Add assert_type (#1103)

4 files changed, 40 insertions, 2 deletions

diff --git a/typing_extensions/CHANGELOG b/typing_extensions/CHANGELOG
index f098464..a9a5980 100644
--- a/typing_extensions/CHANGELOG
+++ b/typing_extensions/CHANGELOG
@@ -1,5 +1,6 @@
 # Unreleased
 
+- Add `typing.assert_type`. Backport from bpo-46480.
 - Drop support for Python 3.6. Original patch by Adam Turner (@AA-Turner).
 
 # Release 4.1.1 (February 13, 2022)
diff --git a/typing_extensions/README.rst b/typing_extensions/README.rst
index d29d9f5..9abed04 100644
--- a/typing_extensions/README.rst
+++ b/typing_extensions/README.rst
@@ -46,6 +46,7 @@ This module currently contains the following:
 - In ``typing`` since Python 3.11
 
   - ``assert_never``
+  - ``assert_type``
   - ``LiteralString`` (see PEP 675)
   - ``Never``
   - ``NotRequired`` (see PEP 655)
diff --git a/typing_extensions/src/test_typing_extensions.py b/typing_extensions/src/test_typing_extensions.py
index 53a8343..b8fe5e3 100644
--- a/typing_extensions/src/test_typing_extensions.py
+++ b/typing_extensions/src/test_typing_extensions.py
@@ -12,7 +12,7 @@ import types
 from unittest import TestCase, main, skipUnless, skipIf
 from test import ann_module, ann_module2, ann_module3
 import typing
-from typing import TypeVar, Optional, Union, Any
+from typing import TypeVar, Optional, Union, Any, AnyStr
 from typing import T, KT, VT  # Not in __all__.
 from typing import Tuple, List, Dict, Iterable, Iterator, Callable
 from typing import Generic, NamedTuple
@@ -23,7 +23,7 @@ from typing_extensions import TypeAlias, ParamSpec, Concatenate, ParamSpecArgs,
 from typing_extensions import Awaitable, AsyncIterator, AsyncContextManager, Required, NotRequired
 from typing_extensions import Protocol, runtime, runtime_checkable, Annotated, overload, final, is_typeddict
 from typing_extensions import TypeVarTuple, Unpack, dataclass_transform, reveal_type, Never, assert_never, LiteralString
-from typing_extensions import get_type_hints, get_origin, get_args
+from typing_extensions import assert_type, get_type_hints, get_origin, get_args
 
 # Flags used to mark tests that only apply after a specific
 # version of the typing module.
@@ -425,6 +425,21 @@ class OverloadTests(BaseTestCase):
         blah()
 
 
+class AssertTypeTests(BaseTestCase):
+
+    def test_basics(self):
+        arg = 42
+        self.assertIs(assert_type(arg, int), arg)
+        self.assertIs(assert_type(arg, Union[str, float]), arg)
+        self.assertIs(assert_type(arg, AnyStr), arg)
+        self.assertIs(assert_type(arg, None), arg)
+
+    def test_errors(self):
+        # Bogus calls are not expected to fail.
+        arg = 42
+        self.assertIs(assert_type(arg, 42), arg)
+        self.assertIs(assert_type(arg, 'hello'), arg)
+
 
 T_a = TypeVar('T_a')
 
diff --git a/typing_extensions/src/typing_extensions.py b/typing_extensions/src/typing_extensions.py
index 5c43354..5698a76 100644
--- a/typing_extensions/src/typing_extensions.py
+++ b/typing_extensions/src/typing_extensions.py
@@ -717,6 +717,27 @@ else:
         """
         return isinstance(tp, tuple(_TYPEDDICT_TYPES))
 
+
+if hasattr(typing, "assert_type"):
+    assert_type = typing.assert_type
+
+else:
+    def assert_type(__val, __typ):
+        """Assert (to the type checker) that the value is of the given type.
+
+        When the type checker encounters a call to assert_type(), it
+        emits an error if the value is not of the specified type::
+
+            def greet(name: str) -> None:
+                assert_type(name, str)  # ok
+                assert_type(name, int)  # type checker error
+
+        At runtime this returns the first argument unchanged and otherwise
+        does nothing.
+        """
+        return __val
+
+
 if hasattr(typing, "Required"):
     get_type_hints = typing.get_type_hints
 else: