Add ignored-parents option to design checker

This allows users to specify classes to ignore while counting parent
classes.

Partially closes pylint-dev#3057

CONTRIBUTORS.txt

@@ -521,3 +521,5 @@ contributors:
 * Yilei Yang: contributor
 
 * Marcin Kurczewski (rr-): contributor
+
+* Rebecca Turner (9999years): contributor

ChangeLog

@@ -14,6 +14,11 @@ What's New in Pylint 2.9.6?
 ===========================
 Release date: TBA
 
+* Added ``ignored-parents`` option to the design checker to ignore classes
+  while counting ancestors.
+
+  Partially closes #3057
+
 ..
   Put bug fixes that should not wait for a new minor version here
 

pylint/checkers/design_analysis.py

@@ -25,6 +25,7 @@
 
 import re
 from collections import defaultdict
+from typing import FrozenSet, List, Set, cast
 
 import astroid
 from astroid import nodes
@@ -237,6 +238,35 @@ def _count_methods_in_class(node):
     return all_methods
 
 
+def _get_parents(
+    node: nodes.ClassDef, ignored_parents: FrozenSet[str]
+) -> Set[nodes.ClassDef]:
+    r"""Get parents of ``node``, excluding ancestors of ``ignored_parents``.
+
+    If we have the following inheritance diagram:
+
+             F
+            /
+        D  E
+         \/
+          B  C
+           \/
+            A      # class A(B, C): ...
+
+    And ``ignored_parents`` is ``{"E"}``, then this function will return
+    ``{A, B, C, D}`` -- both ``E`` and its ancestors are excluded.
+    """
+    parents: Set[nodes.ClassDef] = set()
+    to_explore = cast(List[nodes.ClassDef], list(node.ancestors(recurs=False)))
+    while to_explore:
+        parent = to_explore.pop()
+        if parent.qname() in ignored_parents:
+            continue
+        parents.add(parent)
+        to_explore.extend(parent.ancestors(recurs=False))  # type: ignore
+    return parents
+
+
 class MisdesignChecker(BaseChecker):
     """checks for sign of poor/misdesign:
     * number of methods, attributes, local variables...
@@ -307,6 +337,15 @@ class MisdesignChecker(BaseChecker):
                 "help": "Maximum number of parents for a class (see R0901).",
             },
         ),
+        (
+            "ignored-parents",
+            {
+                "default": (),
+                "type": "csv",
+                "metavar": "<comma separated list of class names>",
+                "help": "List of qualified class names to ignore when countint class parents (see R0901)",
+            },
+        ),
         (
             "max-attributes",
             {
@@ -379,11 +418,11 @@ def _ignored_argument_names(self):
     )
     def visit_classdef(self, node: nodes.ClassDef):
         """check size of inheritance hierarchy and number of instance attributes"""
-        nb_parents = sum(
-            1
-            for ancestor in node.ancestors()
-            if ancestor.qname() not in STDLIB_CLASSES_IGNORE_ANCESTOR
+        parents = _get_parents(
+            node, STDLIB_CLASSES_IGNORE_ANCESTOR.union(self.config.ignored_parents)
         )
+        print(parents)
+        nb_parents = len(parents)
         if nb_parents > self.config.max_parents:
             self.add_message(
                 "too-many-ancestors",

pylintrc

@@ -324,6 +324,9 @@ max-statements=100
 # Maximum number of parents for a class (see R0901).
 max-parents=7
 
+# List of qualified class names to ignore when counting class parents (see R0901).
+ignored-parents=
+
 # Maximum number of attributes for a class (see R0902).
 max-attributes=11
 

tests/checkers/unittest_design.py

@@ -0,0 +1,44 @@
+# Copyright (c) 2021 Rebecca Turner <rturner@starry.com>
+
+# 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
+
+from pathlib import Path
+
+import astroid
+
+from pylint.checkers import design_analysis
+from pylint.testutils import CheckerTestCase, set_config
+
+REGR_DATA_DIR = str(Path(__file__).parent / ".." / "regrtest_data")
+
+
+class TestDesignChecker(CheckerTestCase):
+
+    CHECKER_CLASS = design_analysis.MisdesignChecker
+
+    @set_config(
+        ignored_parents=(".Dddd",),
+        max_parents=1,
+    )
+    def test_too_many_ancestors_ignored_parents_are_skipped(self):
+        """Make sure that classes listed in ``ignored-parents`` aren't counted
+        by the too-many-ancestors message.
+        """
+
+        node = astroid.extract_node(
+            """
+        class Aaaa(object):
+            pass
+        class Bbbb(Aaaa):
+            pass
+        class Cccc(Bbbb):
+            pass
+        class Dddd(Cccc):
+            pass
+        class Eeee(Dddd):
+            pass
+        """
+        )
+        with self.assertNoMessages():
+            self.checker.visit_classdef(node)