bpo-46414: Add typing.reveal_type

Part of the documentation was taken from mypy: https://mypy.readthedocs.io/en/stable/common_issues.html#reveal-type
python · gvanrossum · Feb 2, 2022 · Jan 17, 2022 · Jan 17, 2022 · Jan 17, 2022
commit a4d99eff559e6cf79dff52f69850782dcace4393
@@ -1932,6 +1932,34 @@ 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
+   and returns it unchanged.
+
+   .. versionadded:: 3.11
+
 .. decorator:: overload
 
    The ``@overload`` decorator allows describing functions and methods

@@ -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 NewType
@@ -5108,6 +5109,12 @@ def bar(self):
         self.assertIn('baz', dir(Foo[int]))
 
 
+class RevealTypeTests(BaseTestCase):
+    def test_reveal_type(self):
+        obj = object()
+        self.assertIs(obj, reveal_type(obj))
+
+
 class AllTests(BaseTestCase):
     """Tests for __all__."""
 

@@ -130,6 +130,7 @@ def _idfunc(_, x):
     'overload',
     'ParamSpecArgs',
     'ParamSpecKwargs',
+    'reveal_type',
     'runtime_checkable',
     'Text',
     'TYPE_CHECKING',
@@ -2667,3 +2668,23 @@ class re(metaclass=_DeprecatedType):
 
 re.__name__ = __name__ + '.re'
 sys.modules[re.__name__] = re
+
+
+def reveal_type(obj: T, /) -> T:
 def read(self, n: int = -1) -> AnyStr: 
 def read(self, n: int = -1) -> AnyStr: 
+    """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("Runtime type is", type(obj))
+    return obj
diff --git a/Misc/NEWS.d/next/Library/2022-01-17-10-00-02.bpo-46414.Ld0b_y.rst b/Misc/NEWS.d/next/Library/2022-01-17-10-00-02.bpo-46414.Ld0b_y.rst
@@ -0,0 +1 @@
+Add ``typing.reveal_type()``. Patch by Jelle Zijlstra.
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1 @@
		Add ``typing.reveal_type()``. Patch by Jelle Zijlstra.
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