feat: add stdnum.fr.tva.to_siren to convert french VAT number to SIREN

stdnum/fr/tva.py

@@ -41,6 +41,8 @@
 Traceback (most recent call last):
     ...
 InvalidFormat: ...
+>>> to_siren('Fr 40 303 265 045')
+'303 265 045'
 """
 
 from __future__ import annotations
@@ -101,3 +103,18 @@ def is_valid(number: str) -> bool:
         return bool(validate(number))
     except ValidationError:
         return False
+
+
+def to_siren(number: str) -> str:
+    """Convert the VAT number to a SIREN number.
+
+    The SIREN number is the 9 last digits of the VAT number.
+    """
+    _siren: list[str] = []
+    digit_count = 0
+    for char in reversed(number):
+        if digit_count < 9:
+            _siren.insert(0, char)
+            if isdigits(char):
+                digit_count += 1
+    return ''.join(_siren)

tests/test_fr_tva.doctest

@@ -0,0 +1,39 @@
+test_fr_tva.doctest - more detailed doctests for the stdnum.fr.tva module
+
+Copyright (C) 2025
+
+This library is free software; you can redistribute it and/or
+modify it under the terms of the GNU Lesser General Public
+License as published by the Free Software Foundation; either
+version 2.1 of the License, or (at your option) any later version.
+
+This library is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+Lesser General Public License for more details.
+
+You should have received a copy of the GNU Lesser General Public
+License along with this library; if not, write to the Free Software
+Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+02110-1301 USA
+
+
+This file contains more detailed doctests for the stdnum.fr.tva module.
+
+>>> from stdnum.fr import tva
+>>> from stdnum.exceptions import *
+
+
+>>> tva.validate('Fr 40 303 265 045')
+'40303265045'
+>>> tva.validate('23 334 175 221')
+'23334175221'
+>>> tva.validate('23334175221')
+'23334175221'
+>>> tva.validate('84 323 140 391')
+Traceback (most recent call last):
+    ...
+InvalidChecksum: ...
+>>> tva.to_siren('Fr 40 303 265 045')
+'303 265 045'
+