How to run a simulation

To calculate some legislation variables on people’s situations, you need to create and run a new Simulation. Whether situations are described with test cases or data, OpenFisca looks for two kinds of inputs:

  • how persons are dispatched in other entities,

  • what variables’ values are already known.

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': {
        'h1': {'children': ['Leila'], 'parents': ['Ari', 'Paul']},
        'h2': {'parents': ['Javier']}

This test case defines 4 persons, Ari, Paul, Leila and Javier. They belong to 2 households named h1 and h2. For example, Ari and Paul are parents in h1 and have one child, Leila.

You may add information at the individual level or at the group entity level:

  • known variable values,

  • and definition periods for those variable values.

Let’s say that we want to add a salary to Ari and a rent to h1. Here is the updated test case:

    'persons': {
        'Ari': {
            'salary': {'2011-01': 1000}
        'Paul': {},
        'Leila': {},
        'Javier': {}
    'households': {
        'h1': {
            'children': ['Leila'],
            'parents': ['Ari', 'Paul'],
            'rent': {'2011-01': 300}
        'h2': {'parents': ['Javier']}

Where 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.

It’s a Python dictionary object that we will use to build a Simulation.

Application: calculate two households housing allowances

Let’s assume that you want to calculate households’ housing_allowance for the same period. You have to follow these steps:

  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 test case household.

Which gives:

# -*- 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

You can build a Simulation on multiple data formats. Any well structured tabular input shoud be fine as long as you are able to iterate over its items in Python.


Data sets describe multiple people 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, you will need the following information:

  • unique indentifiers for persons and group entities

    like person_id and household_id columns information in the CSV example

  • if you have multiple entities types (persons, households, …), you need to know how your persons list is dispatched over your group entities

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

  • the name of the corresponding variable in your model for every 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 Person entity the reference period isn’t in the CSV file but it might, for example, come from the CSV creation time and be identical for the whole data set.

Application: calculate persons income tax from a CSV file

Let’s say you are using the country-template, which describes the legislation of a yet to be country.

Let’s also say you have the following data.csv and that you want to calculate income_tax for all persons:


In the following example, we will use the pandas library to access the data.

  1. Install the required libraries, by running in your 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

    You can now access the data.person_salary column values and get:

    >>> 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 your 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))

You are all set! You can now calculate the income_tax variable for each person of your 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, persons’ order is kept:

>>> print(data.person_id.values)

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

Thus, you can get the calculated income_tax of one person. For example, get its value for the 8th person in the list with:

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


Application: calculate households total taxes from a CSV file

In this example, we will manage persons and households entities. To calculate households’ total_taxes, we include persons’ income_tax. So, we need to link the persons list to the households and define their roles.

Let’s say that our persons and households lists are defined in distinct files:

  • data_persons.csv

  • data_households.csv


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

  1. Install the required libraries, by running in your 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 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')

    You can now 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. All you need is the list of persons identifiers:

    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 your group entity Household. All you need here is the list of households identifiers and the role of each person member of the households:

    # ...step 3 code...
    # Instanciate 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 to your calculation like salary values:

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

You are all set! You can now calculate the total_taxes variable for each household of your data_households.csv file and the same period:

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

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