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bpo-43688: Support the limited C API in debug mode (GH-25131)
The limited C API is now supported if Python is built in debug mode (if the Py_DEBUG macro is defined). In the limited C API, the Py_INCREF() and Py_DECREF() functions are now implemented as opaque function calls, rather than accessing directly the PyObject.ob_refcnt member, if Python is built in debug mode and the Py_LIMITED_API macro targets Python 3.10 or newer. It became possible to support the limited C API in debug mode because the PyObject structure is the same in release and debug mode since Python 3.8 (see bpo-36465). The limited C API is still not supported in the --with-trace-refs special build (Py_TRACE_REFS macro).
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‎Doc/whatsnew/3.10.rst‎

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@@ -1315,6 +1315,18 @@ New Features
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to simulate.
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(Contributed by Antoine Pitrou in :issue:`43356`.)
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* The limited C API is now supported if Python is built in debug mode (if the
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``Py_DEBUG`` macro is defined). In the limited C API, the :c:func:`Py_INCREF`
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and :c:func:`Py_DECREF` functions are now implemented as opaque function
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calls, rather than accessing directly the :c:member:`PyObject.ob_refcnt`
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member, if Python is built in debug mode and the ``Py_LIMITED_API`` macro
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targets Python 3.10 or newer. It became possible to support the limited C API
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in debug mode because the :c:type:`PyObject` structure is the same in release
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and debug mode since Python 3.8 (see :issue:`36465`).
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The limited C API is still not supported in the ``--with-trace-refs`` special
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build (``Py_TRACE_REFS`` macro).
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(Contributed by Victor Stinner in :issue:`43688`.)
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Porting to Python 3.10
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----------------------

‎Include/object.h‎

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@@ -54,11 +54,11 @@ whose size is determined when the object is allocated.
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/* Py_DEBUG implies Py_REF_DEBUG. */
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#if defined(Py_DEBUG) && !defined(Py_REF_DEBUG)
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#define Py_REF_DEBUG
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# define Py_REF_DEBUG
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#endif
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#if defined(Py_LIMITED_API) && defined(Py_REF_DEBUG)
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#error Py_LIMITED_API is incompatible with Py_DEBUG, Py_TRACE_REFS, and Py_REF_DEBUG
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#if defined(Py_LIMITED_API) && defined(Py_TRACE_REFS)
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# error Py_LIMITED_API is incompatible with Py_TRACE_REFS
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#endif
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/* PyTypeObject structure is defined in cpython/object.h.
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#define _PyObject_EXTRA_INIT 0, 0,
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#else
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#define _PyObject_HEAD_EXTRA
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#define _PyObject_EXTRA_INIT
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# define _PyObject_HEAD_EXTRA
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# define _PyObject_EXTRA_INIT
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#endif
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/* PyObject_HEAD defines the initial segment of every PyObject. */
@@ -427,21 +427,46 @@ PyAPI_FUNC(void) _Py_NegativeRefcount(const char *filename, int lineno,
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PyAPI_FUNC(void) _Py_Dealloc(PyObject *);
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/*
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These are provided as conveniences to Python runtime embedders, so that
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they can have object code that is not dependent on Python compilation flags.
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*/
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PyAPI_FUNC(void) Py_IncRef(PyObject *);
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PyAPI_FUNC(void) Py_DecRef(PyObject *);
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// Similar to Py_IncRef() and Py_DecRef() but the argument must be non-NULL.
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// Private functions used by Py_INCREF() and Py_DECREF().
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PyAPI_FUNC(void) _Py_IncRef(PyObject *);
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PyAPI_FUNC(void) _Py_DecRef(PyObject *);
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static inline void _Py_INCREF(PyObject *op)
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{
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#if defined(Py_REF_DEBUG) && defined(Py_LIMITED_API) && Py_LIMITED_API+0 >= 0x030A0000
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// Stable ABI for Python 3.10 built in debug mode.
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_Py_IncRef(op);
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#else
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// Non-limited C API and limited C API for Python 3.9 and older access
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// directly PyObject.ob_refcnt.
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#ifdef Py_REF_DEBUG
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_Py_RefTotal++;
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#endif
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op->ob_refcnt++;
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#endif
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}
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#define Py_INCREF(op) _Py_INCREF(_PyObject_CAST(op))
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static inline void _Py_DECREF(
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#ifdef Py_REF_DEBUG
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#if defined(Py_REF_DEBUG) && !(defined(Py_LIMITED_API) && Py_LIMITED_API+0 >= 0x030A0000)
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const char *filename, int lineno,
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#endif
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PyObject *op)
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{
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#if defined(Py_REF_DEBUG) && defined(Py_LIMITED_API) && Py_LIMITED_API+0 >= 0x030A0000
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// Stable ABI for Python 3.10 built in debug mode.
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_Py_DecRef(op);
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#else
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// Non-limited C API and limited C API for Python 3.9 and older access
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// directly PyObject.ob_refcnt.
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#ifdef Py_REF_DEBUG
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_Py_RefTotal--;
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#endif
@@ -455,8 +480,9 @@ static inline void _Py_DECREF(
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else {
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_Py_Dealloc(op);
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}
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#endif
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}
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#ifdef Py_REF_DEBUG
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#if defined(Py_REF_DEBUG) && !(defined(Py_LIMITED_API) && Py_LIMITED_API+0 >= 0x030A0000)
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# define Py_DECREF(op) _Py_DECREF(__FILE__, __LINE__, _PyObject_CAST(op))
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#else
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# define Py_DECREF(op) _Py_DECREF(_PyObject_CAST(op))
@@ -525,13 +551,6 @@ static inline void _Py_XDECREF(PyObject *op)
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#define Py_XDECREF(op) _Py_XDECREF(_PyObject_CAST(op))
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/*
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These are provided as conveniences to Python runtime embedders, so that
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they can have object code that is not dependent on Python compilation flags.
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*/
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PyAPI_FUNC(void) Py_IncRef(PyObject *);
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PyAPI_FUNC(void) Py_DecRef(PyObject *);
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// Create a new strong reference to an object:
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// increment the reference count of the object and return the object.
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PyAPI_FUNC(PyObject*) Py_NewRef(PyObject *obj);

‎Misc/NEWS.d/next/C API/2021-04-01-09-10-42.bpo-43688.G4gs6k.rst‎

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The limited C API is now supported if Python is built in debug mode (if the
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``Py_DEBUG`` macro is defined). In the limited C API, the :c:func:`Py_INCREF`
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and :c:func:`Py_DECREF` functions are now implemented as opaque function calls,
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rather than accessing directly the :c:member:`PyObject.ob_refcnt` member, if
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Python is built in debug mode and the ``Py_LIMITED_API`` macro targets Python
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3.10 or newer. It became possible to support the limited C API in debug mode
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because the :c:type:`PyObject` structure is the same in release and debug mode
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since Python 3.8 (see :issue:`36465`).
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The limited C API is still not supported in the ``--with-trace-refs`` special
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build (``Py_TRACE_REFS`` macro).
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Patch by Victor Stinner.

‎Objects/object.c‎

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@@ -13,6 +13,11 @@
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#include "frameobject.h"
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#include "interpreteridobject.h"
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#ifdef Py_LIMITED_API
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// Prevent recursive call _Py_IncRef() <=> Py_INCREF()
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# error "Py_LIMITED_API macro must not be defined"
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#endif
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#ifdef __cplusplus
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extern "C" {
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#endif
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Py_XDECREF(o);
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}
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void
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_Py_IncRef(PyObject *o)
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{
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Py_INCREF(o);
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}
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void
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_Py_DecRef(PyObject *o)
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{
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Py_DECREF(o);
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}
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PyObject *
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PyObject_Init(PyObject *op, PyTypeObject *tp)
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{

‎PC/python3dll.c‎

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@@ -36,6 +36,8 @@ EXPORT_FUNC(_PyTrash_deposit_object)
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EXPORT_FUNC(_PyTrash_destroy_chain)
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EXPORT_FUNC(_PyTrash_thread_deposit_object)
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EXPORT_FUNC(_PyTrash_thread_destroy_chain)
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EXPORT_FUNC(_Py_IncRef)
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EXPORT_FUNC(_Py_DecRef)
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EXPORT_FUNC(Py_AddPendingCall)
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EXPORT_FUNC(Py_AtExit)
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EXPORT_FUNC(Py_BuildValue)

‎setup.py‎

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@@ -1864,17 +1864,11 @@ def detect_modules(self):
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## # Uncomment these lines if you want to play with xxmodule.c
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## self.add(Extension('xx', ['xxmodule.c']))
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if 'd' not in sysconfig.get_config_var('ABIFLAGS'):
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# Non-debug mode: Build xxlimited with limited API
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self.add(Extension('xxlimited', ['xxlimited.c'],
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define_macros=[('Py_LIMITED_API', '0x030a0000')]))
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self.add(Extension('xxlimited_35', ['xxlimited_35.c'],
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define_macros=[('Py_LIMITED_API', '0x03050000')]))
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else:
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# Debug mode: Build xxlimited with the full API
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# (which is compatible with the limited one)
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self.add(Extension('xxlimited', ['xxlimited.c']))
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self.add(Extension('xxlimited_35', ['xxlimited_35.c']))
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# Limited C API
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self.add(Extension('xxlimited', ['xxlimited.c'],
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define_macros=[('Py_LIMITED_API', '0x030a0000')]))
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self.add(Extension('xxlimited_35', ['xxlimited_35.c'],
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define_macros=[('Py_LIMITED_API', '0x03050000')]))
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def detect_tkinter_fromenv(self):
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# Build _tkinter using the Tcl/Tk locations specified by

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