fix(finance): validate the ISIN check digit

_isin_checksum() never accumulated into `check`, so it stayed 0 and
`check % 10 == 0` was always true. Any 12-character string with a
two-letter country code and a trailing digit was accepted regardless of
its check digit (e.g. US0378331004, an invalid variant of Apple's
US0378331005, validated as a valid ISIN).

Implement the real algorithm: expand letters to digits (A=10..Z=35),
keep digits as-is, then run the Luhn checksum over the result. Also
enforce that the country code is alphabetic and the check digit numeric,
rejecting lowercase/malformed input as requested in the issue.

Cross-checked against python-stdnum: this also revealed that the existing
'valid' test fixture JP000K0VF054 was itself an invalid ISIN (correct
check digit is 5) that only passed because the checksum was never
computed; corrected to JP000K0VF055 and added wrong-check-digit cases.

Fixes #440.

Author: apoorvdarshan

src/validators/finance.py

@@ -32,21 +32,32 @@ def _cusip_checksum(cusip: str):
 
 
 def _isin_checksum(value: str):
-    check, val = 0, None
-
+    # Expand the ISIN to digits: each letter maps to two digits (A=10, ..., Z=35)
+    # while digits are kept as-is. The country code (first two characters) must be
+    # letters and the check digit (last character) must be numeric.
+    digits = ""
     for idx in range(12):
         c = value[idx]
-        if c >= "0" and c <= "9" and idx > 1:
-            val = ord(c) - ord("0")
+        if c >= "0" and c <= "9":
+            if idx < 2:
+                return False
+            digits += c
         elif c >= "A" and c <= "Z":
-            val = 10 + ord(c) - ord("A")
-        elif c >= "a" and c <= "z":
-            val = 10 + ord(c) - ord("a")
+            if idx == 11:
+                return False
+            digits += str(10 + ord(c) - ord("A"))
         else:
             return False
 
+    # Luhn checksum: doubling every second digit from the right.
+    check = 0
+    for idx, digit in enumerate(reversed(digits)):
+        val = int(digit)
         if idx & 1:
-            val += val
+            val *= 2
+            if val > 9:
+                val -= 9
+        check += val
 
     return (check % 10) == 0
 
@@ -82,10 +93,12 @@ def isin(value: str):
     [1]: https://en.wikipedia.org/wiki/International_Securities_Identification_Number
 
     Examples:
+        >>> isin('US0378331005')
+        True
+        >>> isin('US0378331004')
+        ValidationError(func=isin, args={'value': 'US0378331004'})
         >>> isin('037833DP2')
         ValidationError(func=isin, args={'value': '037833DP2'})
-        >>> isin('037833DP3')
-        ValidationError(func=isin, args={'value': '037833DP3'})
 
     Args:
         value: ISIN string to validate.

tests/test_finance.py

@@ -24,13 +24,31 @@ def test_returns_failed_validation_on_invalid_cusip(value: str):
 # ==> ISIN <== #
 
 
-@pytest.mark.parametrize("value", ["US0004026250", "JP000K0VF054", "US0378331005"])
+@pytest.mark.parametrize(
+    "value",
+    ["US0004026250", "JP000K0VF055", "US0378331005", "AU0000XVGZA3", "GB0002634946"],
+)
 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 format but incorrect check digit (previously accepted, see gh-440)
+        "US0378331004",
+        "GB0002634947",
+        "AU0000XVGZA4",
+        "JP000K0VF054",
+        # lowercase country code is not a valid ISIN
+        "us0378331005",
+    ],
+)
 def test_returns_failed_validation_on_invalid_isin(value: str):
     """Test returns failed validation on invalid isin."""
     assert isinstance(isin(value), ValidationError)