Restore the useful part of the python3 checker (#5843)

* Reinstate checks from the python3 checker that are still useful for py3

Closes #5025

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

9 files changed, 68 insertions, 6 deletions

diff --git a/ChangeLog b/ChangeLog
index 76c92a0b7..51bc84fc5 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -53,6 +53,12 @@ Release date: TBA
 
   Closes #5486
 
+* Reinstated checks from the python3 checker that are still useful for python 3
+  (``eq-without-hash``). This is now in the ``pylint.extensions.eq_without_hash`` optional
+  extension.
+
+  Closes #5025
+
 * Added several checkers to deal with unicode security issues
   (see `Trojan Sources <https://trojansource.codes/>`_ and
   `PEP 672 <https://www.python.org/dev/peps/pep-0672/>`_ for details) that also
diff --git a/doc/whatsnew/2.13.rst b/doc/whatsnew/2.13.rst
index e7fc63b88..6fe17d995 100644
--- a/doc/whatsnew/2.13.rst
+++ b/doc/whatsnew/2.13.rst
@@ -110,6 +110,12 @@ Other Changes
 
   Closes #352
 
+* Reinstated checks from the python3 checker that are still useful for python 3
+  (``eq-without-hash``). This is now in the ``pylint.extensions.eq_without_hash`` optional
+  extension.
+
+  Closes #5025
+
 * Fix false-negative for ``assignment-from-none`` checker with list.sort() method.
 
   Closes #5722
diff --git a/pylint/constants.py b/pylint/constants.py
index c3b94863d..1a06ea832 100644
--- a/pylint/constants.py
+++ b/pylint/constants.py
@@ -80,9 +80,7 @@ class DeletedMessage(NamedTuple):
     old_names: List[Tuple[str, str]] = []
 
 
-DELETED_MSGID_PREFIXES = [
-    16,  # the PY3K+ checker, see https://github.com/PyCQA/pylint/pull/4942
-]
+DELETED_MSGID_PREFIXES: List[int] = []
 
 DELETED_MESSAGES = [
     # Everything until the next comment is from the
@@ -139,7 +137,6 @@ DELETED_MESSAGES = [
     DeletedMessage("W1638", "range-builtin-not-iterating"),
     DeletedMessage("W1639", "filter-builtin-not-iterating"),
     DeletedMessage("W1640", "using-cmp-argument"),
-    DeletedMessage("W1641", "eq-without-hash"),
     DeletedMessage("W1642", "div-method"),
     DeletedMessage("W1643", "idiv-method"),
     DeletedMessage("W1644", "rdiv-method"),
diff --git a/pylint/extensions/__init__.py b/pylint/extensions/__init__.py
index c34836101..4af3aefb1 100644
--- a/pylint/extensions/__init__.py
+++ b/pylint/extensions/__init__.py
@@ -10,7 +10,7 @@ if TYPE_CHECKING:
 
 
 def initialize(linter: "PyLinter") -> None:
-    """Initialize linter with checkers in the extensions directory."""
+    """Initialize linter with checkers in the extensions' directory."""
     register_plugins(linter, __path__[0])
 
 
diff --git a/pylint/extensions/code_style.py b/pylint/extensions/code_style.py
index 7082c991c..5a3e3ba17 100644
--- a/pylint/extensions/code_style.py
+++ b/pylint/extensions/code_style.py
@@ -28,7 +28,7 @@ class CodeStyleChecker(BaseChecker):
        i.e. detect a common issue or improve performance
        => it should probably be part of the core checker classes
     2. Is it something that would improve code consistency,
-       maybe because it's slightly better with regards to performance
+       maybe because it's slightly better with regard to performance
        and therefore preferred => this is the right place
     3. Everything else should go into another extension
     """
diff --git a/pylint/extensions/eq_without_hash.py b/pylint/extensions/eq_without_hash.py
new file mode 100644
index 000000000..b0dd6ce46
--- /dev/null
+++ b/pylint/extensions/eq_without_hash.py
@@ -0,0 +1,39 @@
+# Licensed under the GPL: https://www.gnu.org/licenses/old-licenses/gpl-2.0.html
+# For details: https://github.com/PyCQA/pylint/blob/main/LICENSE
+
+"""This is the remnant of the python3 checker. It was removed because
+the transition from python 2 to python3 is behind us, but some checks
+are still useful in python3 after all.
+See https://github.com/PyCQA/pylint/issues/5025
+"""
+
+from astroid import nodes
+
+from pylint import checkers, interfaces
+from pylint.checkers import utils
+from pylint.lint import PyLinter
+
+
+class EqWithoutHash(checkers.BaseChecker):
+
+    __implements__ = interfaces.IAstroidChecker
+    name = "eq-without-hash"
+
+    msgs = {
+        "W1641": (
+            "Implementing __eq__ without also implementing __hash__",
+            "eq-without-hash",
+            "Used when a class implements __eq__ but not __hash__. Objects get "
+            "None as their default __hash__ implementation if they also implement __eq__.",
+        ),
+    }
+
+    @utils.check_messages("eq-without-hash")
+    def visit_classdef(self, node: nodes.ClassDef) -> None:
+        locals_and_methods = set(node.locals).union(x.name for x in node.mymethods())
+        if "__eq__" in locals_and_methods and "__hash__" not in locals_and_methods:
+            self.add_message("eq-without-hash", node=node, confidence=interfaces.HIGH)
+
+
+def register(linter: PyLinter) -> None:
+    linter.register_checker(EqWithoutHash(linter))
diff --git a/tests/functional/ext/eq_without_hash/eq_without_hash.py b/tests/functional/ext/eq_without_hash/eq_without_hash.py
new file mode 100644
index 000000000..a28afc97b
--- /dev/null
+++ b/tests/functional/ext/eq_without_hash/eq_without_hash.py
@@ -0,0 +1,11 @@
+"""Regression test for #5025"""
+
+# pylint: disable=invalid-name, missing-docstring, too-few-public-methods
+
+
+class AClass:  # [eq-without-hash]
+    def __init__(self) -> None:
+        self.x = 5
+
+    def __eq__(self, other: object) -> bool:
+        return isinstance(other, AClass) and other.x == self.x
diff --git a/tests/functional/ext/eq_without_hash/eq_without_hash.rc b/tests/functional/ext/eq_without_hash/eq_without_hash.rc
new file mode 100644
index 000000000..5a4c9fcce
--- /dev/null
+++ b/tests/functional/ext/eq_without_hash/eq_without_hash.rc
@@ -0,0 +1,2 @@
+[MASTER]
+load-plugins=pylint.extensions.eq_without_hash,
diff --git a/tests/functional/ext/eq_without_hash/eq_without_hash.txt b/tests/functional/ext/eq_without_hash/eq_without_hash.txt
new file mode 100644
index 000000000..b7e2dc46d
--- /dev/null
+++ b/tests/functional/ext/eq_without_hash/eq_without_hash.txt
@@ -0,0 +1 @@
+eq-without-hash:6:0:11:62:AClass:Implementing __eq__ without also implementing __hash__:HIGH