bpo-46414: Add typing.reveal_type (#30646)

Co-authored-by: Nikita Sobolev <mail@sobolevn.me>

2 files changed, 31 insertions, 1 deletions

diff --git a/Lib/test/test_typing.py b/Lib/test/test_typing.py
index 090d4c7..85f7406 100644
--- a/Lib/test/test_typing.py
+++ b/Lib/test/test_typing.py
@@ -20,6 +20,7 @@ from typing import cast, runtime_checkable
 from typing import get_type_hints
 from typing import get_origin, get_args
 from typing import is_typeddict
+from typing import reveal_type
 from typing import no_type_check, no_type_check_decorator
 from typing import Type
 from typing import NamedTuple, TypedDict
@@ -34,7 +35,7 @@ import typing
 import weakref
 import types
 
-from test.support import import_helper
+from test.support import import_helper, captured_stderr
 from test import mod_generics_cache
 from test import _typed_dict_helper
 
@@ -5289,6 +5290,14 @@ class SpecialAttrsTests(BaseTestCase):
         self.assertIn('baz', dir(Foo[int]))
 
 
+class RevealTypeTests(BaseTestCase):
+    def test_reveal_type(self):
+        obj = object()
+        with captured_stderr() as stderr:
+            self.assertIs(obj, reveal_type(obj))
+        self.assertEqual(stderr.getvalue(), "Runtime type is 'object'\n")
+
+
 class AllTests(BaseTestCase):
     """Tests for __all__."""
 
diff --git a/Lib/typing.py b/Lib/typing.py
index dac9c6c..0cf9755 100644
--- a/Lib/typing.py
+++ b/Lib/typing.py
@@ -130,6 +130,7 @@ __all__ = [
     'overload',
     'ParamSpecArgs',
     'ParamSpecKwargs',
+    'reveal_type',
     'runtime_checkable',
     'Text',
     'TYPE_CHECKING',
@@ -2675,3 +2676,23 @@ class re(metaclass=_DeprecatedType):
 
 re.__name__ = __name__ + '.re'
 sys.modules[re.__name__] = re
+
+
+def reveal_type(obj: T, /) -> T:
+    """Reveal the inferred type of a variable.
+
+    When a static type checker encounters a call to ``reveal_type()``,
+    it will emit the inferred type of the argument::
+
+        x: int = 1
+        reveal_type(x)
+
+    Running a static type checker (e.g., ``mypy``) on this example
+    will produce output similar to 'Revealed type is "builtins.int"'.
+
+    At runtime, the function prints the runtime type of the
+    argument and returns it unchanged.
+
+    """
+    print(f"Runtime type is {type(obj).__name__!r}", file=sys.stderr)
+    return obj