python-stdnum branch master updated. 2.2-11-g5fd7498

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https://arthurdejong.org/git/python-stdnum/commit/?id=5fd7498d8daa76277ea748d284c909bd38e5383f

commit 5fd7498d8daa76277ea748d284c909bd38e5383f
Author: Andrew Chi <17609957+gccbb02498@users.noreply.github.com>
Date:   Tue Jul 7 16:04:44 2026 +0800

    Support post 2023 Taiwan UBN numbers
    
    The Ministry of Finance relaxed the UBN (統一編號) check digit logic
    effective 2023-04-01 (民國112年): the weighted sum only needs to be
    divisible by 5 instead of 10, to enlarge the pool of assignable numbers.
    
    The special case for a 7 in the 7th position (allowing 1 to be added to
    the sum) is preserved, now checked against divisibility by 5.
    
    See: 
https://www.fia.gov.tw/singlehtml/3?cntId=c4d9cff38c8642ef8872774ee9987283
    Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
    Closes https://github.com/arthurdejong/python-stdnum/pull/501

diff --git a/stdnum/tw/ubn.py b/stdnum/tw/ubn.py
index cf518cc..252e5ef 100644
--- a/stdnum/tw/ubn.py
+++ b/stdnum/tw/ubn.py
@@ -22,9 +22,14 @@ The Unified Business Number (UBN, 統一編號) is the number 
assigned to busine
 within Taiwan for tax (VAT) purposes. The number consists of 8 digits, the
 last being a check digit.
 
+Since 2023-04-01 the Ministry of Finance relaxed the check digit logic so
+that the weighted sum only needs to be divisible by 5 (instead of 10) to
+increase the pool of available numbers.
+
 More information:
 
 * https://zh.wikipedia.org/wiki/統一編號
+* https://www.fia.gov.tw/singlehtml/3?cntId=c4d9cff38c8642ef8872774ee9987283
 * https://findbiz.nat.gov.tw/fts/query/QueryBar/queryInit.do?request_locale=en
 
 >>> validate('00501503')
@@ -61,7 +66,7 @@ def calc_checksum(number: str) -> int:
     # convert to numeric first, then sum individual digits
     weights = (1, 2, 1, 2, 1, 2, 4, 1)
     number = ''.join(str(w * int(n)) for w, n in zip(weights, number))
-    return sum(int(n) for n in number) % 10
+    return sum(int(n) for n in number) % 5
 
 
 def validate(number: str) -> str:
@@ -75,7 +80,8 @@ def validate(number: str) -> str:
     if not isdigits(number):
         raise InvalidFormat()
     checksum = calc_checksum(number)
-    if not (checksum == 0 or (checksum == 9 and number[6] == '7')):
+    # if the 7th digit is 7 the check may also pass when adding 1 to the sum
+    if not (checksum == 0 or (checksum == 4 and number[6] == '7')):
         raise InvalidChecksum()
     return number
 
diff --git a/tests/test_tw_ubn.doctest b/tests/test_tw_ubn.doctest
index 06a4974..766ef65 100644
--- a/tests/test_tw_ubn.doctest
+++ b/tests/test_tw_ubn.doctest
@@ -45,6 +45,20 @@ Traceback (most recent call last):
 InvalidChecksum: ...
 
 
+Since 2023-04-01 the check digit only needs the weighted sum to be divisible
+by 5 (instead of 10), so numbers that were invalid under the old rule may now
+be valid. The special case for a 7 in the 7th position still applies.
+
+>>> ubn.validate('10000004')
+'10000004'
+>>> ubn.validate('10000026')
+'10000026'
+>>> ubn.validate('10000073')
+'10000073'
+>>> ubn.validate('10000171')
+'10000171'
+
+
 These have been found online and should all be valid numbers.
 
 >>> numbers = '''

-----------------------------------------------------------------------

Summary of changes:
 stdnum/tw/ubn.py          | 10 ++++++++--
 tests/test_tw_ubn.doctest | 14 ++++++++++++++
 2 files changed, 22 insertions(+), 2 deletions(-)


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