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gh-98417: Stop modifying PyConfig in sys.set_int_max_str_digits() · gpshead/cpython@3b1997a · GitHub
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pythongh-98417: Stop modifying PyConfig in sys.set_int_max_str_digits()
PyConfig is an input to interpreter initialization; the copy stored on PyInterpreterState records how the interpreter was initialized and must be treated as read-only afterwards. The runtime int<->str digit limit already lives in PyInterpreterState.long_state.max_str_digits and that is what all readers use, so drop the write-back into interp->config.int_max_str_digits from _PySys_SetIntMaxStrDigits(). As a consequence, interpreters created after a runtime limit change now start from the initially-configured limit instead of inheriting the current runtime value, restoring the Python 3.11 behavior. Also document the read-only-after-init constraint on the PyConfig declaration itself, per discussion on the issue. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01UTM742RWwqsDkF5JGSsGWX
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Lines changed: 35 additions & 4 deletions

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‎Include/cpython/initconfig.h‎

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@@ -129,7 +129,16 @@ PyAPI_FUNC(void) PyPreConfig_InitIsolatedConfig(PyPreConfig *config);
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/* --- PyConfig ---------------------------------------------- */
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/* This structure is best documented in the Doc/c-api/init_config.rst file. */
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/* This structure is best documented in the Doc/c-api/init_config.rst file.
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PyConfig is an *input* to interpreter initialization. The copy kept
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on the interpreter state records how the interpreter was initialized.
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Do not use it to store mutable runtime state: a setting that can
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change at runtime belongs on the interpreter state itself, as is done
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for int_max_str_digits (gh-98417). Note that PyConfig_Set() still
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writes some members of the interpreter's copy; no new post-init
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writes should be added. Use the PyConfig_Get()/PyConfig_Set() API to
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access settings at runtime. */
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typedef struct PyConfig {
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int _config_init; /* _PyConfigInitEnum value */
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‎Lib/test/test_int.py‎

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@@ -732,6 +732,24 @@ def test_int_max_str_digits_is_per_interpreter(self):
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after_value = sys.get_int_max_str_digits()
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self.assertEqual(before_value, after_value)
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def test_int_max_str_digits_not_inherited_by_subinterpreter(self):
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# gh-98417: sys.set_int_max_str_digits() changes runtime state
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# only; it must not be written back into the interpreter's
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# initialization config. Interpreters created afterwards start
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# from the initial config, not the current runtime value.
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sentinel = 99_999
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if sys.get_int_max_str_digits() == sentinel:
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sentinel = 88_888
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code = f"""if 1:
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import sys
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if sys.get_int_max_str_digits() == {sentinel}:
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raise AssertionError(
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'subinterpreter inherited a runtime limit change')
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"""
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with support.adjust_int_max_str_digits(sentinel):
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self.assertEqual(support.run_in_subinterp(code), 0,
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'subinterp code failure, check stderr.')
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class IntSubclassStrDigitLimitsTests(IntStrDigitLimitsTests):
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int_class = IntSubclass

‎Misc/NEWS.d/next/Core_and_Builtins/2026-07-02-06-15-00.gh-issue-98417.Qk3vXm.rst‎

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@@ -0,0 +1,7 @@
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:func:`sys.set_int_max_str_digits` no longer writes the new limit back into
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the interpreter's initialization configuration (:c:type:`PyConfig`); the
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runtime limit is stored only on the interpreter state and the configuration
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is treated as read-only after initialization. As a consequence,
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interpreters created after changing the limit at runtime now start with the
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initial configured limit rather than inheriting the runtime value, matching
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the behavior of Python 3.11 and earlier.

‎Python/sysmodule.c‎

Lines changed: 0 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -4701,10 +4701,7 @@ _PySys_SetIntMaxStrDigits(int maxdigits)
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return -1;
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}
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// Set PyInterpreterState.long_state.max_str_digits
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// and PyInterpreterState.config.int_max_str_digits.
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PyInterpreterState *interp = _PyInterpreterState_GET();
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_Py_atomic_store_int(&interp->long_state.max_str_digits, maxdigits);
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_Py_atomic_store_int(&interp->config.int_max_str_digits, maxdigits);
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return 0;
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}

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