Add typing_extensions.reveal_type (#1055)

Author: JelleZijlstra

CHANGELOG

@@ -1,5 +1,6 @@
 # Release 4.x.x
 
+- Add `reveal_type`. Backport from bpo-46414.
 - Runtime support for PEP 681 and `typing_extensions.dataclass_transform`.
 - `Annotated` can now wrap `ClassVar` and `Final`. Backport from
   bpo-46491. Patch by Gregory Beauregard (@GBeauregard).

README.rst

@@ -40,6 +40,10 @@ This module currently contains the following:
   - ``@dataclass_transform()`` (see PEP 681)
   - ``NotRequired`` (see PEP 655)
   - ``Required`` (see PEP 655)
+
+- In ``typing`` since Python 3.11
+
+  - ``reveal_type``
   - ``Self`` (see PEP 673)
 
 - In ``typing`` since Python 3.10

src/test_typing_extensions.py

@@ -22,7 +22,7 @@
 from typing_extensions import TypeAlias, ParamSpec, Concatenate, ParamSpecArgs, ParamSpecKwargs, TypeGuard
 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 dataclass_transform
+from typing_extensions import dataclass_transform, reveal_type
 try:
     from typing_extensions import get_type_hints
 except ImportError:
@@ -2346,6 +2346,12 @@ def cached(self): ...
         self.assertIs(True, Methods.cached.__final__)
 
 
+class RevealTypeTests(BaseTestCase):
+    def test_reveal_type(self):
+        obj = object()
+        self.assertIs(obj, reveal_type(obj))
+
+
 class DataclassTransformTests(BaseTestCase):
     def test_decorator(self):
         def create_model(*, frozen: bool = False, kw_only: bool = True):

src/typing_extensions.py

@@ -78,6 +78,7 @@ def _check_generic(cls, parameters):
     'NewType',
     'overload',
     'Protocol',
+    'reveal_type',
     'runtime',
     'runtime_checkable',
     'Text',
@@ -2343,6 +2344,29 @@ class Movie(TypedDict):
     Required = _Required(_root=True)
     NotRequired = _NotRequired(_root=True)
 
+if hasattr(typing, "reveal_type"):
+    reveal_type = typing.reveal_type
+else:
+    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
+
+
 if hasattr(typing, 'dataclass_transform'):
     dataclass_transform = typing.dataclass_transform
 else: