fix(finance): isin() actually validates the check digit

_isin_checksum never accumulated into `check` (the accumulation line
present in _cusip_checksum was missing), and the per-character loop it
copied is not the ISIN scheme anyway. `check` stayed 0, so isin()
returned True for every 12-character string -- e.g. isin('US0378331004')
(wrong check digit) and isin('XX0000000000').

Reimplement with the documented ISIN algorithm: expand letters to digits
(A=10..Z=35) and run Luhn over the result. Verified against real ISINs
(US0378331005, AU0000XVGZA3, GB0002634946).

The existing 'valid' fixture 'JP000K0VF054' was itself invalid (its
correct check digit is 5) -- corrected to 'JP000K0VF055'. Added a
wrong-check-digit case to the invalid fixtures and a valid example to
the docstring.

Author: Jordan-Bourillot

src/validators/finance.py

@@ -32,21 +32,27 @@ def _cusip_checksum(cusip: str):
 
 
 def _isin_checksum(value: str):
-    check, val = 0, None
+    if not (value[:2].isalpha() and value[-1].isdigit()):
+        return False
 
-    for idx in range(12):
-        c = value[idx]
-        if c >= "0" and c <= "9" and idx > 1:
-            val = ord(c) - ord("0")
-        elif c >= "A" and c <= "Z":
-            val = 10 + ord(c) - ord("A")
-        elif c >= "a" and c <= "z":
-            val = 10 + ord(c) - ord("a")
+    # Expand letters to numbers (A=10, ..., Z=35), then run the Luhn algorithm.
+    digits = ""
+    for char in value:
+        if char.isdigit():
+            digits += char
+        elif char.isalpha():
+            digits += str(10 + ord(char.upper()) - ord("A"))
         else:
             return False
 
+    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
 
@@ -82,8 +88,8 @@ def isin(value: str):
     [1]: https://en.wikipedia.org/wiki/International_Securities_Identification_Number
 
     Examples:
-        >>> isin('037833DP2')
-        ValidationError(func=isin, args={'value': '037833DP2'})
+        >>> isin('US0378331005')
+        True
         >>> isin('037833DP3')
         ValidationError(func=isin, args={'value': '037833DP3'})
 

tests/test_finance.py

@@ -24,13 +24,13 @@ 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"])
 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", "US0378331004"])
 def test_returns_failed_validation_on_invalid_isin(value: str):
     """Test returns failed validation on invalid isin."""
     assert isinstance(isin(value), ValidationError)