taxe-fonciere-python

Python translation of the French property tax calculator (taxe foncière), originally written in COBOL by DGFiP (Direction Générale des Finances Publiques).

What is this

The French government publishes the source code of its property tax calculation engine as COBOL programs on data.gouv.fr. This repository contains:

1. Original COBOL source — versions 2018, 2020, 2021, 2022, 2023 (from official etalab repository)
2. Python translation — aligned with the 2023 COBOL version (EFITA3B3 / EFITA3N3)
3. Tests — 24 pytest tests verifying the translation against manual calculations

Structure

src/
├── python/                 ← Python translation
│   ├── models.py           ← Data structures (from COBOL copybooks)
│   ├── calc_bati.py        ← Built property calculator (EFITA3B3)
│   ├── calc_non_bati.py    ← Non-built property calculator (EFITA3N3)
│   ├── rounding.py         ← COBOL COMPUTE ROUNDED emulation
│   └── rates.py            ← Tax rate provider (mock)
├── cobol/                  ← Original COBOL 2018
├── cobol-2020/             ← COBOL 2020
├── cobol-2021/             ← COBOL 2021
├── cobol-2022/             ← COBOL 2022
└── cobol-2023/             ← COBOL 2023 (current reference)

tests/
├── test_bati.py            ← 11 tests for built property
└── test_non_bati.py        ← 13 tests for non-built property

Key translation decisions

• COMPUTE ROUNDED → Python Decimal with ROUND_HALF_UP (not banker's rounding)
• COPY REPLACING → Python dataclasses with explicit field mapping
• CALL USING → Python function calls
• ORGANIZATION INDEXED (EFITAUX2) → Mock rate provider class
• All monetary values are integers (euros, no cents) — matches COBOL PIC S9(10)

Changes from 2018 to 2023

The COBOL source evolved between 2018 and 2023 with two notable changes:

1. Département tax removed from bâti (2020 reform) — the departmental property tax on built properties was abolished and transferred to communes
2. TOTOFR flag — new mechanism for TEOM (waste collection tax) transfer regime: when TAU-R-TOTOFR = 'O', OM cotisations go into the 3% frais bucket instead of 8%

Running tests

pip install pytest
pytest tests/ -v

All 24 tests pass:

• Validation checks (year, property type, numeric bases, OM zones)
• Cotisation calculations (commune, syndicat, EPCI, GEMAPI, TASA, TSE, OM)
• Frais de gestion (3%, 8%, 9% fee tiers with rééquilibrage)
• CAAA (droit fixe for département 57, droit proportionnel)
• Saint-Martin special case (971/127)
• Dégrévement jeunes agriculteurs
• TOTOFR transfer regime (both modes)

Origin

Original COBOL source: etalab/taxe_fonciere on GitLab (also GitHub mirror, frozen at 2018).

Translation produced using the M7 system as part of the CobolD project — verified COBOL-to-Python migration.

License

Original COBOL source is under CeCILL v2.1 (French open source license, GPL-compatible). Python translation is under the same license — see LICENSE.