OpenFisca extensions

Extensions allow you to define new variables or parameters for a tax and benefit system, while keeping their code separated from the main country package. They can only add variables and parameters to the tax and benefit system: they cannot modify or neutralize existing ones.

They are for instance used to code local prestations.

Extensions are sometimes confused with another mechanism: reforms. Read more about their respective uses.

Extensions can be manually loaded to a tax and benefit system using the load_extension method.

Extension architecture

The architecture of an extension folder is the following:

{extension_name}/ # The folder name is by convention the name of the extension.
    {extension_name}/ # Empty file.
    {extension_name}/{some_formula}.py # File containing formulas
    {extension_name}/parameters # Optional parameters directory.
    {extension_name}/tests/{some_formula}.yaml # Optional test files

All python files located directly in {extension_name}/ are imported in the tax and benefit system.

The syntax of the formulas within extension python files is the same than in the general country package formulas (e.g. from openfisca_france.model.base import *).

Variables inside an extension should not have the same name than any existing formula, nor than any formula in another extension being used.