Improve/cli exit codes

README.md

@@ -44,6 +44,9 @@ Invalid numbers print:
 invalid
 ```
 
+The command exits with status `0` for a valid number and status `1` for an
+invalid number, so it can be used directly in shell scripts and CI checks.
+
 Run the test suite:
 
 ```bash

luhn.py

@@ -87,11 +87,14 @@ def append_check_digit(payload: NumberLike) -> str:
 
 if __name__ == "__main__":
     import argparse
+    import sys
 
     parser = argparse.ArgumentParser(
         description="Validate a number with the Luhn algorithm."
     )
     parser.add_argument("number", help="number to validate")
     args = parser.parse_args()
 
-    print("valid" if is_valid(args.number) else "invalid")
+    valid = is_valid(args.number)
+    print("valid" if valid else "invalid")
+    sys.exit(0 if valid else 1)

test_luhn.py

@@ -1,4 +1,7 @@
+import subprocess
+import sys
 import unittest
+from pathlib import Path
 
 from luhn import append_check_digit, calculate_check_digit, checksum, is_valid
 
@@ -41,6 +44,24 @@ def test_rejects_unsupported_types(self):
                     with self.assertRaises(ValueError):
                         function(value)
 
+    def test_cli_exit_status_reflects_validity(self):
+        script = Path(__file__).with_name("luhn.py")
+
+        for number, expected_output, expected_status in (
+            ("79927398713", "valid", 0),
+            ("79927398714", "invalid", 1),
+        ):
+            with self.subTest(number=number):
+                result = subprocess.run(
+                    [sys.executable, str(script), number],
+                    capture_output=True,
+                    text=True,
+                    check=False,
+                )
+
+                self.assertEqual(result.stdout.strip(), expected_output)
+                self.assertEqual(result.returncode, expected_status)
+
 
 if __name__ == "__main__":
     unittest.main()