Add fr.tva.to_siren() function

The function can convert a French VAT number to a SIREN.

Closes #480

Author: fmichel-adista

stdnum/fr/tva.py

@@ -41,6 +41,8 @@
 Traceback (most recent call last):
     ...
 InvalidFormat: ...
+>>> to_siren('Fr 40 303 265 045')
+'303265045'
 """
 
 from __future__ import annotations
@@ -101,3 +103,15 @@ 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.
+    """
+    number = compact(number)
+    if number[2:5] == '000':
+        # numbers from Monaco are valid TVA but not SIREN
+        raise InvalidComponent()
+    return number[2:]

tests/test_fr_tva.doctest

@@ -0,0 +1,44 @@
+test_fr_tva.doctest - more detailed doctests for the stdnum.fr.tva module
+
+Copyright (C) 2025 Fabien Michel
+
+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')
+'303265045'
+>>> tva.to_siren('23 334 175 221')
+'334175221'
+>>> tva.to_siren('FR 53 0000 04605')  # Monaco VAT code canot be converted
+Traceback (most recent call last):
+    ...
+InvalidComponent: ...