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gh-121115: Skip __index__ in PyLong_AsNativeBytes by default (GH-121118)
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‎Doc/c-api/long.rst

+11-4
Original file line numberDiff line numberDiff line change
@@ -405,14 +405,13 @@ distinguished from a number. Use :c:func:`PyErr_Occurred` to disambiguate.
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Passing zero to *n_bytes* will return the size of a buffer that would
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be large enough to hold the value. This may be larger than technically
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necessary, but not unreasonably so.
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necessary, but not unreasonably so. If *n_bytes=0*, *buffer* may be
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``NULL``.
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.. note::
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Passing *n_bytes=0* to this function is not an accurate way to determine
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the bit length of a value.
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If *n_bytes=0*, *buffer* may be ``NULL``.
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the bit length of the value.
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To get at the entire Python value of an unknown size, the function can be
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called twice: first to determine the buffer size, then to fill it::
@@ -462,6 +461,7 @@ distinguished from a number. Use :c:func:`PyErr_Occurred` to disambiguate.
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.. c:macro:: Py_ASNATIVEBYTES_NATIVE_ENDIAN ``3``
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.. c:macro:: Py_ASNATIVEBYTES_UNSIGNED_BUFFER ``4``
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.. c:macro:: Py_ASNATIVEBYTES_REJECT_NEGATIVE ``8``
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.. c:macro:: Py_ASNATIVEBYTES_ALLOW_INDEX ``16``
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============================================= ======
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Specifying ``Py_ASNATIVEBYTES_NATIVE_ENDIAN`` will override any other endian
@@ -483,6 +483,13 @@ distinguished from a number. Use :c:func:`PyErr_Occurred` to disambiguate.
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provided there is enough space for at least one sign bit, regardless of
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whether ``Py_ASNATIVEBYTES_UNSIGNED_BUFFER`` was specified.
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If ``Py_ASNATIVEBYTES_ALLOW_INDEX`` is specified and a non-integer value is
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passed, its :meth:`~object.__index__` method will be called first. This may
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result in Python code executing and other threads being allowed to run, which
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could cause changes to other objects or values in use. When *flags* is
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``-1``, this option is not set, and non-integer values will raise
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:exc:`TypeError`.
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.. note::
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With the default *flags* (``-1``, or *UNSIGNED_BUFFER* without

‎Include/cpython/longobject.h

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@@ -10,6 +10,7 @@ PyAPI_FUNC(PyObject*) PyLong_FromUnicodeObject(PyObject *u, int base);
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#define Py_ASNATIVEBYTES_NATIVE_ENDIAN 3
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#define Py_ASNATIVEBYTES_UNSIGNED_BUFFER 4
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#define Py_ASNATIVEBYTES_REJECT_NEGATIVE 8
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#define Py_ASNATIVEBYTES_ALLOW_INDEX 16
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/* PyLong_AsNativeBytes: Copy the integer value to a native variable.
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buffer points to the first byte of the variable.
@@ -20,8 +21,10 @@ PyAPI_FUNC(PyObject*) PyLong_FromUnicodeObject(PyObject *u, int base);
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* 2 - native endian
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* 4 - unsigned destination (e.g. don't reject copying 255 into one byte)
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* 8 - raise an exception for negative inputs
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If flags is -1 (all bits set), native endian is used and value truncation
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behaves most like C (allows negative inputs and allow MSB set).
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* 16 - call __index__ on non-int types
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If flags is -1 (all bits set), native endian is used, value truncation
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behaves most like C (allows negative inputs and allow MSB set), and non-int
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objects will raise a TypeError.
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Big endian mode will write the most significant byte into the address
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directly referenced by buffer; little endian will write the least significant
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byte into that address.

‎Lib/test/test_capi/test_long.py

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Original file line numberDiff line numberDiff line change
@@ -496,8 +496,9 @@ def test_long_asnativebytes(self):
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"PyLong_AsNativeBytes(v, <unknown>, 0, -1)")
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self.assertEqual(buffer, b"\x5a",
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"buffer overwritten when it should not have been")
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# Also check via the __index__ path
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self.assertEqual(expect, asnativebytes(Index(v), buffer, 0, -1),
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# Also check via the __index__ path.
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# We pass Py_ASNATIVEBYTES_NATIVE_ENDIAN | ALLOW_INDEX
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self.assertEqual(expect, asnativebytes(Index(v), buffer, 0, 3 | 16),
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"PyLong_AsNativeBytes(Index(v), <unknown>, 0, -1)")
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self.assertEqual(buffer, b"\x5a",
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"buffer overwritten when it should not have been")
@@ -607,6 +608,12 @@ def test_long_asnativebytes(self):
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with self.assertRaises(ValueError):
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asnativebytes(-1, buffer, 0, 8)
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# Ensure omitting Py_ASNATIVEBYTES_ALLOW_INDEX raises on __index__ value
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with self.assertRaises(TypeError):
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asnativebytes(Index(1), buffer, 0, -1)
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with self.assertRaises(TypeError):
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asnativebytes(Index(1), buffer, 0, 3)
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# Check a few error conditions. These are validated in code, but are
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# unspecified in docs, so if we make changes to the implementation, it's
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# fine to just update these tests rather than preserve the behaviour.

‎Misc/NEWS.d/next/Core and Builtins/2024-06-28-10-02-58.gh-issue-121115.EeSLfc.rst

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@@ -0,0 +1,3 @@
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:c:func:`PyLong_AsNativeBytes` no longer uses :meth:`~object.__index__`
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methods by default. The ``Py_ASNATIVEBYTES_ALLOW_INDEX`` flag has been added
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to allow it.

‎Objects/longobject.c

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Original file line numberDiff line numberDiff line change
@@ -1128,13 +1128,17 @@ PyLong_AsNativeBytes(PyObject* vv, void* buffer, Py_ssize_t n, int flags)
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if (PyLong_Check(vv)) {
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v = (PyLongObject *)vv;
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}
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else {
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else if (flags != -1 && (flags & Py_ASNATIVEBYTES_ALLOW_INDEX)) {
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v = (PyLongObject *)_PyNumber_Index(vv);
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if (v == NULL) {
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return -1;
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}
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do_decref = 1;
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}
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else {
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PyErr_Format(PyExc_TypeError, "expect int, got %T", vv);
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return -1;
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}
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if ((flags != -1 && (flags & Py_ASNATIVEBYTES_REJECT_NEGATIVE))
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&& _PyLong_IsNegative(v)) {

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