Implement PEP 800 (@disjoint_base)

https://peps.python.org/pep-0800/

- Recognize the @disjoint_base decorator
- Error if a class definition has incompatible disjoint bases
- Recognize that classes with incompatible disjoint bases cannot exist
- Check in stubtest that @disjoint_base is correctly applied

Author: JelleZijlstra

mypy/checker.py

@@ -171,6 +171,7 @@
 from mypy.typeanal import check_for_explicit_any, has_any_from_unimported_type, make_optional_type
 from mypy.typeops import (
     bind_self,
+    can_have_shared_disjoint_base,
     coerce_to_literal,
     custom_special_method,
     erase_def_to_union_or_bound,
@@ -2658,6 +2659,8 @@ def visit_class_def(self, defn: ClassDef) -> None:
         for base in typ.mro[1:]:
             if base.is_final:
                 self.fail(message_registry.CANNOT_INHERIT_FROM_FINAL.format(base.name), defn)
+        if not can_have_shared_disjoint_base(typ.bases):
+            self.fail(message_registry.INCOMPATIBLE_DISJOINT_BASES.format(typ.name), defn)
         with self.tscope.class_scope(defn.info), self.enter_partial_types(is_class=True):
             old_binder = self.binder
             self.binder = ConditionalTypeBinder(self.options)
@@ -5826,6 +5829,10 @@ def _make_fake_typeinfo_and_full_name(
             format_type_distinctly(*base_classes, options=self.options, bare=True), "and"
         )
 
+        if not can_have_shared_disjoint_base(base_classes):
+            errors.append((pretty_names_list, "have distinct disjoint bases"))
+            return None
+
         new_errors = []
         for base in base_classes:
             if base.type.is_final:

mypy/message_registry.py

@@ -239,6 +239,11 @@ def with_additional_msg(self, info: str) -> ErrorMessage:
 )
 CANNOT_MAKE_DELETABLE_FINAL: Final = ErrorMessage("Deletable attribute cannot be final")
 
+# Disjoint bases
+INCOMPATIBLE_DISJOINT_BASES: Final = ErrorMessage(
+    'Class "{}" has incompatible disjoint bases'
+)
+
 # Enum
 ENUM_MEMBERS_ATTR_WILL_BE_OVERRIDDEN: Final = ErrorMessage(
     'Assigned "__members__" will be overridden by "Enum" internally'

mypy/nodes.py

@@ -3004,6 +3004,7 @@ class is generic then it will be a type constructor of higher kind.
         "_mro_refs",
         "bad_mro",
         "is_final",
+        "is_disjoint_base",
         "declared_metaclass",
         "metaclass_type",
         "names",
@@ -3055,6 +3056,7 @@ class is generic then it will be a type constructor of higher kind.
     _mro_refs: list[str] | None
     bad_mro: bool  # Could not construct full MRO
     is_final: bool
+    is_disjoint_base: bool
 
     declared_metaclass: mypy.types.Instance | None
     metaclass_type: mypy.types.Instance | None
@@ -3209,6 +3211,7 @@ class is generic then it will be a type constructor of higher kind.
         "is_protocol",
         "runtime_protocol",
         "is_final",
+        "is_disjoint_base",
         "is_intersection",
     ]
 
@@ -3241,6 +3244,7 @@ def __init__(self, names: SymbolTable, defn: ClassDef, module_name: str) -> None
         self.type_var_tuple_suffix: int | None = None
         self.add_type_vars()
         self.is_final = False
+        self.is_disjoint_base = False
         self.is_enum = False
         self.fallback_to_any = False
         self.meta_fallback_to_any = False

mypy/semanal.py

@@ -254,6 +254,7 @@
     ASSERT_TYPE_NAMES,
     DATACLASS_TRANSFORM_NAMES,
     DEPRECATED_TYPE_NAMES,
+    DISJOINT_BASE_DECORATOR_NAMES,
     FINAL_DECORATOR_NAMES,
     FINAL_TYPE_NAMES,
     IMPORTED_REVEAL_TYPE_NAMES,
@@ -2188,6 +2189,8 @@ def analyze_class_decorator_common(
         """
         if refers_to_fullname(decorator, FINAL_DECORATOR_NAMES):
             info.is_final = True
+        elif refers_to_fullname(decorator, DISJOINT_BASE_DECORATOR_NAMES):
+            info.is_disjoint_base = True
         elif refers_to_fullname(decorator, TYPE_CHECK_ONLY_NAMES):
             info.is_type_check_only = True
         elif (deprecated := self.get_deprecated(decorator)) is not None:

mypy/stubtest.py

@@ -17,6 +17,7 @@
 import os
 import pkgutil
 import re
+import struct
 import symtable
 import sys
 import traceback
@@ -465,6 +466,82 @@ class SubClass(runtime):  # type: ignore[misc]
             stub_desc=repr(stub),
         )
 
+SIZEOF_PYOBJECT = struct.calcsize("P")
+
+
+def _shape_differs(t1: type[object], t2: type[object]) -> bool:
+    """Check whether two types differ in shape.
+
+    Mirrors the shape_differs() function in typeobject.c in CPython."""
+    if sys.version_info >= (3, 12):
+        return (
+            t1.__basicsize__ != t2.__basicsize__ or t1.__itemsize__ != t2.__itemsize__
+        )
+    else:
+        # CPython had more complicated logic before 3.12:
+        # https://github.com/python/cpython/blob/f3c6f882cddc8dc30320d2e73edf019e201394fc/Objects/typeobject.c#L2224
+        # We attempt to mirror it here well enough to support the most common cases.
+        if t1.__itemsize__ or t2.__itemsize__:
+            return (
+                t1.__basicsize__ != t2.__basicsize__
+                or t1.__itemsize__ != t2.__itemsize__
+            )
+        t_size = t1.__basicsize__
+        if (
+            # TODO: better understanding of attributes of type objects
+            not t2.__weakrefoffset__  # static analysis: ignore[value_always_true]
+            and t1.__weakrefoffset__ + SIZEOF_PYOBJECT == t_size
+        ):
+            t_size -= SIZEOF_PYOBJECT
+        if (
+            not t2.__dictoffset__  # static analysis: ignore[value_always_true]
+            and t1.__dictoffset__ + SIZEOF_PYOBJECT == t_size
+        ):
+            t_size -= SIZEOF_PYOBJECT
+        if (
+            not t2.__weakrefoffset__  # static analysis: ignore[value_always_true]
+            and t2.__weakrefoffset__ == t_size
+        ):
+            t_size -= SIZEOF_PYOBJECT
+        return t_size != t2.__basicsize__
+
+
+def _is_disjoint_base(typ: type[object]) -> bool:
+    """Return whether a type is a disjoint base at runtime, mirroring CPython's logic in typeobject.c.
+
+    See PEP 800."""
+    if typ is object:
+        return True
+    base = typ.__base__
+    assert base is not None, f"Type {typ} has no base"
+    return _shape_differs(typ, base)
+
+
+
+def _verify_disjoint_base(
+    stub: nodes.TypeInfo, runtime: type[object], object_path: list[str]
+) -> Iterator[Error]:
+    # If it's final, doesn't matter whether it's a disjoint base or not
+    if stub.is_final:
+        return
+    is_disjoint_runtime = _is_disjoint_base(runtime)
+    if is_disjoint_runtime and not stub.is_disjoint_base:
+        yield Error(
+            object_path,
+            "is a disjoint base at runtime, but isn't marked with @disjoint_base in the stub",
+            stub,
+            runtime,
+            stub_desc=repr(stub),
+        )
+    elif not is_disjoint_runtime and stub.is_disjoint_base:
+        yield Error(
+            object_path,
+            "is marked with @disjoint_base in the stub, but isn't a disjoint base at runtime",
+            stub,
+            runtime,
+            stub_desc=repr(stub),
+        )
+
 
 def _verify_metaclass(
     stub: nodes.TypeInfo, runtime: type[Any], object_path: list[str], *, is_runtime_typeddict: bool
@@ -534,6 +611,7 @@ def verify_typeinfo(
         return
 
     yield from _verify_final(stub, runtime, object_path)
+    yield from _verify_disjoint_base(stub, runtime, object_path)
     is_runtime_typeddict = stub.typeddict_type is not None and is_typeddict(runtime)
     yield from _verify_metaclass(
         stub, runtime, object_path, is_runtime_typeddict=is_runtime_typeddict

mypy/test/teststubtest.py

@@ -1613,6 +1613,60 @@ def test_not_subclassable(self) -> Iterator[Case]:
             error="CannotBeSubclassed",
         )
 
+    @collect_cases
+    def test_disjoint_base(self) -> Iterator[Case]:
+        yield Case(
+            stub="""
+            class A: pass
+            """,
+            runtime="""
+            class A: pass
+            """,
+            error=None,
+        )
+        yield Case(
+            stub="""
+            from typing_extensions import disjoint_base
+
+            @disjoint_base
+            class B: pass
+            """,
+            runtime="""
+            class B: pass
+            """,
+            error="test_module.B",
+        )
+        yield Case(
+            stub="""
+            from typing_extensions import Self
+
+            class mytakewhile:
+                def __new__(cls, predicate: object, iterable: object, /) -> Self: ...
+                def __iter__(self) -> Self: ...
+                def __next__(self) -> object: ...
+            """,
+            runtime="""
+            from itertools import takewhile as mytakewhile
+            """,
+            # Should have @disjoint_base
+            error="test_module.mytakewhile",
+        )
+        yield Case(
+            stub="""
+            from typing_extensions import disjoint_base, Self
+
+            @disjoint_base
+            class mycorrecttakewhile:
+                def __new__(cls, predicate: object, iterable: object, /) -> Self: ...
+                def __iter__(self) -> Self: ...
+                def __next__(self) -> object: ...
+            """,
+            runtime="""
+            from itertools import takewhile as mycorrecttakewhile
+            """,
+            error=None,
+        )
+
     @collect_cases
     def test_has_runtime_final_decorator(self) -> Iterator[Case]:
         yield Case(

mypy/typeops.py

@@ -1253,3 +1253,42 @@ def named_type(fullname: str) -> Instance:
             )
         )
     return subtype
+
+
+def _is_disjoint_base(info: TypeInfo) -> bool:
+    # It either has the @disjoint_base decorator or defines nonempty __slots__.
+    return info.is_disjoint_base or bool(info.slots)
+
+
+def _get_disjoint_base_of(instance: Instance) -> TypeInfo | None:
+    """Returns the disjoint base of the given instance, if it exists."""
+    if _is_disjoint_base(instance.type):
+        return instance.type
+    for base in instance.type.mro:
+        if _is_disjoint_base(base):
+            return base
+    return None
+
+
+def can_have_shared_disjoint_base(instances: list[Instance]) -> bool:
+    """Returns whether the given instances can share a disjoint base.
+
+    This means that a child class of these classes can exist at runtime.
+    """
+    # Ignore None disjoint bases (which are `object`).
+    disjoint_bases = [
+        base for instance in instances if (base := _get_disjoint_base_of(instance)) is not None
+    ]
+    if not disjoint_bases:
+        # All are `object`.
+        return True
+
+    candidate = disjoint_bases[0]
+    for base in disjoint_bases[1:]:
+        if candidate.has_base(base.fullname):
+            continue
+        elif base.has_base(candidate.fullname):
+            candidate = base
+        else:
+            return False
+    return True

mypy/types.py

@@ -119,9 +119,15 @@
 # Supported Unpack type names.
 UNPACK_TYPE_NAMES: Final = ("typing.Unpack", "typing_extensions.Unpack")
 
-# Supported @deprecated type names
+# Supported @deprecated decorator names
 DEPRECATED_TYPE_NAMES: Final = ("warnings.deprecated", "typing_extensions.deprecated")
 
+# Supported @disjoint_base decorator names
+DISJOINT_BASE_DECORATOR_NAMES: Final = (
+    "typing.disjoint_base",
+    "typing_extensions.disjoint_base",
+)
+
 # We use this constant in various places when checking `tuple` subtyping:
 TUPLE_LIKE_INSTANCE_NAMES: Final = (
     "builtins.tuple",

test-data/unit/check-final.test

@@ -1272,3 +1272,24 @@ if FOO is not None:
 
     def func() -> int:
         return FOO
+
+[case testDisjointBase]
+from typing_extensions import disjoint_base
+
+@disjoint_base
+class Disjoint1: pass
+
+@disjoint_base
+class Disjoint2: pass
+
+@disjoint_base
+class DisjointChild(Disjoint1): pass
+
+class C1: pass
+class C2(Disjoint1, C1): pass
+class C3(DisjointChild, Disjoint1): pass
+
+class C4(Disjoint1, Disjoint2):  # E: Class "C4" has incompatible disjoint bases
+    pass
+
+[builtins fixtures/tuple.pyi]

test-data/unit/check-isinstance.test

@@ -2551,6 +2551,33 @@ def f2(x: T2) -> T2:
         return C()
 [builtins fixtures/isinstance.pyi]
 
+[case testIsInstanceDisjointBase]
+# flags: --warn-unreachable
+from typing_extensions import disjoint_base
+
+@disjoint_base
+class Disjoint1: pass
+@disjoint_base
+class Disjoint2: pass
+@disjoint_base
+class Disjoint3(Disjoint1): pass
+class Child(Disjoint1): pass
+class Unrelated: pass
+
+def f(d1: Disjoint1, u: Unrelated, c: Child) -> object:
+    if isinstance(d1, Disjoint2): # E: Subclass of "Disjoint1" and "Disjoint2" cannot exist: have distinct disjoint bases
+        return u  # E: Statement is unreachable
+    if isinstance(u, Disjoint1):  # OK
+        return d1
+    if isinstance(c, Disjoint3):  # OK
+        return c
+    if isinstance(c, Disjoint2):  # E: Subclass of "Child" and "Disjoint2" cannot exist: have distinct disjoint bases
+        return c  # E: Statement is unreachable
+    return d1
+
+[builtins fixtures/isinstance.pyi]
+
+
 [case testIsInstanceAdHocIntersectionUsage]
 # flags: --warn-unreachable
 class A: pass