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

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

1 files changed, 21 insertions, 0 deletions

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