What’s New In Python 3.11

Release

3.11.0a4

Date

January 18, 2022

This article explains the new features in Python 3.11, compared to 3.10.

For full details, see the changelog.

Note

Prerelease users should be aware that this document is currently in draft form. It will be updated substantially as Python 3.11 moves towards release, so it’s worth checking back even after reading earlier versions.

Summary – Release highlights

PEP-654: Exception Groups and except*. (Contributed by Irit Katriel in bpo-45292.)

New Features

Enhanced error locations in tracebacks

When printing tracebacks, the interpreter will now point to the exact expression that caused the error instead of just the line. For example:

Traceback (most recent call last):
  File "distance.py", line 11, in <module>
    print(manhattan_distance(p1, p2))
          ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "distance.py", line 6, in manhattan_distance
    return abs(point_1.x - point_2.x) + abs(point_1.y - point_2.y)
                           ^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'x'

Previous versions of the interpreter would point to just the line making it ambiguous which object was None. These enhanced errors can also be helpful when dealing with deeply nested dictionary objects and multiple function calls,

Traceback (most recent call last):
  File "query.py", line 37, in <module>
    magic_arithmetic('foo')
    ^^^^^^^^^^^^^^^^^^^^^^^
  File "query.py", line 18, in magic_arithmetic
    return add_counts(x) / 25
           ^^^^^^^^^^^^^
  File "query.py", line 24, in add_counts
    return 25 + query_user(user1) + query_user(user2)
                ^^^^^^^^^^^^^^^^^
  File "query.py", line 32, in query_user
    return 1 + query_count(db, response['a']['b']['c']['user'], retry=True)
                               ~~~~~~~~~~~~~~~~~~^^^^^
TypeError: 'NoneType' object is not subscriptable

as well as complex arithmetic expressions:

Traceback (most recent call last):
  File "calculation.py", line 54, in <module>
    result = (x / y / z) * (a / b / c)
              ~~~~~~^~~
ZeroDivisionError: division by zero

See PEP 657 for more details. (Contributed by Pablo Galindo, Batuhan Taskaya and Ammar Askar in bpo-43950.)

Note

This feature requires storing column positions in code objects which may result in a small increase of disk usage of compiled Python files or interpreter memory usage. To avoid storing the extra information and/or deactivate printing the extra traceback information, the -X no_debug_ranges command line flag or the PYTHONNODEBUGRANGES environment variable can be used.

Column information for code objects

The information used by the enhanced traceback feature is made available as a general API that can be used to correlate bytecode instructions with source code. This information can be retrieved using:

The -X no_debug_ranges option and the environment variable PYTHONNODEBUGRANGES can be used to disable this feature.

See PEP 657 for more details. (Contributed by Pablo Galindo, Batuhan Taskaya and Ammar Askar in bpo-43950.)

Exceptions can be enriched with a string __note__

The __note__ field was added to BaseException. It is None by default but can be set to a string which is added to the exception’s traceback. (Contributed by Irit Katriel in bpo-45607.)

Other Language Changes

Other CPython Implementation Changes

  • Special methods complex.__complex__() and bytes.__bytes__() are implemented to support typing.SupportsComplex and typing.SupportsBytes protocols. (Contributed by Mark Dickinson and Dong-hee Na in bpo-24234.)

  • siphash13 is added as a new internal hashing algorithms. It’s has similar security properties as siphash24 but it is slightly faster for long inputs. str, bytes, and some other types now use it as default algorithm for hash(). PEP 552 hash-based pyc files now use siphash13, too. (Contributed by Inada Naoki in bpo-29410.)

  • When an active exception is re-raised by a raise statement with no parameters, the traceback attached to this exception is now always sys.exc_info()[1].__traceback__. This means that changes made to the traceback in the current except clause are reflected in the re-raised exception. (Contributed by Irit Katriel in bpo-45711.)

  • The interpreter state’s representation of handled exceptions (a.k.a exc_info, or _PyErr_StackItem) now has only the exc_value field, exc_type and exc_traceback have been removed as their values can be derived from exc_value. (Contributed by Irit Katriel in bpo-45711.)

  • A new command line option for the Windows installer AppendPath has beend added. It behaves similiar to PrependPath but appends the install and scripts directories instead of prepending it. (Contributed by Bastian Neuburger in bpo-44934.)

New Modules

  • None yet.

Improved Modules

fractions

  • Support PEP 515-style initialization of Fraction from string. (Contributed by Sergey B Kirpichev in bpo-44258.)

  • Fraction now implements an __int__ method, so that an isinstance(some_fraction, typing.SupportsInt) check passes. (Contributed by Mark Dickinson in bpo-44547.)

inspect

math

  • Add math.exp2(): return 2 raised to the power of x. (Contributed by Gideon Mitchell in bpo-45917.)

  • Add math.cbrt(): return the cube root of x. (Contributed by Ajith Ramachandran in bpo-44357.)

  • The behaviour of two math.pow() corner cases was changed, for consistency with the IEEE 754 specification. The operations math.pow(0.0, -math.inf) and math.pow(-0.0, -math.inf) now return inf. Previously they raised ValueError. (Contributed by Mark Dickinson in bpo-44339.)

operator

  • A new function operator.call has been added, such that operator.call(obj, *args, **kwargs) == obj(*args, **kwargs). (Contributed by Antony Lee in bpo-44019.)

os

  • On Windows, os.urandom() now uses BCryptGenRandom(), instead of CryptGenRandom() which is deprecated. (Contributed by Dong-hee Na in bpo-44611.)

sqlite3

sys

  • sys.exc_info() now derives the type and traceback fields from the value (the exception instance), so when an exception is modified while it is being handled, the changes are reflected in the results of subsequent calls to exc_info(). (Contributed by Irit Katriel in bpo-45711.)

  • Add sys.exception() which returns the active exception instance (equivalent to sys.exc_info()[1]). (Contributed by Irit Katriel in bpo-46328.)

threading

time

  • On Unix, time.sleep() now uses the clock_nanosleep() or nanosleep() function, if available, which has a resolution of 1 nanosecond (10-9 seconds), rather than using select() which has a resolution of 1 microsecond (10-6 seconds). (Contributed by Benjamin Szőke and Victor Stinner in bpo-21302.)

  • On Windows 8.1 and newer, time.sleep() now uses a waitable timer based on high-resolution timers which has a resolution of 100 nanoseconds (10-7 seconds). Previously, it had a resolution of 1 millisecond (10-3 seconds). (Contributed by Benjamin Szőke, Dong-hee Na, Eryk Sun and Victor Stinner in bpo-21302 and bpo-45429.)

unicodedata

  • The Unicode database has been updated to version 14.0.0. (bpo-45190).

fcntl

  • On FreeBSD, the F_DUP2FD and F_DUP2FD_CLOEXEC flags respectively are supported, the former equals to dup2 usage while the latter set the FD_CLOEXEC flag in addition.

Optimizations

  • Compiler now optimizes simple C-style formatting with literal format containing only format codes %s, %r and %a and makes it as fast as corresponding f-string expression. (Contributed by Serhiy Storchaka in bpo-28307.)

  • “Zero-cost” exceptions are implemented. The cost of try statements is almost eliminated when no exception is raised. (Contributed by Mark Shannon in bpo-40222.)

  • Method calls with keywords are now faster due to bytecode changes which avoid creating bound method instances. Previously, this optimization was applied only to method calls with purely positional arguments. (Contributed by Ken Jin and Mark Shannon in bpo-26110, based on ideas implemented in PyPy.)

  • Pure ASCII strings are now normalized in constant time by unicodedata.normalize(). (Contributed by Dong-hee Na in bpo-44987.)

  • math functions comb() and perm() are now up to 10 times or more faster for large arguments (the speed up is larger for larger k). (Contributed by Serhiy Storchaka in bpo-37295.)

CPython bytecode changes

  • Replaced all numeric BINARY_* and INPLACE_* instructions with a single BINARY_OP implementation.

  • Replaced the three call instructions: CALL_FUNCTION, CALL_FUNCTION_KW and CALL_METHOD with CALL_NO_KW, CALL_KW and PRECALL_METHOD. This decouples the argument shifting for methods from the handling of keyword arguments and allows better specialization of calls.

  • Removed COPY_DICT_WITHOUT_KEYS and GEN_START.

  • MATCH_CLASS and MATCH_KEYS no longer push an additional boolean value indicating whether the match succeeded or failed. Instead, they indicate failure with None (where a tuple of extracted values would otherwise be).

  • Added COPY, which pushes the i-th item to the top of the stack. The item is not removed from its original location.

  • Add POP_JUMP_IF_NOT_NONE and POP_JUMP_IF_NONE opcodes to speed up conditional jumps.

  • JUMP_IF_NOT_EXC_MATCH no longer pops the active exception.

Deprecated

  • The lib2to3 package and 2to3 tool are now deprecated and may not be able to parse Python 3.10 or newer. See the PEP 617 (New PEG parser for CPython). (Contributed by Victor Stinner in bpo-40360.)

  • webbrowser.MacOSX is deprecated and will be removed in Python 3.13. It is untested and undocumented and also not used by webbrowser itself. (Contributed by Dong-hee Na in bpo-42255.)

  • The behavior of returning a value from a TestCase and IsolatedAsyncioTestCase test methods (other than the default None value), is now deprecated.

  • Deprecated the following unittest functions, scheduled for removal in Python 3.13:

    • unittest.findTestCases()

    • unittest.makeSuite()

    • unittest.getTestCaseNames()

    Use TestLoader method instead:

    (Contributed by Erlend E. Aasland in bpo-5846.)

  • The turtle.RawTurtle.settiltangle() is deprecated since Python 3.1, it now emits a deprecation warning and will be removed in Python 3.13. Use turtle.RawTurtle.tiltangle() instead (it was earlier incorrectly marked as deprecated, its docstring is now corrected). (Contributed by Hugo van Kemenade in bpo-45837.)

Removed

  • smtpd.MailmanProxy is now removed as it is unusable without an external module, mailman. (Contributed by Dong-hee Na in bpo-35800.)

  • The binhex module, deprecated in Python 3.9, is now removed. The following binascii functions, deprecated in Python 3.9, are now also removed:

    • a2b_hqx(), b2a_hqx();

    • rlecode_hqx(), rledecode_hqx().

    The binascii.crc_hqx() function remains available.

    (Contributed by Victor Stinner in bpo-45085.)

  • The distutils bdist_msi command, deprecated in Python 3.9, is now removed. Use bdist_wheel (wheel packages) instead. (Contributed by Hugo van Kemenade in bpo-45124.)

  • Due to significant security concerns, the reuse_address parameter of asyncio.loop.create_datagram_endpoint(), disabled in Python 3.9, is now entirely removed. This is because of the behavior of the socket option SO_REUSEADDR in UDP. (Contributed by Hugo van Kemenade in bpo-45129.)

  • Removed __getitem__() methods of xml.dom.pulldom.DOMEventStream, wsgiref.util.FileWrapper and fileinput.FileInput, deprecated since Python 3.9. (Contributed by Hugo van Kemenade in bpo-45132.)

  • Removed many old deprecated unittest features:

    • TestCase method aliases failUnlessEqual, failIfEqual, failUnless, failIf, failUnlessRaises, failUnlessAlmostEqual, failIfAlmostEqual (deprecated in Python 3.1), assertEquals, assertNotEquals, assert_, assertAlmostEquals, assertNotAlmostEquals, assertRegexpMatches, assertRaisesRegexp (deprecated in Python 3.2), and assertNotRegexpMatches (deprecated in Python 3.5).

    • Undocumented and broken TestCase method assertDictContainsSubset (deprecated in Python 3.2).

    • Undocumented <unittest.TestLoader.loadTestsFromModule> TestLoader.loadTestsFromModule() parameter use_load_tests (deprecated and ignored since Python 3.2).

    • An alias of the TextTestResult class: _TextTestResult (deprecated in Python 3.2).

    (Contributed by Serhiy Storchaka in bpo-45162.)

  • The following deprecated functions and methods are removed in the gettext module: lgettext(), ldgettext(), lngettext() and ldngettext().

    Function bind_textdomain_codeset(), methods output_charset() and set_output_charset(), and the codeset parameter of functions translation() and install() are also removed, since they are only used for the l*gettext() functions. (Contributed by Dong-hee Na and Serhiy Storchaka in bpo-44235.)

  • Removed from the configparser module: the SafeConfigParser class, the filename property of the ParsingError class, the readfp() method of the ConfigParser class, deprecated since Python 3.2. (Contributed by Hugo van Kemenade in bpo-45173.)

  • The @asyncio.coroutine decorator enabling legacy generator-based coroutines to be compatible with async/await code. The function has been deprecated since Python 3.8 and the removal was initially scheduled for Python 3.10. Use async def instead. (Contributed by Illia Volochii in bpo-43216.)

  • asyncio.coroutines.CoroWrapper used for wrapping legacy generator-based coroutine objects in the debug mode. (Contributed by Illia Volochii in bpo-43216.)

  • Removed the deprecated split() method of _tkinter.TkappType. (Contributed by Erlend E. Aasland in bpo-38371.)

  • Removed from the inspect module:

    (Contributed by Hugo van Kemenade in bpo-45320.)

  • Remove namespace package support from unittest discovery. It was introduced in Python 3.4 but has been broken since Python 3.7. (Contributed by Inada Naoki in bpo-23882.)

Porting to Python 3.11

This section lists previously described changes and other bugfixes that may require changes to your code.

Changes in the Python API

  • Prohibited passing non-concurrent.futures.ThreadPoolExecutor executors to loop.set_default_executor() following a deprecation in Python 3.8. (Contributed by Illia Volochii in bpo-43234.)

  • open(), io.open(), codecs.open() and fileinput.FileInput no longer accept 'U' (“universal newline”) in the file mode. This flag was deprecated since Python 3.3. In Python 3, the “universal newline” is used by default when a file is open in text mode. The newline parameter of open() controls how universal newlines works. (Contributed by Victor Stinner in bpo-37330.)

  • The pdb module now reads the .pdbrc configuration file with the 'utf-8' encoding. (Contributed by Srinivas Reddy Thatiparthy (శ్రీనివాస్ రెడ్డి తాటిపర్తి) in bpo-41137.)

  • 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 Value comparisons for information on total ordering). It’s generally true that the result of sorting simply isn’t well-defined in the absence of a total ordering on list elements.

Build Changes

  • CPython can now be built with the ThinLTO option via --with-lto=thin. (Contributed by Dong-hee Na and Brett Holman in bpo-44340.)

  • libpython is no longer linked against libcrypt. (Contributed by Mike Gilbert in bpo-45433.)

  • Building Python now requires a C99 <math.h> header file providing the following functions: copysign(), hypot(), isfinite(), isinf(), isnan(), round(). (Contributed by Victor Stinner in bpo-45440.)

  • Freelists for object structs can now be disabled. A new configure option --without-freelists can be used to disable all freelists except empty tuple singleton. (Contributed by Christian Heimes in bpo-45522)

  • Modules/Setup and Modules/makesetup have been improved and tied up. Extension modules can now be built through makesetup. All except some test modules can be linked statically into main binary or library. (Contributed by Brett Cannon and Christian Heimes in bpo-45548, bpo-45570, bpo-45571, and bpo-43974.)

  • Build dependencies, compiler flags, and linker flags for most stdlib extension modules are now detected by configure. libffi, libnsl, libsqlite3, zlib, bzip2, liblzma, libcrypt, and uuid flags are detected by pkg-config (when available). (Contributed by Christian Heimes and Erlend Egeberg Aasland in bpo-bpo-45847, bpo-45747, and bpo-45763.)

  • CPython now has experimental support for cross compiling to WebAssembly platform wasm32-emscripten. The effort is inspired by previous work like Pyodide. (Contributed by Christian Heimes and Ethan Smith in bpo-40280.)

  • CPython will now use 30-bit digits by default for the Python int implementation. Previously, the default was to use 30-bit digits on platforms with SIZEOF_VOID_P >= 8, and 15-bit digits otherwise. It’s still possible to explicitly request use of 15-bit digits via either the --enable-big-digits option to the configure script or (for Windows) the PYLONG_BITS_IN_DIGIT variable in PC/pyconfig.h, but this option may be removed at some point in the future. (Contributed by Mark Dickinson in bpo-45569.)

C API Changes

New Features

Porting to Python 3.11

  • The old trashcan macros (Py_TRASHCAN_SAFE_BEGIN/Py_TRASHCAN_SAFE_END) are now deprecated. They should be replaced by the new macros Py_TRASHCAN_BEGIN and Py_TRASHCAN_END.

    A tp_dealloc function that has the old macros, such as:

    static void
    mytype_dealloc(mytype *p)
    {
        PyObject_GC_UnTrack(p);
        Py_TRASHCAN_SAFE_BEGIN(p);
        ...
        Py_TRASHCAN_SAFE_END
    }
    

    should migrate to the new macros as follows:

    static void
    mytype_dealloc(mytype *p)
    {
        PyObject_GC_UnTrack(p);
        Py_TRASHCAN_BEGIN(p, mytype_dealloc)
        ...
        Py_TRASHCAN_END
    }
    

    Note that Py_TRASHCAN_BEGIN has a second argument which should be the deallocation function it is in.

    To support older Python versions in the same codebase, you can define the following macros and use them throughout the code (credit: these were copied from the mypy codebase):

    #if PY_MAJOR_VERSION >= 3 && PY_MINOR_VERSION >= 8
    #  define CPy_TRASHCAN_BEGIN(op, dealloc) Py_TRASHCAN_BEGIN(op, dealloc)
    #  define CPy_TRASHCAN_END(op) Py_TRASHCAN_END
    #else
    #  define CPy_TRASHCAN_BEGIN(op, dealloc) Py_TRASHCAN_SAFE_BEGIN(op)
    #  define CPy_TRASHCAN_END(op) Py_TRASHCAN_SAFE_END(op)
    #endif
    
  • The PyType_Ready() function now raises an error if a type is defined with the Py_TPFLAGS_HAVE_GC flag set but has no traverse function (PyTypeObject.tp_traverse). (Contributed by Victor Stinner in bpo-44263.)

  • Heap types with the Py_TPFLAGS_IMMUTABLETYPE flag can now inherit the PEP 590 vectorcall protocol. Previously, this was only possible for static types. (Contributed by Erlend E. Aasland in bpo-43908)

  • Since Py_TYPE() is changed to a inline static function, Py_TYPE(obj) = new_type must be replaced with Py_SET_TYPE(obj, new_type): see the Py_SET_TYPE() function (available since Python 3.9). For backward compatibility, this macro can be used:

    #if PY_VERSION_HEX < 0x030900A4 && !defined(Py_SET_TYPE)
    static inline void _Py_SET_TYPE(PyObject *ob, PyTypeObject *type)
    { ob->ob_type = type; }
    #define Py_SET_TYPE(ob, type) _Py_SET_TYPE((PyObject*)(ob), type)
    #endif
    

    (Contributed by Victor Stinner in bpo-39573.)

  • Since Py_SIZE() is changed to a inline static function, Py_SIZE(obj) = new_size must be replaced with Py_SET_SIZE(obj, new_size): see the Py_SET_SIZE() function (available since Python 3.9). For backward compatibility, this macro can be used:

    #if PY_VERSION_HEX < 0x030900A4 && !defined(Py_SET_SIZE)
    static inline void _Py_SET_SIZE(PyVarObject *ob, Py_ssize_t size)
    { ob->ob_size = size; }
    #define Py_SET_SIZE(ob, size) _Py_SET_SIZE((PyVarObject*)(ob), size)
    #endif
    

    (Contributed by Victor Stinner in bpo-39573.)

  • <Python.h> no longer includes the header files <stdlib.h>, <stdio.h>, <errno.h> and <string.h> when the Py_LIMITED_API macro is set to 0x030b0000 (Python 3.11) or higher. C extensions should explicitly include the header files after #include <Python.h>. (Contributed by Victor Stinner in bpo-45434.)

  • The non-limited API files cellobject.h, classobject.h, context.h, funcobject.h, genobject.h and longintrepr.h have been moved to the Include/cpython directory. Moreover, the eval.h header file was removed. These files must not be included directly, as they are already included in Python.h: Include Files. If they have been included directly, consider including Python.h instead. (Contributed by Victor Stinner in bpo-35134.)

  • The PyUnicode_CHECK_INTERNED() macro has been excluded from the limited C API. It was never usable there, because it used internal structures which are not available in the limited C API. (Contributed by Victor Stinner in bpo-46007.)

  • Changes of the PyFrameObject structure members:

    • f_code: removed, use PyFrame_GetCode() instead. Warning: the function returns a strong reference, need to call Py_DECREF().

    • f_back: changed, use PyFrame_GetBack().

    • f_builtins: removed, use PyObject_GetAttrString(frame, "f_builtins").

    • f_globals: removed, use PyObject_GetAttrString(frame, "f_globals").

    • f_locals: removed, use PyObject_GetAttrString(frame, "f_locals").

    • f_lasti: removed, use PyObject_GetAttrString(frame, "f_lasti").

    • f_valuesstack: removed.

    • f_stackdepth: removed.

    • f_gen: removed.

    • f_iblock: removed.

    • f_state: removed.

    • f_blockstack: removed.

    • f_localsplus: removed.

    The Python frame object is now created lazily. A side effect is that the f_back member must not be accessed directly, since its value is now also computed lazily. The PyFrame_GetBack() function must be called instead.

    Code defining PyFrame_GetCode() on Python 3.8 and older:

    #if PY_VERSION_HEX < 0x030900B1
    static inline PyCodeObject* PyFrame_GetCode(PyFrameObject *frame)
    {
        Py_INCREF(frame->f_code);
        return frame->f_code;
    }
    #endif
    

    Code defining PyFrame_GetBack() on Python 3.8 and older:

    #if PY_VERSION_HEX < 0x030900B1
    static inline PyFrameObject* PyFrame_GetBack(PyFrameObject *frame)
    {
        Py_XINCREF(frame->f_back);
        return frame->f_back;
    }
    #endif
    

    Or use the pythoncapi_compat project to get these functions on old Python functions.

  • Changes of the PyThreadState structure members:

    Code defining PyThreadState_GetFrame() on Python 3.8 and older:

    #if PY_VERSION_HEX < 0x030900B1
    static inline PyFrameObject* PyThreadState_GetFrame(PyThreadState *tstate)
    {
        Py_XINCREF(tstate->frame);
        return tstate->frame;
    }
    #endif
    

    Code defining PyThreadState_EnterTracing() and PyThreadState_LeaveTracing() on Python 3.10 and older:

    #if PY_VERSION_HEX < 0x030B00A2
    static inline void PyThreadState_EnterTracing(PyThreadState *tstate)
    {
        tstate->tracing++;
    #if PY_VERSION_HEX >= 0x030A00A1
        tstate->cframe->use_tracing = 0;
    #else
        tstate->use_tracing = 0;
    #endif
    }
    
    static inline void PyThreadState_LeaveTracing(PyThreadState *tstate)
    {
        int use_tracing = (tstate->c_tracefunc != NULL || tstate->c_profilefunc != NULL);
        tstate->tracing--;
    #if PY_VERSION_HEX >= 0x030A00A1
        tstate->cframe->use_tracing = use_tracing;
    #else
        tstate->use_tracing = use_tracing;
    #endif
    }
    #endif
    

    Or use the pythoncapi_compat project to get these functions on old Python functions.

Deprecated

Removed

  • PyFrame_BlockSetup() and PyFrame_BlockPop() have been removed. (Contributed by Mark Shannon in bpo-40222.)

  • Remove the following math macros using the errno variable:

    • Py_ADJUST_ERANGE1()

    • Py_ADJUST_ERANGE2()

    • Py_OVERFLOWED()

    • Py_SET_ERANGE_IF_OVERFLOW()

    • Py_SET_ERRNO_ON_MATH_ERROR()

    (Contributed by Victor Stinner in bpo-45412.)

  • Remove Py_UNICODE_COPY() and Py_UNICODE_FILL() macros, deprecated since Python 3.3. Use PyUnicode_CopyCharacters() or memcpy() (wchar_t* string), and PyUnicode_Fill() functions instead. (Contributed by Victor Stinner in bpo-41123.)

  • Remove the pystrhex.h header file. It only contains private functions. C extensions should only include the main <Python.h> header file. (Contributed by Victor Stinner in bpo-45434.)

  • Remove the Py_FORCE_DOUBLE() macro. It was used by the Py_IS_INFINITY() macro. (Contributed by Victor Stinner in bpo-45440.)

  • The following items are no longer available when Py_LIMITED_API is defined:

    These are not part of the limited API.

    (Contributed by Victor Stinner in bpo-45474.)

  • Exclude PyWeakref_GET_OBJECT() from the limited C API. It never worked since the PyWeakReference structure is opaque in the limited C API. (Contributed by Victor Stinner in bpo-35134.)