Assert number of messages in test_messages_documentation.py (#6145)

Co-authored-by: Daniël van Noord <13665637+DanielNoord@users.noreply.github.com>

Author: matusvalo

doc/test_messages_documentation.py

@@ -4,7 +4,18 @@
 
 """Functional tests for the code examples in the messages documentation."""
 
-from collections import Counter
+import sys
+
+if sys.version_info[:2] >= (3, 9):
+    from collections import Counter
+else:
+    from collections import Counter as _Counter
+
+    class Counter(_Counter):
+        def total(self):
+            return len(tuple(self.elements()))
+
+
 from pathlib import Path
 from typing import Counter as CounterType
 from typing import List, Optional, TextIO, Tuple
@@ -75,6 +86,12 @@ def __init__(self, test_file: Tuple[str, Path]) -> None:
     def runTest(self) -> None:
         self._runTest()
 
+    def is_good_test_file(self) -> bool:
+        return self._test_file[1].name == "good.py"
+
+    def is_bad_test_file(self) -> bool:
+        return self._test_file[1].name == "bad.py"
+
     @staticmethod
     def get_expected_messages(stream: TextIO) -> MessageCounter:
         """Parse a file and get expected messages."""
@@ -114,6 +131,10 @@ def _runTest(self) -> None:
         self._linter.check([str(self._test_file[1])])
         expected_messages = self._get_expected()
         actual_messages = self._get_actual()
+        if self.is_good_test_file():
+            assert actual_messages.total() == 0  # type: ignore[attr-defined]
+        if self.is_bad_test_file():
+            assert actual_messages.total() > 0  # type: ignore[attr-defined]
         assert expected_messages == actual_messages