PEP 620
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Victor Stinner
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pep-0620.rst
103
pep-0620.rst
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@ -12,15 +12,16 @@ Abstract
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Introduce C API incompatible changes to hide implementation details.
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Hiding implementation details reduces backward compatibility issues and
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so ease evolution of CPython internals and addition of new features.
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Once most implementation details will be hidden, evolution of CPython
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internals would be less limited by C API backward compatibility issues.
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It will be way easier to add new features.
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It makes possible to experiment in CPython more advanced optimizations
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than just micro-optimizations.
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than just micro-optimizations, like tagged pointers.
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Define a process to reduce the number of broken C extensions.
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The implementation of this PEP is expected to be done slowly over
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The implementation of this PEP is expected to be done carefully over
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multiple Python versions. It already started in Python 3.7 and most
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changes are already completed. The `Process to introduce incompatible
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changes to the C API`_ didactes the rythm.
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@ -32,14 +33,12 @@ Motivation
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The C API blocks CPython evolutions
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-----------------------------------
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Adding or removing members of C structures is causing multiple
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implementation issues.
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Adding or removing members of C structures is causing multiple backward
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compatibility issues.
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Adding a new member breaks the stable ABI (PEP 384), especially for
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types declared statically (e.g. ``static PyTypeObject MyType =
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{...};``).
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In Python 3.4, the PEP 442 "Safe object finalization" added
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{...};``). In Python 3.4, the PEP 442 "Safe object finalization" added
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``tp_finalize`` member at the end of the ``PyTypeObject`` structure. For
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ABI backward compatibility, a new ``Py_TPFLAGS_HAVE_FINALIZE`` type flag
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was required to announce if the type structure contains the
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@ -256,34 +255,44 @@ re-implement this private function to support this C extension.
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**STATUS**: Completed (in Python 3.9)
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In Python 3.9, 4 private functions have been moved to the internal C API
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and are no longer exported:
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Private functions moved to the internal C API in Python 3.8:
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* ``_PyDebug_PrintTotalRefs()``
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* ``_Py_AddToAllObjects()``
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* ``_Py_PrintReferenceAddresses()``
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* ``_Py_PrintReferences()``
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* ``_PyObject_GC_TRACK()``, ``_PyObject_GC_UNTRACK()``
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In Python 3.9, the public "clear free list" functions have been renamed
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to private functions and moved to the internal C API:
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Macros and functions excluded from the limited C API in Python 3.9:
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* ``PyAsyncGen_ClearFreeLists()()``
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* ``PyContext_ClearFreeList()()``
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* ``PyDict_ClearFreeList()()``
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* ``PyFloat_ClearFreeList()()``
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* ``PyFrame_ClearFreeList()()``
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* ``PyList_ClearFreeList()()``
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* ``PyTuple_ClearFreeList()()``
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* ``_PyObject_SIZE()``, ``_PyObject_VAR_SIZE()``
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* ``PyThreadState_DeleteCurrent()``
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* ``PyFPE_START_PROTECT()``, ``PyFPE_END_PROTECT()``
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* ``_Py_NewReference()``, ``_Py_ForgetReference()``
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* ``_PyTraceMalloc_NewReference()``
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* ``_Py_GetRefTotal()``
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And the following functions have been simply removed:
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Private functions moved to the internal C API in Python 3.9:
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* ``PyMethod_ClearFreeList()`` and ``PyCFunction_ClearFreeList()``:
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the free lists of bound method objects have been removed (in Python 3.9).
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* ``PySet_ClearFreeList()``: the set free list has been removed in
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Python 3.4.
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* ``PyUnicode_ClearFreeList()``: the Unicode free list has been removed
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in Python 3.3.
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* GC functions like ``_Py_AS_GC()``, ``_PyObject_GC_IS_TRACKED()``
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and ``_PyGCHead_NEXT()``
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* ``_Py_AddToAllObjects()`` (not exported)
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* ``_PyDebug_PrintTotalRefs()``, ``_Py_PrintReferences()``,
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``_Py_PrintReferenceAddresses()`` (not exported)
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Public "clear free list" functions moved to the internal C API an
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renamed to private functions and in Python 3.9:
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* ``PyAsyncGen_ClearFreeLists()``
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* ``PyContext_ClearFreeList()``
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* ``PyDict_ClearFreeList()``
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* ``PyFloat_ClearFreeList()``
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* ``PyFrame_ClearFreeList()``
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* ``PyList_ClearFreeList()``
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* ``PyTuple_ClearFreeList()``
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* Functions simply removed:
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* ``PyMethod_ClearFreeList()`` and ``PyCFunction_ClearFreeList()``:
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bound method free list removed in Python 3.9.
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* ``PySet_ClearFreeList()``: set free list removed in Python 3.4.
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* ``PyUnicode_ClearFreeList()``: Unicode free list removed
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in Python 3.3.
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Convert macros to static inline functions
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-----------------------------------------
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@ -320,11 +329,18 @@ Macros converted to static inline functions in Python 3.8:
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Macros converted to regular functions in Python 3.9:
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* ``Py_EnterRecursiveCall()``, ``Py_LeaveRecursiveCall()``
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(added to the limited C API)
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* ``PyObject_INIT()``, ``PyObject_INIT_VAR()``
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In Python 3.9, the trashcan macros are now calling functions which hide
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implementation details, rather than accessing directly members of the
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``PyThreadState`` structure.
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* ``PyObject_GET_WEAKREFS_LISTPTR()``
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* ``PyObject_CheckBuffer()``
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* ``PyIndex_Check()``
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* ``PyObject_IS_GC()``
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* ``PyObject_NEW()`` (alias to ``PyObject_New()``),
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``PyObject_NEW_VAR()`` (alias to ``PyObject_NewVar()``)
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* ``PyType_HasFeature()`` (always call ``PyType_GetFlags()``)
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* ``Py_TRASHCAN_BEGIN_CONDITION()`` and ``Py_TRASHCAN_END()`` macros
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now call functions which hide implementation details, rather than
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accessing directly members of the ``PyThreadState`` structure.
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Make structures opaque
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----------------------
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@ -373,13 +389,16 @@ structures opaque:
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* ``PyObject``: `bpo-39573 <https://bugs.python.org/issue39573>`_
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* ``PyTypeObject``: `bpo-40170 <https://bugs.python.org/issue40170>`_
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* ``PyFrameObject``: `bpo-40421 <https://bugs.python.org/issue40421>`_.
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Python 3.9 adds ``PyFrame_GetCode()`` and ``PyFrame_GetBack()``
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getter functions, and moves ``PyFrame_GetLineNumber`` to the limited C
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API.
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* ``PyThreadState``: `bpo-39947 <https://bugs.python.org/issue39947>`_.
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Python 3.9 adds 3 getter functions: ``PyThreadState_GetFrame()``,
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``PyThreadState_GetID()`` and ``PyThreadState_GetInterpreter()``.
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* ``PyFrameObject``: `bpo-40421 <https://bugs.python.org/issue40421>`_
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* Python 3.9 adds ``PyFrame_GetCode()`` and ``PyFrame_GetBack()``
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getter functions, and moves ``PyFrame_GetLineNumber`` to the limited
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C API.
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* ``PyThreadState``: `bpo-39947 <https://bugs.python.org/issue39947>`_
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* Python 3.9 adds 3 getter functions: ``PyThreadState_GetFrame()``,
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``PyThreadState_GetID()``, ``PyThreadState_GetInterpreter()``.
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Disallow using Py_TYPE() as l-value
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-----------------------------------
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