Don't return None in Code 39 calculate_checksum

Author: maresb

barcode/codex.py

@@ -59,12 +59,14 @@ def get_fullcode(self) -> str:
         """:returns: The full code as it will be encoded."""
         return self.code
 
-    def calculate_checksum(self):
+    def calculate_checksum(self) -> str:
         check = sum(code39.MAP[x][0] for x in self.code) % 43
         for k, v in code39.MAP.items():
             if check == v[0]:
                 return k
-        return None
+        raise RuntimeError(
+            "All possible values for the checksum should have been included in the map."
+        )
 
     def build(self):
         chars = [code39.EDGE]