Fix linterstats.get_module_message_count() (#9146)

Co-authored-by: Jacob Walls <jacobtylerwalls@gmail.com>

doc/whatsnew/fragments/9145.bugfix

@@ -0,0 +1,3 @@
+Restore "errors / warnings by module" section to report output (with `-ry`).
+
+Closes #9145

pylint/lint/report_functions.py

@@ -6,9 +6,11 @@
 
 import collections
 from collections import defaultdict
+from typing import cast
 
 from pylint import checkers, exceptions
 from pylint.reporters.ureports.nodes import Section, Table
+from pylint.typing import MessageTypesFullName
 from pylint.utils import LinterStats
 
 
@@ -54,6 +56,7 @@ def report_messages_by_module_stats(
         raise exceptions.EmptyReportError()
     by_mod: defaultdict[str, dict[str, int | float]] = collections.defaultdict(dict)
     for m_type in ("fatal", "error", "warning", "refactor", "convention"):
+        m_type = cast(MessageTypesFullName, m_type)
         total = stats.get_global_message_count(m_type)
         for module in module_stats.keys():
             mod_total = stats.get_module_message_count(module, m_type)

pylint/utils/linterstats.py

@@ -292,9 +292,11 @@ def get_global_message_count(self, type_name: str) -> int:
         """Get a global message count."""
         return getattr(self, type_name, 0)
 
-    def get_module_message_count(self, modname: str, type_name: str) -> int:
+    def get_module_message_count(
+        self, modname: str, type_name: MessageTypesFullName
+    ) -> int:
         """Get a module message count."""
-        return getattr(self.by_module[modname], type_name, 0)
+        return self.by_module[modname].get(type_name, 0)
 
     def increase_single_message_count(self, type_name: str, increase: int) -> None:
         """Increase the message type count of an individual message type."""

tests/test_self.py

@@ -988,6 +988,11 @@ def test_can_list_directories_without_dunder_init(tmp_path: Path) -> None:
             stderr=subprocess.PIPE,
         )
 
+    def test_warnings_by_module(self) -> None:
+        path = join(HERE, "regrtest_data", "unused_variable.py")
+        expected = "errors / warnings by module"
+        self._test_output([path, "-ry"], expected_output=expected)
+
     @pytest.mark.needs_two_cores
     def test_jobs_score(self) -> None:
         path = join(HERE, "regrtest_data", "unused_variable.py")