Python has the concept of "immortal" interned strings: PyUnicode_InternImmortal(). The feature was first introduced in the Python 2 "str" (bytes) type, bpo-576101 (commit 45ec02aed14685c353e55841b5acbc0dadee76f8). New PyString_InternImmortal() function. commit 45ec02aed14685c353e55841b5acbc0dadee76f8 Author: Guido van Rossum <guido@python.org> Date: Mon Aug 19 21:43:18 2002 +0000 SF patch 576101, by Oren Tirosh: alternative implementation of interning. I modified Oren's patch significantly, but the basic idea and most of the implementation is unchanged. Interned strings created with PyString_InternInPlace() are now mortal, and you must keep a reference to the resulting string around; use the new function PyString_InternImmortal() to create immortal interned strings. Later, the feature was added to the PyUnicodeObject type, new PyUnicode_InternImmortal() function: commit 1680713e524016d93a94114c4a874ad71a090b95 Author: Walter Dörwald <walter@livinglogic.de> Date: Fri May 25 13:52:07 2007 +0000 Add interning of unicode strings by copying the functionality from stringobject.c. Intern "True" and "False" in bool_repr() again as it was in the 8bit string era. Since Python 3.10, (mortal) interned strings are cleared at Python exit in Py_Finalize(). It avoids leaking memory when Python is embedded in an application: bpo-1635741. commit 666ecfb0957a2fa0df5e2bd03804195de74bdfbf Author: Victor Stinner <vstinner@python.org> Date: Thu Jul 2 01:19:57 2020 +0200 bpo-1635741: Release Unicode interned strings at exit (GH-21269) * PyUnicode_InternInPlace() now ensures that interned strings are ready. * Add _PyUnicode_ClearInterned(). * Py_Finalize() now releases Unicode interned strings: call _PyUnicode_ClearInterned(). -- PyUnicode_InternImmortal() is not used in the Python standard library. I propose to start deprecating the function and remove it in Python 3.12 (PEP 387 requires a deprecation for 2 releases). In Python 3.10, calling the function will emit a DeprecationWarning at runtime. Note: PyString_InternImmortal() (for bytes strings) has been removed from Python 3.0.

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I proposed PR 22486 to deprecate the function.

 New changeset 583ee5a5b1971a18ebeb877948ce6264da0cc8aa by Victor Stinner in branch 'master': bpo-41692: Deprecate PyUnicode_InternImmortal() (GH-22486) https://github.com/python/cpython/commit/583ee5a5b1971a18ebeb877948ce6264da0cc8aa 

The function is now deprecated. Thanks for the review INADA-san, I close the issue. Let's meet in Python 3.12 to remove it ;-)

For the record, I noticed PyUnicode_InternImmortal() is a stable ABI. We may need to keep the function to avoid dynamic link errors. But we can still change its implementation to just raise an exception.

I cannot find "PyUnicode_InternImmortal" pattern in the source code of the PyPI top 5000 projects (December 1, 2021). I only found a false positive in frozendict-2.1.1: frozendict/src/3_10/cpython_src/Include/unicodeobject.h: // PyUnicode_InternImmortal() is deprecated since Python 3.10 frozendict/src/3_10/cpython_src/Include/unicodeobject.h: Py_DEPRECATED(3.10) PyAPI_FUNC(void) PyUnicode_InternImmortal(PyObject **); frozendict/src/3_6/cpython_src/Include/unicodeobject.h: PyAPI_FUNC(void) PyUnicode_InternImmortal(PyObject **); frozendict/src/3_7/cpython_src/Include/unicodeobject.h: PyAPI_FUNC(void) PyUnicode_InternImmortal(PyObject **); frozendict/src/3_8/cpython_src/Include/unicodeobject.h: PyAPI_FUNC(void) PyUnicode_InternImmortal(PyObject **); frozendict/src/3_9/cpython_src/Include/unicodeobject.h: PyAPI_FUNC(void) PyUnicode_InternImmortal(PyObject **); These are copies of the Python unicodeobject.h header files, but the PyUnicode_InternImmortal() function is not called by frozendict. I used my download_pypi_top.py and search_pypi_top.py tools which can be found at: https://github.com/vstinner/misc/tree/main/cpython