python · uriyyo · Jul 30, 2021 · Jul 30, 2021 · Jul 30, 2021 · Oct 20, 2021
@@ -272,6 +272,13 @@ time
   a resolution of 1 millisecond (10\ :sup:`-3` seconds).
   (Contributed by Benjamin Szőke and Victor Stinner in :issue:`21302`.)
 
+typing
+------
+
+* :func:`typing.cast` now has C implementation. It is now around 2 times faster
+  to call it.
+  (Contributed by Yurii Karabas in :issue:`44775`.)
+
 unicodedata
 -----------
 
@@ -480,6 +487,10 @@ Changes in the Python API
   the ``'utf-8'`` encoding.
   (Contributed by Srinivas Reddy Thatiparthy (శ్రీనివాస్  రెడ్డి తాటిపర్తి) in :issue:`41137`.)
 
+* :func:`typing.cast` no longer accepts keyword arguments. Note that static
+  checkers have always rejected this. Now it's also enforced at runtime.
+  (Contributed by Yurii Karabas in :issue:`44775`.)
+
 * When sorting using tuples as keys, the order of the result may differ
   from earlier releases if the tuple elements don't define a total
   ordering (see :ref:`expressions-value-comparisons` for

@@ -32,8 +32,9 @@
 import typing
 import weakref
 import types
+import pydoc
 
-from test.support import import_helper
+from test.support import captured_stdout, import_helper
 from test import mod_generics_cache
 from test import _typed_dict_helper
 
@@ -2539,9 +2540,10 @@ def func(x): ...
         self.assertIs(func, final(func))
 
 
-class CastTests(BaseTestCase):
-
+class CastTests:
     def test_basics(self):
+        cast = self.module.cast
+
         self.assertEqual(cast(int, 42), 42)
         self.assertEqual(cast(float, 42), 42)
         self.assertIs(type(cast(float, 42)), int)
@@ -2552,10 +2554,39 @@ def test_basics(self):
         self.assertEqual(cast(None, 42), 42)
 
     def test_errors(self):
+        cast = self.module.cast
+
         # Bogus calls are not expected to fail.
         cast(42, 42)
         cast('hello', 42)
 
+    def test_keyword_arguments(self):
+        with self.assertRaises(TypeError):
+            self.module.cast(type=int, val=10)
+
+        with self.assertRaises(TypeError):
+            self.module.cast(int, val=10)
+
+
+class CastPythonTests(CastTests, BaseTestCase):
+    module = py_typing
+
+
+@skipUnless(c_typing, 'requires _typing')
+class CastCTests(CastTests, BaseTestCase):
+    module = c_typing
+
+    def test_help(self):
+        def _get_doc(func):
+            with captured_stdout() as stdout:
+                pydoc.help(func)
+
+            # skip first line because it different for regular and built-in
+            _, doc = stdout.getvalue().split("\n", 1)
+            return doc
+
+        self.assertEqual(_get_doc(c_typing.cast), _get_doc(py_typing.cast))
+
 
 class ForwardRefTests(BaseTestCase):
 

@@ -1714,16 +1714,18 @@ def close(self): ...
     cls._is_runtime_protocol = True
     return cls
 
+try:
+    from _typing import cast
+except ImportError:
+    def cast(typ, val, /):
+        """Cast a value to a type.
 
-def cast(typ, val):
-    """Cast a value to a type.
-
-    This returns the value unchanged.  To the type checker this
-    signals that the return value has the designated type, but at
-    runtime we intentionally don't check anything (we want this
-    to be as fast as possible).
-    """
-    return val
+        This returns the value unchanged.  To the type checker this
+        signals that the return value has the designated type, but at
+        runtime we intentionally don't check anything (we want this
+        to be as fast as possible).
+        """
+        return val
 
 
 def _get_defaults(func):

diff --git a/Misc/NEWS.d/next/Library/2021-07-30-12-35-10.bpo-44775.gDLT2g.rst b/Misc/NEWS.d/next/Library/2021-07-30-12-35-10.bpo-44775.gDLT2g.rst
@@ -0,0 +1,2 @@
+Speed-up ``typing.cast`` by implementing it in C. Also disallow passing keyword
+arguments to ``typing.cast``. Patch provided by Yurii Karabas.
@@ -27,9 +27,33 @@ _typing__idfunc(PyObject *module, PyObject *x)
     return x;
 }
 
+/*[clinic input]
+_typing.cast -> object
+
+    typ: object
+    val: object
+    /
+
+Cast a value to a type.
+
+This returns the value unchanged.  To the type checker this
+signals that the return value has the designated type, but at
+runtime we intentionally don't check anything (we want this
+to be as fast as possible).
+[clinic start generated code]*/
+
+static PyObject *
+_typing_cast_impl(PyObject *module, PyObject *typ, PyObject *val)
+/*[clinic end generated code: output=11224a3fa037a9a1 input=bde696783400a5b0]*/
+{
+    Py_INCREF(val);
+    return val;
+}
+
 
 static PyMethodDef typing_methods[] = {
     _TYPING__IDFUNC_METHODDEF
+    _TYPING_CAST_METHODDEF
     {NULL, NULL, 0, NULL}
 };
 

diff --git a/Modules/clinic/_typingmodule.c.h b/Modules/clinic/_typingmodule.c.h
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,2 @@
		Speed-up ``typing.cast`` by implementing it in C. Also disallow passing keyword
		arguments to ``typing.cast``. Patch provided by Yurii Karabas.