How to run a simulation

To calculate legislation variables on people’s situations, a Simulation needs to be created and run. OpenFisca looks for two kinds of inputs:

  • how persons are dispatched in other entities,

  • what variables’ values are already known.

This is true for both test cases and data, the two approaches to running Simulations.

How to run a simulation on a test case

Test cases can be expressed in Python, or in JSON when using the web API (see the specific section of the documentation).

Test cases

A test case describes persons and other entities with their variables values. It’s the usual solution to define a small number of situations.

Here is an example of test case (in Python):

    'persons': {'Ari': {}, 'Paul': {}, 'Leila': {}, 'Javier': {}},
    'households': {
        'household_1': {'children': ['Leila'], 'parents': ['Ari', 'Paul']},
        'household_2': {'parents': ['Javier']}

This test case defines 4 persons, Ari, Paul, Leila and Javier. They belong to 2 households named household_1 and household_2. For example, Ari and Paul are parents in household_1 and have one child, Leila.

Information that can be added at the individual level or at the group entity level:

  • known variable values,

  • and definition periods for those variable values.

To add a salary to Ari and rent to household_1 would look like this:

    'persons': {
        'Ari': {
            'salary': {'2011-01': 1000}
        'Paul': {},
        'Leila': {},
        'Javier': {}
    'households': {
        'household_1': {
            'children': ['Leila'],
            'parents': ['Ari', 'Paul'],
            'rent': {'2011-01': 300}
        'household_2': {'parents': ['Javier']}

The salary and rent names come from the salary and rent variables of the OpenFisca-Country-Template. In this model:

  • salary is a Person entity variable defined on a monthly basis,

  • rent is a Household entity variable defined on a monthly basis as well.

Application: calculate two households housing allowances

The following steps calculate two households’ housing_allowance for the same period:

  1. Load a tax and benefit system like OpenFisca-Country-Template.

  2. Initialise a SimulationBuilder.

  3. Create a Simulation using, for example, the build_from_entities(...) function.

  4. Calculate the housing_allowance and print its value for every household.


# -*- coding: utf-8 -*-

from openfisca_core.simulation_builder import SimulationBuilder
from openfisca_country_template import CountryTaxBenefitSystem

    # ... whole test case ...

tax_benefit_system = CountryTaxBenefitSystem()

simulation_builder = SimulationBuilder()
simulation = simulation_builder.build_from_entities(tax_benefit_system, TEST_CASE)

housing_allowance = simulation.calculate('housing_allowance', '2011-01')

print("households", simulation.household.ids)
print("housing_allowance", housing_allowance)

How to run a simulation on data

The following is an example of how to build a Simulation from multiple data formats. Any well structured tabular input should be fine as long as it’s possible to iterate over its items in Python.


The data describes multiple people’s situations. It could define a whole population. This data could come from a survey with aggregated data, data files extracted from a database, etc.

Here is a survey example. It typically goes from 50 000 households to 500 000.

Here is a minimal example of data (in CSV format):


As for the test case content, it needs the following information:

  • unique identifiers for persons and group entities

    like the person_id and household_id columns information in the CSV example

  • if there have multiple entities types (persons, households, …), there needs to be a way that the persons list is dispatched over the group entities

    in the CSV example, every person_id is associated with a household_id on the same line

  • the name of the corresponding variables in the model for each set of values

    person_salary values become salary values in OpenFisca-Country-Template model

  • the period and entity for every set of values

    person_salary and person_age belong to the Person entity. The period isn’t in the CSV file, but it may for example be the CSV creation time and identical for the whole data set.

Application: calculate persons income tax from a CSV file

Given the following data.csv here is how to calculate income_tax for all persons:


In the following example, the pandas library can be leveraged to access the data.

  1. Install the required libraries, by running in a console:

    python --version # Python 3.7.0 or greater should be installed on your computer
    pip install --upgrade pip openfisca_country_template pandas
  2. Load the country-template legislation and the content of the data.csv file with the pandas library:

    import pandas
    from openfisca_country_template import CountryTaxBenefitSystem
    tax_benefit_system = CountryTaxBenefitSystem()
    data = pandas.read_csv('./data.csv')  # pandas.DataFrame object
    length = len(data)  # ignores CSV header

    Now access the data.person_salary column values:

    >>> print(data.person_salary)
    0     2694
    1     2720
    2     3865
    3     1300
    4        0
    5        0
    6     2884
    7     1200
    8     3386
    9     2929
    10       0
    11    1600
    Name: person_salary, dtype: int64
  3. Build a simulation according to the data’s length with SimulationBuilder and configure the simulation:

    from openfisca_core.simulation_builder import SimulationBuilder
    import numpy
    # ...step 2 code...
    simulation = SimulationBuilder().build_default_simulation(tax_benefit_system, length)
    period = '2018-01'
    simulation.set_input('salary', period, numpy.array(data.person_salary))

It is now possible to calculate the income_tax variable for each person described in the data.csv file for the same period:

income_tax = simulation.calculate('income_tax', period)

income_tax is an instance of numpy.ndarray as you can see with:

>>> print(income_tax)

array([404.1     408.00003      579.75    195.00002   0.        0.      432.6
       1.        507.90002      439.35      0.      240.00002],

And, the persons’ order is kept:

>>> print(data.person_id.values)

array([ 1  2  3  4  5  6  7 12  8  9 10 11])

The following example calculates the income_tax of the 8th person in the list:

>>> print(income_tax.item(7))  # person_id == 12


Application: calculate households total taxes from a CSV file

This example will manage the persons and households entities. To calculate households’ total_taxes, it requires each persons’ income_tax. First link the persons list to the households and define their roles.

Persons and households lists in this example are defined in distinct files:

  • data_persons.csv

  • data_households.csv


where household_id is used as a pivot item linking these files contents.

  1. Install the required libraries, by running in a console:

    python --version # Python 3.9 or greater should be installed on your computer
    pip install --upgrade pip openfisca_country_template pandas
  2. Load the country-template legislation and the content of the CSV files with the pandas library:

    import pandas
    from openfisca_country_template import CountryTaxBenefitSystem
    tax_benefit_system = CountryTaxBenefitSystem()
    data_persons = pandas.read_csv('./data_persons.csv')
    data_households = pandas.read_csv('./data_households.csv')

    Access the entity identifiers columns values:

    persons_ids = data_persons.person_id
    households_ids = data_households.household_id
  3. Initialise a simulation builder with the Person entity. The list of persons identifiers is required as follows:

    from openfisca_core.simulation_builder import SimulationBuilder
    import numpy as numpy
    # ...step 2 code...
    sb = SimulationBuilder()
    persons_ids = data_persons.person_id
    sb.declare_person_entity('person', persons_ids)
  4. Configure the simulation builder with the group entity Household. The list of households identifiers and the role of each person member of the households is required:

    # ...step 3 code...
    # Instantiate the household entity:
    households_ids = data_households.household_id
    household_instance = sb.declare_entity('household', households_ids)
    # Join households data on persons:
    persons_households = data_persons.household_id
    persons_households_roles = data_persons.person_role_in_household
    sb.join_with_persons(household_instance, persons_households, persons_households_roles)
  5. Create a simulation from the configured builder and set other inputs like the salary values:

    simulation =
    period = '2019-03'
    simulation.set_input('salary', period, data_persons.person_salary)

This now allows calculation of the total_taxes variable for each household of the data_households.csv file over the given period:

total_taxes = simulation.calculate('total_taxes', period)

Note: This example assumes that the calculated variable and its input values share the same period.