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

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

Doc/library/typing.rst

@@ -1932,6 +1932,37 @@ Functions and decorators
    runtime we intentionally don't check anything (we want this
    to be as fast as possible).
 
+.. function:: reveal_type(obj)
+
+   Reveal the inferred static type of an expression.
+
+   When a static type checker encounters a call to this function,
+   it emits a diagnostic with the type of the argument. For example::
+
+      x: int = 1
+      reveal_type(x)  # Revealed type is "builtins.int"
+
+   This can be useful when you want to debug how your type checker
+   handles a particular piece of code.
+
+   The function returns its argument unchanged, which allows using
+   it within an expression::
+
+      x = reveal_type(1)  # Revealed type is "builtins.int"
+
+   Most type checkers support ``reveal_type()`` anywhere, even if the
+   name is not imported from ``typing``. Importing the name from
+   ``typing`` allows your code to run without runtime errors and
+   communicates intent more clearly.
+
+   At runtime, this function prints the runtime type of its argument to stderr
+   and returns it unchanged::
+
+      x = reveal_type(1)  # prints "Runtime type is int"
+      print(x)  # prints "1"
+
+   .. versionadded:: 3.11
+
 .. decorator:: overload
 
    The ``@overload`` decorator allows describing functions and methods

Lib/test/test_typing.py

@@ -20,6 +20,7 @@
 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 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 @@ def bar(self):
         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__."""
 

Lib/typing.py

@@ -130,6 +130,7 @@ def _idfunc(_, x):
     '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

Misc/NEWS.d/next/Library/2022-01-17-10-00-02.bpo-46414.Ld0b_y.rst

@@ -0,0 +1 @@
+Add :func:`typing.reveal_type`. Patch by Jelle Zijlstra.