Fix erroneous calculation in calculate_checksum

See: #213

Author: WhyNotHugo

barcode/ean.py

@@ -86,8 +86,10 @@ def calculate_checksum(self) -> int:
         def sum_(x, y):
             return int(x) + int(y)
 
-        evensum = reduce(sum_, self.ean[-2::-2])
-        oddsum = reduce(sum_, self.ean[-1::-2])
+        ean_without_checksum = self.ean[: self.digits]
+
+        evensum = reduce(sum_, ean_without_checksum[-2::-2])
+        oddsum = reduce(sum_, ean_without_checksum[-1::-2])
         return (10 - ((evensum + oddsum * 3) % 10)) % 10
 
     def build(self):

tests/test_ean.py

@@ -0,0 +1,11 @@
+from __future__ import annotations
+
+from barcode.ean import EAN13
+
+
+def test_ean_checksum() -> None:
+    ean = EAN13("842169142322")  # input has 12 digits
+    assert ean.calculate_checksum() == 0
+
+    ean = EAN13("8421691423220")  # input has 13 digits
+    assert ean.calculate_checksum() == 0