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fix: ISSN checksum returns 11 instead of 0 when weighted sum is divisible by 11 #262

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Merged
WhyNotHugo merged 1 commit into from
Jun 25, 2026
+14 −1
Merged

fix: ISSN checksum returns 11 instead of 0 when weighted sum is divisible by 11 #262

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2 changes: 1 addition & 1 deletion barcode/isxn.py
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Original file line number Diff line number Diff line change
Expand Up @@ -113,7 +113,7 @@ def _calculate_checksum(self):
11
- sum(x * int(y) for x, y in enumerate(reversed(self.issn[:7]), start=2))
% 11
)
) % 11
if tmp == 10:
return "X"

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13 changes: 13 additions & 0 deletions tests/test_checksums.py
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Original file line number Diff line number Diff line change
Expand Up @@ -46,3 +46,16 @@ def test_isbn13_checksum() -> None:
def test_gs1_128_checksum() -> None:
gs1_128 = get_barcode("gs1_128", "00376401856400470087")
assert gs1_128.get_fullcode() == "00376401856400470087"


def test_issn_checksum_zero() -> None:
# When the weighted sum is divisible by 11, the checksum should be 0,
# not 11. Regression test for issue #238.
issn = get_barcode("issn", "6727893")
assert issn.issn == "67278930" # type: ignore[attr-defined]


def test_issn_checksum_x() -> None:
# When (11 - weighted_sum % 11) % 11 == 10, checksum should be "X".
issn = get_barcode("issn", "0000006")
assert issn.issn == "0000006X" # type: ignore[attr-defined]
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