Add checkers for typing.final for Python version 3.8 or later (#5133)

* Add checkers for typing.final for Python version 3.8 or later

- overridden-final-method
- subclassed-final-class

Closes #3197

ChangeLog

@@ -11,6 +11,10 @@ Release date: TBA
 ..
   Put new features here and also in 'doc/whatsnew/2.12.rst'
 
+* Add checkers ``overridden-final-method`` & ``subclassed-final-class``
+
+  Closes #3197
+
 * Added support for ``ModuleNotFoundError`` (``import-error`` and ``no-name-in-module``).
   ``ModuleNotFoundError`` inherits from ``ImportError`` and was added in Python ``3.6``
 

doc/whatsnew/2.12.rst

@@ -12,6 +12,14 @@ Summary -- Release highlights
 New checkers
 ============
 
+* Checkers for ``typing.final``
+
+  * Added ``overridden-final-method``: Emitted when a method which is annotated with ``typing.final`` is overridden
+
+  * Added ``subclassed-final-class``: Emitted when a class which is annotated with ``typing.final`` is subclassed
+
+  Closes #3197
+
 
 Removed checkers
 ================

pylint/checkers/classes.py

@@ -79,6 +79,7 @@
     safe_infer,
     unimplemented_abstract_methods,
 )
+from pylint.constants import PY38_PLUS
 from pylint.interfaces import IAstroidChecker
 from pylint.utils import get_global_option
 
@@ -649,6 +650,16 @@ def _has_same_layout_slots(slots, assigned_value):
         "unused-private-member",
         "Emitted when a private member of a class is defined but not used.",
     ),
+    "W0239": (
+        "Method %r overrides a method decorated with typing.final which is defined in class %r",
+        "overridden-final-method",
+        "Used when a method decorated with typing.final has been overridden.",
+    ),
+    "W0240": (
+        "Class %r is a subclass of a class decorated with typing.final: %r",
+        "subclassed-final-class",
+        "Used when a class decorated with typing.final has been subclassed.",
+    ),
     "E0236": (
         "Invalid object %r in __slots__, must contain only non empty strings",
         "invalid-slots-object",
@@ -860,6 +871,7 @@ def visit_classdef(self, node: nodes.ClassDef) -> None:
                 self.add_message("no-init", args=node, node=node)
         self._check_slots(node)
         self._check_proper_bases(node)
+        self._check_typing_final(node)
         self._check_consistent_mro(node)
 
     def _check_consistent_mro(self, node):
@@ -898,6 +910,23 @@ def _check_proper_bases(self, node):
                     "useless-object-inheritance", args=node.name, node=node
                 )
 
+    def _check_typing_final(self, node: nodes.ClassDef) -> None:
+        """Detect that a class does not subclass a class decorated with `typing.final`"""
+        if not PY38_PLUS:
+            return
+        for base in node.bases:
+            ancestor = safe_infer(base)
+            if not ancestor:
+                continue
+            if isinstance(ancestor, nodes.ClassDef) and decorated_with(
+                ancestor, ["typing.final"]
+            ):
+                self.add_message(
+                    "subclassed-final-class",
+                    args=(node.name, ancestor.name),
+                    node=node,
+                )
+
     @check_messages("unused-private-member", "attribute-defined-outside-init")
     def leave_classdef(self, node: nodes.ClassDef) -> None:
         """close a class node:
@@ -1347,6 +1376,12 @@ def _check_invalid_overridden_method(self, function_node, parent_function_node):
                 args=(function_node.name, "non-async", "async"),
                 node=function_node,
             )
+        if decorated_with(parent_function_node, ["typing.final"]) and PY38_PLUS:
+            self.add_message(
+                "overridden-final-method",
+                args=(function_node.name, parent_function_node.parent.name),
+                node=function_node,
+            )
 
     def _check_slots(self, node):
         if "__slots__" not in node.locals:

tests/functional/o/overridden_final_method_py38.py

@@ -0,0 +1,17 @@
+"""Since Python version 3.8, a method decorated with typing.final cannot be
+overridden"""
+
+# pylint: disable=no-init, import-error, invalid-name, using-constant-test, useless-object-inheritance
+# pylint: disable=missing-docstring, too-few-public-methods
+
+from typing import final
+
+class Base:
+    @final
+    def my_method(self):
+        pass
+
+
+class Subclass(Base):
+    def my_method(self): # [overridden-final-method]
+        pass

tests/functional/o/overridden_final_method_py38.rc

@@ -0,0 +1,2 @@
+[testoptions]
+min_pyver=3.8

tests/functional/o/overridden_final_method_py38.txt

@@ -0,0 +1 @@
+overridden-final-method:16:4:Subclass.my_method:Method 'my_method' overrides a method decorated with typing.final which is defined in class 'Base':HIGH

tests/functional/s/subclassed_final_class_py38.py

@@ -0,0 +1,16 @@
+"""Since Python version 3.8, a class decorated with typing.final cannot be
+subclassed """
+
+# pylint: disable=no-init, import-error, invalid-name, using-constant-test, useless-object-inheritance
+# pylint: disable=missing-docstring, too-few-public-methods
+
+from typing import final
+
+
+@final
+class Base:
+    pass
+
+
+class Subclass(Base): # [subclassed-final-class]
+    pass

tests/functional/s/subclassed_final_class_py38.rc

@@ -0,0 +1,2 @@
+[testoptions]
+min_pyver=3.8

tests/functional/s/subclassed_final_class_py38.txt

@@ -0,0 +1 @@
+subclassed-final-class:15:0:Subclass:"Class 'Subclass' is a subclass of a class decorated with typing.final: 'Base'":HIGH