[IMPR] Ensure that _cmpkey method is defined in subclasses of ComparableMixin

Change-Id: I91921c8f4548de62b834e223865f23a05ce63117
---
M pywikibot/tools/__init__.py
1 file changed, 21 insertions(+), 2 deletions(-)

diff --git a/pywikibot/tools/__init__.py b/pywikibot/tools/__init__.py
index 1c06290..023b922 100644
--- a/pywikibot/tools/__init__.py
+++ b/pywikibot/tools/__init__.py
@@ -1,9 +1,10 @@
 """Miscellaneous helper functions (not wiki-dependent)."""
 #
-# (C) Pywikibot team, 2008-2022
+# (C) Pywikibot team, 2008-2023
 #
 # Distributed under the terms of the MIT license.
 #
+import abc
 import bz2
 import gzip
 import hashlib
@@ -226,13 +227,22 @@
 
 
 # From http://python3porting.com/preparing.html
-class ComparableMixin:
+class ComparableMixin(abc.ABC):
 
     """Mixin class to allow comparing to other objects which are comparable.
 
     .. versionadded:: 3.0
     """
 
+    @abc.abstractmethod
+    def _cmpkey(self) -> Any:
+        """Abstract method to return key for comparison of objects.
+
+        This ensures that ``_cmpkey`` method is defined in subclass.
+
+        .. versionadded:: 8.2
+        """
+
     def __lt__(self, other):
         """Compare if self is less than other."""
         return other > self._cmpkey()

