gh-117516: Implement typing.TypeIs (#117517)

See PEP 742.

Co-authored-by: Carl Meyer <[email protected]>
Co-authored-by: Bénédikt Tran <[email protected]>

Author: 3 people

Doc/library/typing.rst

@@ -1385,22 +1385,23 @@ These can be used as types in annotations. They all support subscription using
    .. versionadded:: 3.9
 
 
-.. data:: TypeGuard
+.. data:: TypeIs
 
-   Special typing construct for marking user-defined type guard functions.
+   Special typing construct for marking user-defined type predicate functions.
 
-   ``TypeGuard`` can be used to annotate the return type of a user-defined
-   type guard function.  ``TypeGuard`` only accepts a single type argument.
-   At runtime, functions marked this way should return a boolean.
+   ``TypeIs`` can be used to annotate the return type of a user-defined
+   type predicate function.  ``TypeIs`` only accepts a single type argument.
+   At runtime, functions marked this way should return a boolean and take at
+   least one positional argument.
 
-   ``TypeGuard`` aims to benefit *type narrowing* -- a technique used by static
+   ``TypeIs`` aims to benefit *type narrowing* -- a technique used by static
    type checkers to determine a more precise type of an expression within a
    program's code flow.  Usually type narrowing is done by analyzing
    conditional code flow and applying the narrowing to a block of code.  The
-   conditional expression here is sometimes referred to as a "type guard"::
+   conditional expression here is sometimes referred to as a "type predicate"::
 
       def is_str(val: str | float):
-          # "isinstance" type guard
+          # "isinstance" type predicate
           if isinstance(val, str):
               # Type of ``val`` is narrowed to ``str``
               ...
@@ -1409,8 +1410,73 @@ These can be used as types in annotations. They all support subscription using
               ...
 
    Sometimes it would be convenient to use a user-defined boolean function
-   as a type guard.  Such a function should use ``TypeGuard[...]`` as its
-   return type to alert static type checkers to this intention.
+   as a type predicate.  Such a function should use ``TypeIs[...]`` or
+   :data:`TypeGuard` as its return type to alert static type checkers to
+   this intention.  ``TypeIs`` usually has more intuitive behavior than
+   ``TypeGuard``, but it cannot be used when the input and output types
+   are incompatible (e.g., ``list[object]`` to ``list[int]``) or when the
+   function does not return ``True`` for all instances of the narrowed type.
+
+   Using  ``-> TypeIs[NarrowedType]`` tells the static type checker that for a given
+   function:
+
+   1. The return value is a boolean.
+   2. If the return value is ``True``, the type of its argument
+      is the intersection of the argument's original type and ``NarrowedType``.
+   3. If the return value is ``False``, the type of its argument
+      is narrowed to exclude ``NarrowedType``.
+
+   For example::
+
+        from typing import assert_type, final, TypeIs
+
+        class Parent: pass
+        class Child(Parent): pass
+        @final
+        class Unrelated: pass
+
+        def is_parent(val: object) -> TypeIs[Parent]:
+            return isinstance(val, Parent)
+
+        def run(arg: Child | Unrelated):
+            if is_parent(arg):
+                # Type of ``arg`` is narrowed to the intersection
+                # of ``Parent`` and ``Child``, which is equivalent to
+                # ``Child``.
+                assert_type(arg, Child)
+            else:
+                # Type of ``arg`` is narrowed to exclude ``Parent``,
+                # so only ``Unrelated`` is left.
+                assert_type(arg, Unrelated)
+
+   The type inside ``TypeIs`` must be consistent with the type of the
+   function's argument; if it is not, static type checkers will raise
+   an error.  An incorrectly written ``TypeIs`` function can lead to
+   unsound behavior in the type system; it is the user's responsibility
+   to write such functions in a type-safe manner.
+
+   If a ``TypeIs`` function is a class or instance method, then the type in
+   ``TypeIs`` maps to the type of the second parameter after ``cls`` or
+   ``self``.
+
+   In short, the form ``def foo(arg: TypeA) -> TypeIs[TypeB]: ...``,
+   means that if ``foo(arg)`` returns ``True``, then ``arg`` is an instance
+   of ``TypeB``, and if it returns ``False``, it is not an instance of ``TypeB``.
+
+   ``TypeIs`` also works with type variables.  For more information, see
+   :pep:`742` (Narrowing types with ``TypeIs``).
+
+   .. versionadded:: 3.13
+
+
+.. data:: TypeGuard
+
+   Special typing construct for marking user-defined type predicate functions.
+
+   Type predicate functions are user-defined functions that return whether their
+   argument is an instance of a particular type.
+   ``TypeGuard`` works similarly to :data:`TypeIs`, but has subtly different
+   effects on type checking behavior (see below).
 
    Using  ``-> TypeGuard`` tells the static type checker that for a given
    function:
@@ -1419,6 +1485,8 @@ These can be used as types in annotations. They all support subscription using
    2. If the return value is ``True``, the type of its argument
       is the type inside ``TypeGuard``.
 
+   ``TypeGuard`` also works with type variables.  See :pep:`647` for more details.
+
    For example::
 
          def is_str_list(val: list[object]) -> TypeGuard[list[str]]:
@@ -1433,23 +1501,19 @@ These can be used as types in annotations. They all support subscription using
                  # Type of ``val`` remains as ``list[object]``.
                  print("Not a list of strings!")
 
-   If ``is_str_list`` is a class or instance method, then the type in
-   ``TypeGuard`` maps to the type of the second parameter after ``cls`` or
-   ``self``.
-
-   In short, the form ``def foo(arg: TypeA) -> TypeGuard[TypeB]: ...``,
-   means that if ``foo(arg)`` returns ``True``, then ``arg`` narrows from
-   ``TypeA`` to ``TypeB``.
-
-   .. note::
-
-      ``TypeB`` need not be a narrower form of ``TypeA`` -- it can even be a
-      wider form. The main reason is to allow for things like
-      narrowing ``list[object]`` to ``list[str]`` even though the latter
-      is not a subtype of the former, since ``list`` is invariant.
-      The responsibility of writing type-safe type guards is left to the user.
-
-   ``TypeGuard`` also works with type variables.  See :pep:`647` for more details.
+   ``TypeIs`` and ``TypeGuard`` differ in the following ways:
+
+   * ``TypeIs`` requires the narrowed type to be a subtype of the input type, while
+     ``TypeGuard`` does not.  The main reason is to allow for things like
+     narrowing ``list[object]`` to ``list[str]`` even though the latter
+     is not a subtype of the former, since ``list`` is invariant.
+   * When a ``TypeGuard`` function returns ``True``, type checkers narrow the type of the
+     variable to exactly the ``TypeGuard`` type. When a ``TypeIs`` function returns ``True``,
+     type checkers can infer a more precise type combining the previously known type of the
+     variable with the ``TypeIs`` type. (Technically, this is known as an intersection type.)
+   * When a ``TypeGuard`` function returns ``False``, type checkers cannot narrow the type of
+     the variable at all. When a ``TypeIs`` function returns ``False``, type checkers can narrow
+     the type of the variable to exclude the ``TypeIs`` type.
 
    .. versionadded:: 3.10
 

Doc/whatsnew/3.13.rst

@@ -87,6 +87,10 @@ Interpreter improvements:
   Performance improvements are modest -- we expect to be improving this
   over the next few releases.
 
+New typing features:
+
+* :pep:`742`: :data:`typing.TypeIs` was added, providing more intuitive
+  type narrowing behavior.
 
 New Features
 ============

Lib/test/test_typing.py

@@ -38,7 +38,7 @@
 from typing import Self, LiteralString
 from typing import TypeAlias
 from typing import ParamSpec, Concatenate, ParamSpecArgs, ParamSpecKwargs
-from typing import TypeGuard
+from typing import TypeGuard, TypeIs
 import abc
 import textwrap
 import typing
@@ -5207,6 +5207,7 @@ def test_subclass_special_form(self):
             Literal[1, 2],
             Concatenate[int, ParamSpec("P")],
             TypeGuard[int],
+            TypeIs[range],
         ):
             with self.subTest(msg=obj):
                 with self.assertRaisesRegex(
@@ -6748,6 +6749,7 @@ class C(Generic[T]): pass
         self.assertEqual(get_args(NotRequired[int]), (int,))
         self.assertEqual(get_args(TypeAlias), ())
         self.assertEqual(get_args(TypeGuard[int]), (int,))
+        self.assertEqual(get_args(TypeIs[range]), (range,))
         Ts = TypeVarTuple('Ts')
         self.assertEqual(get_args(Ts), ())
         self.assertEqual(get_args((*Ts,)[0]), (Ts,))
@@ -9592,6 +9594,56 @@ def test_no_isinstance(self):
             issubclass(int, TypeGuard)
 
 
+class TypeIsTests(BaseTestCase):
+    def test_basics(self):
+        TypeIs[int]  # OK
+
+        def foo(arg) -> TypeIs[int]: ...
+        self.assertEqual(gth(foo), {'return': TypeIs[int]})
+
+        with self.assertRaises(TypeError):
+            TypeIs[int, str]
+
+    def test_repr(self):
+        self.assertEqual(repr(TypeIs), 'typing.TypeIs')
+        cv = TypeIs[int]
+        self.assertEqual(repr(cv), 'typing.TypeIs[int]')
+        cv = TypeIs[Employee]
+        self.assertEqual(repr(cv), 'typing.TypeIs[%s.Employee]' % __name__)
+        cv = TypeIs[tuple[int]]
+        self.assertEqual(repr(cv), 'typing.TypeIs[tuple[int]]')
+
+    def test_cannot_subclass(self):
+        with self.assertRaisesRegex(TypeError, CANNOT_SUBCLASS_TYPE):
+            class C(type(TypeIs)):
+                pass
+        with self.assertRaisesRegex(TypeError, CANNOT_SUBCLASS_TYPE):
+            class D(type(TypeIs[int])):
+                pass
+        with self.assertRaisesRegex(TypeError,
+                                    r'Cannot subclass typing\.TypeIs'):
+            class E(TypeIs):
+                pass
+        with self.assertRaisesRegex(TypeError,
+                                    r'Cannot subclass typing\.TypeIs\[int\]'):
+            class F(TypeIs[int]):
+                pass
+
+    def test_cannot_init(self):
+        with self.assertRaises(TypeError):
+            TypeIs()
+        with self.assertRaises(TypeError):
+            type(TypeIs)()
+        with self.assertRaises(TypeError):
+            type(TypeIs[Optional[int]])()
+
+    def test_no_isinstance(self):
+        with self.assertRaises(TypeError):
+            isinstance(1, TypeIs[int])
+        with self.assertRaises(TypeError):
+            issubclass(int, TypeIs)
+
+
 SpecialAttrsP = typing.ParamSpec('SpecialAttrsP')
 SpecialAttrsT = typing.TypeVar('SpecialAttrsT', int, float, complex)
 
@@ -9691,6 +9743,7 @@ def test_special_attrs(self):
             typing.Optional: 'Optional',
             typing.TypeAlias: 'TypeAlias',
             typing.TypeGuard: 'TypeGuard',
+            typing.TypeIs: 'TypeIs',
             typing.TypeVar: 'TypeVar',
             typing.Union: 'Union',
             typing.Self: 'Self',
@@ -9705,6 +9758,7 @@ def test_special_attrs(self):
             typing.Literal[True, 2]: 'Literal',
             typing.Optional[Any]: 'Optional',
             typing.TypeGuard[Any]: 'TypeGuard',
+            typing.TypeIs[Any]: 'TypeIs',
             typing.Union[Any]: 'Any',
             typing.Union[int, float]: 'Union',
             # Incompatible special forms (tested in test_special_attrs2)