fix: correct broken ISIN checksum (validator accepted any check digit)

_isin_checksum computed a per-character val in the loop but never added it
into check, which stayed 0. So (check % 10) == 0 was always True and isin()
accepted any 12-character string with valid characters regardless of its
check digit. The existing invalid-ISIN tests all tripped the earlier length
or character guards, so the checksum branch was never actually exercised.

Reimplement the ISO 6166 algorithm: require a numeric check digit, expand
each letter to its two-digit value (A=10..Z=35), then run a right-to-left
Luhn sum over the expanded digits. Validated against python-stdnum's
isin.is_valid over 100k+ country-code-valid inputs with zero mismatches.
Also dropped JP000K0VF054 from the valid samples (it is not a valid ISIN;
it only passed because the checksum did nothing) and added wrong-check-digit
cases to the invalid set.

Author: DucMinhNe

src/validators/finance.py

@@ -32,21 +32,32 @@ def _cusip_checksum(cusip: str):
 
 
 def _isin_checksum(value: str):
-    check, val = 0, None
+    # The check digit (last character) is always numeric.
+    if not value[-1].isdecimal():
+        return False
 
+    # Expand the code into a string of digits, mapping each letter to its
+    # two-digit value (A=10, ..., Z=35) and leaving digits unchanged.
+    digits = ""
     for idx in range(12):
         c = value[idx]
         if c >= "0" and c <= "9" and idx > 1:
-            val = ord(c) - ord("0")
+            digits += c
         elif c >= "A" and c <= "Z":
-            val = 10 + ord(c) - ord("A")
+            digits += str(10 + ord(c) - ord("A"))
         elif c >= "a" and c <= "z":
-            val = 10 + ord(c) - ord("a")
+            digits += str(10 + ord(c) - ord("a"))
         else:
             return False
 
+    # Luhn checksum over the expanded digit string: starting from the
+    # rightmost digit, double every second digit and sum the resulting digits.
+    check = 0
+    for idx, c in enumerate(reversed(digits)):
+        val = ord(c) - ord("0")
         if idx & 1:
             val += val
+        check += (val // 10) + (val % 10)
 
     return (check % 10) == 0
 

tests/test_finance.py

@@ -24,13 +24,25 @@ def test_returns_failed_validation_on_invalid_cusip(value: str):
 # ==> ISIN <== #
 
 
-@pytest.mark.parametrize("value", ["US0004026250", "JP000K0VF054", "US0378331005"])
+@pytest.mark.parametrize("value", ["US0004026250", "US0378331005", "GB0002634946", "DE000BAY0017"])
 def test_returns_true_on_valid_isin(value: str):
     """Test returns true on valid isin."""
     assert isin(value)
 
 
-@pytest.mark.parametrize("value", ["010378331005", "XCVF", "00^^^1234", "A000009"])
+@pytest.mark.parametrize(
+    "value",
+    [
+        "010378331005",
+        "XCVF",
+        "00^^^1234",
+        "A000009",
+        # valid length and characters but wrong check digit
+        "US0378331006",
+        "US0004026251",
+        "GB0002634947",
+    ],
+)
 def test_returns_failed_validation_on_invalid_isin(value: str):
     """Test returns failed validation on invalid isin."""
     assert isinstance(isin(value), ValidationError)