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How to contribute

Getting the latest source code

The OpenSesame source code is hosted on GitHub:

GitHub provides a straightforward way for collaborating on a project. If you're not familiar with GitHub, you may want to take a look at their help site:

The best (and easiest) way to contribute code is as follows:

  1. Create a GitHub account.
  2. Create a fork of OpenSesame
  3. Modify your fork.
  4. Send a 'pull request', asking for your changes to be merged back into the main repository.

Each major version of OpenSesame has its own branch. For example, the ising branch contains the code for 3.0 Interactive Ising. The master branch contains the code for the latest stable release.

Developing a plugin or extension

For plugin or extension development, see:

Translate the user interface

For instructions on how to translate the user interface, see:

Coding-style guidelines

The goal is to maintain a readable and consistent code base. Therefore, please consider the following style guidelines when contributing code:

Exception handling

Exceptions should be handled via the libopensesame.exceptions.osexception class. For example:

from libopensesame.exceptions import osexception
raise osexception(u'An error occurred')

Printing debug output

Debug output should be handled via libopensesame.debug.msg(), and is shown only when OpenSesame is started with the --debug command-line argument. For example:

from libopensesame import debug
debug.msg(u'This will be shown only in debug mode')


Indentation should be tab based. This is the most important style guideline of all, because mixed indentation causes trouble and is time consuming to correct.

Names, doc-strings, and line wrapping

  • Names should be lower case, with words separated by underscorses.
  • Each function should be accompanied by an informative doc string, of the format shown below. If a doc-string is redundant, for example, because a function overrides another function that has a doc-string, please indicate where the full doc-string can be found.
  • Please do not have lines of code extend beyond 79 characters (where a tab counts as 4 characters), with the exception of long strings that are awkward to break up.
def a_function(argument, keyword=None):

        This is a YAMLDoc-style docstring, which allows for a full specification
        of arguments. See also <>.

        argument:   This is an argument.

        keyword:    This is a keyword.

        This function returns some values.


def a_simple_function():

    """This is a simple doc-string"""


Writing Python 2 and 3 compatible code

Code should be compatible with Python 2.7 and 3.4 and above. To make it easer to write Python 2 and 3 compatible code, a few tricks are included in the py3compat module, which should always be imported in your script like so:

from libopensesame.py3compat import *

This module:

  • Remaps the Python-2 str and unicode types to the (roughly) equivalent Python-3 bytes and str types. Therefore you should code with str objects in most cases and bytes object in special cases.
  • Adds the following functions:
  • safe_decode(s, enc='utf-8', errors='strict') turns any object into a str object
  • safe_encode(s, enc='utf-8', errors='strict') turns any object into a bytes object
  • Adds a py3 variable, which is True when running on Python 3 and False when running on Python 2.
  • Adds a basestr object when running on Python 3.

Unicode and strings

Assure that all functionality is Unicode safe. For new code, use only Unicode strings internally.

my_value = 'a string' # not preferred
my_value = u'a string' # preferred

For more information, see:


With the exception of the guidelines shown above, please adhere to the following standard: