 4-space indentation

The correct editor settings for Python indentation

For the usage of editors for GRASS GIS Python programming, see

PEP8 standard Style

Follow PEP8 standard and use the pep8 tool to check compliance of your code to this standard. You can either install it with pip3 or it is offered for your operating system as package python3-pep8 python3-pycodestyle.

Update 2020:

  • pep8 has been renamed to pycodestyle-3
    • Use of the pep8 tool will be removed in future, please install and use pycodestyle-3 instead.

Note that not all code is currently compliant to complete PEP8, so we are using a custom configuration stored in tools/pep8config.txt (here shipped within the GRASS GIS source code), so use:

pycodestyle-3 --config=grass_master/tools/pep8config.txt directory_to_check

Alternatively, you can use pycodestyle-3 with --diff option to check just the parts of the code you have changed:

git diff | pycodestyle-3 --diff --config=grass_master/tools/pep8config.txt

The best practice is to use pycodestyle-3 with default configuration (i.e., without custom configuration file) for new files and new code in old files.

Do not fix (intentionally or unintentionally) existing style issues in code (at lines) you are not changing. If you are fixing style issues, do it in a separate commit.

Summary of the most important rules:

  • Make sure a new line is at the end of each file.
  • Use three double quotes for docstrings ("""..."""). Use double quotes for translatable (user visible) strings, single quotes for the rest.
  • Remove trailing whitespace from the end of lines. Empty lines should not contain any spaces.
  • Put space between operators:
# use this:
angle = angle * pi / 180
# not this:
angle = angle*pi/180
  • Do not use space around parentheses:
# use this:
grass.run_command('g.region', raster='myrast')
# not this:
grass.run_command( 'g.region', raster='myrast' )
  • Do not use space around '=' in a keyword argument:
# use this:
grass.run_command('g.region', raster='myrast')
# not this:
grass.run_command( 'g.region', raster = 'myrast' )
  • Use space after comma:
# use this:
a = [1, 2, 3]
# not this:
a = [1,2,3]
  • Good practice is to use named parameters in functions:
# use this:
dlg = wx.FileDialog(parent=self, message=_("Choose file to save current workspace"),
                    wildcard=_("GRASS Workspace File (*.gxw)|*.gxw"), style=wx.FD_SAVE)
# not this:
dlg = wx.FileDialog(self, _("Choose file to save current workspace"),
                     _("GRASS Workspace File (*.gxw)|*.gxw"), wx.FD_SAVE)

Writing the code

Temporary files

Create and use secure temporary files and directories. Use the grass.tempfile() or grass.tempdir() functions to do this. e.g.

# setup temporary file
TMP = grass.tempfile()
if TMP is None:
    grass.fatal("Unable to create temporary files")

TODO: this needs to be fixed, it's more complicated

Temporary region

If your script needs to modify computational region, use the following functions:

# now you can safely modify the region
grass.run_command('g.region', raster='input')
# and when you are done:

Note that changing computational region is usually not necessary and not even expected. Typically, user sets region before running the script and expects all computations to be done within this region.

Checking inputs of a module

Use grass.findfile() when there is a need to test if a map exists.

# test for input raster map
result = grass.find_file(name=map_name, element='cell', quiet=True)
if not result['file']
  grass.fatal("Raster map <%s> not found" % map_name)

# test for input vector map
result = grass.find_file(name=map_name, element='vector', quiet=True)
if not result['file']
  grass.fatal("Vector map <%s> not found" % map_name)

... and so forth. See 'g.manual g.findfile' for details.

Overwrite maps

Do not use overwrite=True when calling a module from a Python script, if to overwrite or not should be automatically detected based on calling of the script with --o or without.


For any informational output, use the grass.message() function. For error messages should be used grass.fatal_error() or grass.error() and for warnings grass.warning(). For debugging purposes grass.debug().

# normal message:

# verbose message:
grass.verbose(_("Computation finished successfully"))

# warning:
grass.warning(_("No input values found, using default values"))

# error:
grass.error(_("No map found"))

# fatal error:
# prints error and exits or raises exception (use set_raise_on_error to set the behavior)
grass.fatal_error("No map found, exiting")

# debug output (use g.gisenv to enable/disable)
# debug level is 1 to 5 (5 is most detailed)
grass.debug(_("Our calculated value is: %d" % value), 3)

Do not use the print statement (print function in Python 3) for informational output. This is reserved for standard module output if it has one.


To enable translating of messages to other languages (than English), use full strings, e.g.

if ...:
    win.SetLabel(_("Name for new 3D raster map to create"))
    win.SetLabel(_("Name for new raster map to create"))

instead of constructing string from several parts:

if ...:
    maplabel = 'raster map'
    maplabel = '3D raster map'
win.SetLabel(_("Name for new %s to create") % maplabel)

Sometimes the string might have different translation depending on the context (is it a verb or a noun? matching ending of a word for particular gender; etc). To help translators, it is suggested to add a comment explaining the context of string. The comment must start with GTC keyword and must be on a line before string:

self.bwizard = wx.Button(...,
    # GTC New location
    label = _("N&ew"))

# GTC %s will be replaced with name of current shell
grass.message(_("Running through %s") % shellname)

See also locale/README for more information on translation process and related issues.

Adding description and keywords

Each module needs to have a description and at least 3 keywords. Here an example from scripts/g.extension/

#% label: Maintains GRASS Addons extensions in local GRASS installation.
#% description: Downloads and installs extensions from GRASS Addons repository or other source into the local GRASS installation or removes installed extensions.
#% keyword: general
#% keyword: installation
#% keyword: extensions
#% keyword: addons
#% keyword: download


  • the first keyword is the module family (g.list belongs to "general") which go to the module family index in the manual
  • the second keyword is the overall topic which go to the topic index in the manual
  • the third* (and more) keyword is describing further keywords which go to the keyword index in the manual

These index manual pages are autogenerated during the build process of GRASS GIS.

Dependencies on external Python libraries

With dependencies on external, non-standard modules should use lazy imports: ​

Documentation and comments

Comment your classes and functions with docstrings. Use Sphinx (reStructuredText) syntax.

Comment also the code itself such as the meaning of variables, conditions etc.

Take the time to add comments throughout your code explaining what the code is doing. It will save a huge amount of time and frustration for other programmers that may have to change your code in the future.

Checking the code

Use tools such as pylint and pep8 to check your code (both style and correctness). Just note that default settings of these tools is not fully compatible with wxGUI/wxPython style and that some of the reported errors may not apply to your code.


See also

Related submitting rules

GRASS GIS documentation

External documentation

Last modified 13 months ago Last modified on Nov 17, 2020, 2:03:10 PM