bpo-46480: add typing.assert_type (GH-30843)

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
Co-authored-by: David Foster <david@dafoster.net>

1 files changed, 17 insertions, 0 deletions

diff --git a/Lib/typing.py b/Lib/typing.py
index dd68e71..6930f5d 100644
--- a/Lib/typing.py
+++ b/Lib/typing.py
@@ -118,6 +118,7 @@ __all__ = [
 
     # One-off things.
     'AnyStr',
+    'assert_type',
     'assert_never',
     'cast',
     'final',
@@ -2093,6 +2094,22 @@ def cast(typ, val):
     return val
 
 
+def assert_type(val, typ, /):
+    """Assert (to the type checker) that the value is of the given type.
+
+    When the type checker encounters a call to assert_type(), it
+    emits an error if the value is not of the specified type::
+
+        def greet(name: str) -> None:
+            assert_type(name, str)  # ok
+            assert_type(name, int)  # type checker error
+
+    At runtime this returns the first argument unchanged and otherwise
+    does nothing.
+    """
+    return val
+
+
 _allowed_types = (types.FunctionType, types.BuiltinFunctionType,
                   types.MethodType, types.ModuleType,
                   WrapperDescriptorType, MethodWrapperType, MethodDescriptorType)