Implement the identity function in the operator module

AA-Turner · AA-Turner · commit bd9aef9102ac · 2025-04-10T05:04:43.000+01:00
diff --git a/Doc/library/operator.rst b/Doc/library/operator.rst
@@ -384,6 +384,13 @@ expect a function argument.
           return caller
 
 
+.. function:: identity(obj, /)
+
+   Returns *obj* unchanged.
+
+   .. versionadded:: next
+
+
 .. _operator-map:
 
 Mapping Operators to Functions
diff --git a/Lib/operator.py b/Lib/operator.py
@@ -12,7 +12,7 @@
 
 __all__ = ['abs', 'add', 'and_', 'attrgetter', 'call', 'concat', 'contains', 'countOf',
            'delitem', 'eq', 'floordiv', 'ge', 'getitem', 'gt', 'iadd', 'iand',
-           'iconcat', 'ifloordiv', 'ilshift', 'imatmul', 'imod', 'imul',
+           'iconcat', 'identity', 'ifloordiv', 'ilshift', 'imatmul', 'imod', 'imul',
            'index', 'indexOf', 'inv', 'invert', 'ior', 'ipow', 'irshift',
            'is_', 'is_none', 'is_not', 'is_not_none', 'isub', 'itemgetter', 'itruediv',
            'ixor', 'le', 'length_hint', 'lshift', 'lt', 'matmul', 'methodcaller', 'mod',
@@ -231,6 +231,10 @@ def length_hint(obj, default=0):
 
 # Other Operations ************************************************************#
 
+def identity(obj, /):
+    """Return the argument unchanged."""
+    return obj
+
 def call(obj, /, *args, **kwargs):
     """Same as obj(*args, **kwargs)."""
     return obj(*args, **kwargs)
diff --git a/Lib/test/test_operator.py b/Lib/test/test_operator.py
@@ -614,6 +614,27 @@ class Y: pass
             operator.length_hint(X(2), "abc")
         self.assertEqual(operator.length_hint(Y(), 10), 10)
 
+    def test_identity(self):
+        operator = self.module
+        a = 'spam'
+        b = []
+        b.append(b)
+        c = None
+        d = object()
+        try:
+            1/0
+        except ZeroDivisionError as exc:
+            e = exc
+        f = {a, c, d, 'ham' * 1000}
+
+        self.assertRaises(TypeError, operator.identity)
+        self.assertIs(operator.identity(a), a)
+        self.assertIs(operator.identity(b), b)
+        self.assertIs(operator.identity(c), c)
+        self.assertIs(operator.identity(d), d)
+        self.assertIs(operator.identity(e), e)
+        self.assertIs(operator.identity(f), f)
+
     def test_call(self):
         operator = self.module
 
diff --git a/Lib/test/test_typing.py b/Lib/test/test_typing.py
@@ -7868,14 +7868,6 @@ def foo(a: A) -> Optional[BaseException]:
         self.assertIsNone(foo(None))
 
 
-class TestModules(TestCase):
-    func_names = ['_idfunc']
-
-    def test_c_functions(self):
-        for fname in self.func_names:
-            self.assertEqual(getattr(typing, fname).__module__, '_typing')
-
-
 class NewTypeTests(BaseTestCase):
     @classmethod
     def setUpClass(cls):
diff --git a/Lib/typing.py b/Lib/typing.py
@@ -30,7 +30,6 @@
 from types import GenericAlias
 
 from _typing import (
-    _idfunc,
     TypeVar,
     ParamSpec,
     TypeVarTuple,
@@ -3359,7 +3358,7 @@ def name_by_id(user_id: UserId) -> str:
         num = UserId(5) + 1     # type: int
     """
 
-    __call__ = _idfunc
+    __call__ = operator.identity
 
     def __init__(self, name, tp):
         self.__qualname__ = name
diff --git a/Misc/NEWS.d/next/Library/2025-04-10-04-27-41.gh-issue-132341.wwXXVD.rst b/Misc/NEWS.d/next/Library/2025-04-10-04-27-41.gh-issue-132341.wwXXVD.rst
@@ -0,0 +1,2 @@
+Add :func:`operator.identity`, the identity function.
+Patch by Adam Turner.
diff --git a/Modules/_operator.c b/Modules/_operator.c
@@ -915,6 +915,22 @@ _operator__compare_digest_impl(PyObject *module, PyObject *a, PyObject *b)
     return PyBool_FromLong(rc);
 }
 
+/*[clinic input]
+_operator.identity -> object
+
+    obj: object
+    /
+
+Return the argument unchanged.
+[clinic start generated code]*/
+
+static PyObject *
+_operator_identity(PyObject *module, PyObject *obj)
+/*[clinic end generated code: output=deede47105fe0dd8 input=72f5a615c2a823c2]*/
+{
+    return Py_NewRef(obj);
+}
+
 PyDoc_STRVAR(_operator_call__doc__,
 "call($module, obj, /, *args, **kwargs)\n"
 "--\n"
@@ -994,6 +1010,7 @@ static struct PyMethodDef operator_methods[] = {
     _OPERATOR_GE_METHODDEF
     _OPERATOR__COMPARE_DIGEST_METHODDEF
     _OPERATOR_LENGTH_HINT_METHODDEF
+    _OPERATOR_IDENTITY_METHODDEF
     _OPERATOR_CALL_METHODDEF
     {NULL,              NULL}           /* sentinel */
 
diff --git a/Modules/_typingmodule.c b/Modules/_typingmodule.c
@@ -9,36 +9,6 @@
 #include "internal/pycore_typevarobject.h"
 #include "internal/pycore_unionobject.h"  // _PyUnion_Type
 #include "pycore_pystate.h"       // _PyInterpreterState_GET()
-#include "clinic/_typingmodule.c.h"
-
-/*[clinic input]
-module _typing
-
-[clinic start generated code]*/
-/*[clinic end generated code: output=da39a3ee5e6b4b0d input=1db35baf1c72942b]*/
-
-/* helper function to make typing.NewType.__call__ method faster */
-
-/*[clinic input]
-_typing._idfunc -> object
-
-    x: object
-    /
-
-[clinic start generated code]*/
-
-static PyObject *
-_typing__idfunc(PyObject *module, PyObject *x)
-/*[clinic end generated code: output=63c38be4a6ec5f2c input=49f17284b43de451]*/
-{
-    return Py_NewRef(x);
-}
-
-
-static PyMethodDef typing_methods[] = {
-    _TYPING__IDFUNC_METHODDEF
-    {NULL, NULL, 0, NULL}
-};
 
 PyDoc_STRVAR(typing_doc,
 "Primitives and accelerators for the typing module.\n");
@@ -85,7 +55,7 @@ static struct PyModuleDef typingmodule = {
         "_typing",
         typing_doc,
         0,
-        typing_methods,
+        NULL,
         _typingmodule_slots,
         NULL,
         NULL,
diff --git a/Modules/clinic/_operator.c.h b/Modules/clinic/_operator.c.h
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 @@`
	`1`	+Add :func:`operator.identity`, the identity function.
	`2`	`+Patch by Adam Turner.`