Make Y060 more aggressive (#432)

Author: AlexWaygood

ERRORCODES.md

@@ -62,7 +62,7 @@ The following warnings are currently emitted by default:
 | Y057 | Do not use `typing.ByteString` or `collections.abc.ByteString`. These types have unclear semantics, and are deprecated; use  `typing_extensions.Buffer` or a union such as `bytes \| bytearray \| memoryview` instead. See [PEP 688](https://peps.python.org/pep-0688/) for more details.
 | Y058 | Use `Iterator` rather than `Generator` as the return value for simple `__iter__` methods, and `AsyncIterator` rather than `AsyncGenerator` as the return value for simple `__aiter__` methods. Using `(Async)Iterator` for these methods is simpler and more elegant, and reflects the fact that the precise kind of iterator returned from an `__iter__` method is usually an implementation detail that could change at any time, and should not be relied upon.
 | Y059 | `Generic[]` should always be the last base class, if it is present in a class's bases tuple. At runtime, if `Generic[]` is not the final class in a the bases tuple, this [can cause the class creation to fail](https://github.com/python/cpython/issues/106102). In a stub file, however, this rule is enforced purely for stylistic consistency.
-| Y060 | Redundant inheritance from `Generic[]`. For example, `class Foo(Iterable[_T], Generic[_T]): ...` can be written more simply as `class Foo(Iterable[_T]): ...`.<br><br>To avoid false-positive errors, and to avoid complexity in the implementation, this check is deliberately conservative: it only looks at classes that have exactly two bases.
+| Y060 | Redundant inheritance from `Generic[]`. For example, `class Foo(Iterable[_T], Generic[_T]): ...` can be written more simply as `class Foo(Iterable[_T]): ...`.<br><br>To avoid false-positive errors, and to avoid complexity in the implementation, this check is deliberately conservative: it only flags classes where all subscripted bases have identical code inside their subscript slices.
 | Y061 | Do not use `None` inside a `Literal[]` slice. For example, use `Literal["foo"] \| None` instead of `Literal["foo", None]`. While both are legal according to [PEP 586](https://peps.python.org/pep-0586/), the former is preferred for stylistic consistency.
 
 ## Warnings disabled by default

pyi.py

@@ -12,7 +12,7 @@
 from copy import deepcopy
 from dataclasses import dataclass
 from functools import partial
-from itertools import chain, zip_longest
+from itertools import chain, groupby, zip_longest
 from keyword import iskeyword
 from typing import TYPE_CHECKING, Any, ClassVar, NamedTuple, Union
 
@@ -62,6 +62,15 @@ class TypeVarInfo(NamedTuple):
     name: str
 
 
+def all_equal(iterable: Iterable[object]) -> bool:
+    """Returns True if all the elements are equal to each other.
+
+    Adapted from the CPython itertools documentation."""
+    g = groupby(iterable)
+    next(g, True)
+    return not next(g, False)
+
+
 _MAPPING_SLICE = "KeyType, ValueType"
 
 # Y022: Use stdlib imports instead of aliases from typing/typing_extensions
@@ -1570,26 +1579,27 @@ def _check_class_bases(self, bases: list[ast.expr]) -> None:
         Y040_encountered = False
         Y059_encountered = False
         Generic_basenode: ast.Subscript | None = None
+        subscript_bases: list[ast.Subscript] = []
         num_bases = len(bases)
 
         for i, base_node in enumerate(bases, start=1):
             if not Y040_encountered and _is_builtins_object(base_node):
                 self.error(base_node, Y040)
                 Y040_encountered = True
-            if isinstance(base_node, ast.Subscript) and _is_Generic(base_node.value):
-                Generic_basenode = base_node
-                if i < num_bases and not Y059_encountered:
-                    Y059_encountered = True
-                    self.error(base_node, Y059)
-
-        if (
-            Generic_basenode is not None
-            and num_bases == 2
-            and isinstance(bases[0], ast.Subscript)
-            and isinstance(bases[1], ast.Subscript)
-            and ast.dump(bases[0].slice) == ast.dump(bases[1].slice)
-        ):
-            self.error(Generic_basenode, Y060)
+            if isinstance(base_node, ast.Subscript):
+                subscript_bases.append(base_node)
+                if _is_Generic(base_node.value):
+                    Generic_basenode = base_node
+                    if i < num_bases and not Y059_encountered:
+                        Y059_encountered = True
+                        self.error(base_node, Y059)
+
+        if Generic_basenode is not None:
+            assert subscript_bases
+            if len(subscript_bases) > 1 and all_equal(
+                ast.dump(subscript_base.slice) for subscript_base in subscript_bases
+            ):
+                self.error(Generic_basenode, Y060)
 
     def visit_ClassDef(self, node: ast.ClassDef) -> None:
         if node.name.startswith("_") and not self.in_class.active:

tests/classdefs.pyi

@@ -14,6 +14,7 @@ from collections.abc import (
     Generator,
     Iterable,
     Iterator,
+    Mapping,
 )
 from enum import EnumMeta
 from typing import Any, Generic, TypeVar, overload
@@ -190,22 +191,39 @@ def __imul__(self, other: Any) -> list[str]: ...
 _S = TypeVar("_S")
 _T = TypeVar("_T")
 
+class BadGeneric(Generic[_T], int): ...  # Y059 "Generic[]" should always be the last base class
 class GoodGeneric(Generic[_T]): ...
-class GoodGeneric2(int, typing.Generic[_T]): ...
-class BreaksAtRuntimeButWhatever(int, str, typing_extensions.Generic[_T]): ...
-class DoesntTypeCheckButWhatever(Iterable[int], str, Generic[_T]): ...
 
-class BadGeneric(Generic[_T], int): ...  # Y059 "Generic[]" should always be the last base class
 class BadGeneric2(int, typing.Generic[_T], str): ...  # Y059 "Generic[]" should always be the last base class
+class GoodGeneric2(int, typing.Generic[_T]): ...
+
 class BadGeneric3(typing_extensions.Generic[_T], int, str): ...  # Y059 "Generic[]" should always be the last base class
+class GoodGeneric3(int, str, typing_extensions.Generic[_T]): ...
+
 class BadGeneric4(Generic[_T], Iterable[int], str): ...  # Y059 "Generic[]" should always be the last base class
+class GoodGeneric4(Iterable[int], str, Generic[_T]): ...
 
 class RedundantGeneric1(Iterable[_T], Generic[_T]): ...  # Y060 Redundant inheritance from "Generic[]"; class would be inferred as generic anyway
+class Corrected1(Iterable[_T]): ...
+
 class RedundantGeneric2(Generic[_S], GoodGeneric[_S]): ...  # Y059 "Generic[]" should always be the last base class  # Y060 Redundant inheritance from "Generic[]"; class would be inferred as generic anyway
+class Corrected2(GoodGeneric[_S]): ...
+
+class RedundantGeneric3(int, Iterator[_T], str, float, memoryview, bytes, Generic[_T]): ...  # Y060 Redundant inheritance from "Generic[]"; class would be inferred as generic anyway
+class Corrected3(int, Iterator[_T], str, float, memoryview, bytes): ...
+
+class RedundantGeneric4(Iterable[_T], Iterator[_T], Generic[_T]): ...  # Y060 Redundant inheritance from "Generic[]"; class would be inferred as generic anyway
+class Corrected4(Iterable[_T], Iterator[_T]): ...
 
-# Strictly speaking these inheritances from Generic are "redundant",
+class BadAndRedundantGeneric(object, Generic[_T], Container[_T]): ...  # Y040 Do not inherit from "object" explicitly, as it is redundant in Python 3  # Y059 "Generic[]" should always be the last base class  # Y060 Redundant inheritance from "Generic[]"; class would be inferred as generic anyway
+class Corrected5(Container[_T]): ...
+
+# Strictly speaking this inheritance from Generic is "redundant",
 # but people may consider it more readable to explicitly inherit from Generic,
-# so we deliberately don't flag them with Y060
-class GoodGeneric3(Container[_S], Iterator[_T], Generic[_S, _T]): ...
-class GoodGeneric4(int, Iterator[_T], str, Generic[_T]): ...
-class BadGeneric5(object, Generic[_T], Container[_T]): ...  # Y040 Do not inherit from "object" explicitly, as it is redundant in Python 3  # Y059 "Generic[]" should always be the last base class
+# so we deliberately don't flag it with Y060
+class GoodGeneric5(Container[_S], Iterator[_T], Generic[_S, _T]): ...
+
+# And these definitely arent't redundant,
+# since the order of the type variables is changed via the inheritance from Generic:
+class GoodGeneric6(Container[_S], Iterator[_T], Generic[_T, _S]): ...
+class GoodGeneric7(Mapping[_S, _T], Generic[_T, _S]): ...