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Author vstinner
Recipients methane, serhiy.storchaka, vstinner
Date 2020-09-02.13:59:25
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Message-id <>
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 <>
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 <>
Date:   Fri May 25 13:52:07 2007 +0000

    Add interning of unicode strings by copying the functionality from
    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 <>
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
    * 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.
Date User Action Args
2020-09-02 13:59:25vstinnersetrecipients: + vstinner, methane, serhiy.storchaka
2020-09-02 13:59:25vstinnersetmessageid: <>
2020-09-02 13:59:25vstinnerlinkissue41692 messages
2020-09-02 13:59:25vstinnercreate