gh-128509: Add sys._is_immortal for identifying immortal objects

Doc/glossary.rst

@@ -658,6 +658,9 @@ Glossary
       and therefore it is never deallocated while the interpreter is running.
       For example, :const:`True` and :const:`None` are immortal in CPython.
 
+      Immortal objects can be identified via :func:`sys._is_immortal`, or
+      via :c:func:`PyUnstable_IsImmortal` in the C API.
+
    immutable
       An object with a fixed value.  Immutable objects include numbers, strings and
       tuples.  Such an object cannot be altered.  A new object has to

Doc/library/sys.rst

@@ -855,6 +855,11 @@ always available. Unless explicitly noted otherwise, all variables are read-only
    reflect the actual number of references.  Consequently, do not rely
    on the returned value to be accurate, other than a value of 0 or 1.
 
+   .. impl-detail::
+
+      :term:`Immortal <immortal>` objects with a large reference count can be
+      identified via :func:`_is_immortal`.
+
    .. versionchanged:: 3.12
       Immortal objects have very large refcounts that do not match
       the actual number of references to the object.
@@ -1264,6 +1269,24 @@ always available. Unless explicitly noted otherwise, all variables are read-only
 
    .. versionadded:: 3.12
 
+.. function:: _is_immortal(op)
+
+   Return :const:`True` if the given object is :term:`immortal`, :const:`False`
+   otherwise.
+
+   .. note::
+
+      Objects that are immortal (and thus return ``True`` upon being passed
+      to this function) are not guaranteed to be immortal in future versions,
+      and vice versa for mortal objects.
+
+   .. versionadded:: next
+
+   .. impl-detail::
+
+      This function should be used for specialized purposes only.
+      It is not guaranteed to exist in all implementations of Python.
+
 .. function:: _is_interned(string)
 
    Return :const:`True` if the given string is "interned", :const:`False`

Doc/whatsnew/3.14.rst

@@ -649,9 +649,13 @@ sys
   which only exists in specialized builds of Python, may now return objects
   from other interpreters than the one it's called in.
 
+* Add :func:`sys._is_immortal` for determining if an object is :term:`immortal`.
+  (Contributed by Peter Bierma in :gh:`128509`.)
+
 * On FreeBSD, :data:`sys.platform` doesn't contain the major version anymore.
   It is always ``'freebsd'``, instead of ``'freebsd13'`` or ``'freebsd14'``.
 
+
 sys.monitoring
 --------------
 

Misc/NEWS.d/next/Library/2025-01-04-20-51-48.gh-issue-128509.3gr_-O.rst

@@ -0,0 +1,2 @@
+Add :func:`sys._is_immortal` for identifying :term:`immortal` objects at
+runtime.

Python/sysmodule.c

@@ -972,6 +972,23 @@ sys__is_interned_impl(PyObject *module, PyObject *string)
     return PyUnicode_CHECK_INTERNED(string);
 }
 
+/*[clinic input]
+sys._is_immortal -> bool
+
+  op: object
+  /
+
+Return True if the given object is "immortal" per PEP 683.
+
+This function should be used for specialized purposes only.
+[clinic start generated code]*/
+
+static int
+sys__is_immortal_impl(PyObject *module, PyObject *op)
+/*[clinic end generated code: output=c2f5d6a80efb8d1a input=4609c9bf5481db76]*/
+{
+    return PyUnstable_IsImmortal(op);
+}
 
 /*
  * Cached interned string objects used for calling the profile and
@@ -2588,6 +2605,7 @@ static PyMethodDef sys_methods[] = {
     SYS__GETFRAMEMODULENAME_METHODDEF
     SYS_GETWINDOWSVERSION_METHODDEF
     SYS__ENABLELEGACYWINDOWSFSENCODING_METHODDEF
+    SYS__IS_IMMORTAL_METHODDEF
     SYS_INTERN_METHODDEF
     SYS__IS_INTERNED_METHODDEF
     SYS_IS_FINALIZING_METHODDEF