Sync pre-release CPython main branch from GitHub (2026-06-16)

Summary:
Imported python/cpython `3.16.0a0` from upstream rev [`9e863fa`](https://www.github.com/python/cpython/commit/9e863fab283eddca9c2a8f9d1ee30f4dc243e314) (committed 2026-06-16 23:16:06+00:00).

# Commit Info

- Base: (`3.16.0a0`) - [`11f032f`](https://www.github.com/python/cpython/commit/11f032f904c8019b332a3c367f335e05cde63628) (commit date: 2026-06-16 04:51:39+00:00)
- Imported: (`3.16.0a0`) - [`9e863fa`](https://www.github.com/python/cpython/commit/9e863fab283eddca9c2a8f9d1ee30f4dc243e314) (commit date: 2026-06-16 23:16:06+00:00)

# Noteworthy file changes

- Stdlib files (1 added)
- Test files (1 added)
- Low-signal files (5 added) (NEWS.d, docs, .github)
- Other files (1 added)

Complete list of added/removed files: https://www.internalfb.com/intern/everpaste/?color=0&handle=GP_FhCIL2Elt5IgEAKEe8HPhSkYhbr0LAAAz

Reviewed By: itamaro

Differential Revision: D108840683

fbshipit-source-id: 5071fc26eb88ff1e39c4707d04b39cf1cd9d5ba9

Author: ambv

.pre-commit-config.yaml

@@ -1,6 +1,6 @@
 repos:
   - repo: https://github.com/astral-sh/ruff-pre-commit
-    rev: e05c5c0818279e5ac248ac9e954431ba58865e61  # frozen: v0.15.7
+    rev: 3b3f7c3f57fe9925356faf5fe6230835138be230  # frozen: v0.15.17
     hooks:
       - id: ruff-check
         name: Run Ruff (lint) on Platforms/Apple/
@@ -10,6 +10,11 @@ repos:
         name: Run Ruff (lint) on Doc/
         args: [--exit-non-zero-on-fix]
         files: ^Doc/
+      - id: ruff-check
+        name: Run Ruff (lint) on Lib/
+        args: [--exit-non-zero-on-fix]
+        files: ^Lib/
+        exclude: ^Lib/test/
       - id: ruff-check
         name: Run Ruff (lint) on Lib/test/
         args: [--exit-non-zero-on-fix]
@@ -18,6 +23,11 @@ repos:
         name: Run Ruff (lint) on Platforms/WASI/
         args: [--exit-non-zero-on-fix, --config=Platforms/WASI/.ruff.toml]
         files: ^Platforms/WASI/
+      - id: ruff-check
+        name: Run Ruff (lint) on Tools/
+        args: [--exit-non-zero-on-fix]
+        files: ^Tools/
+        exclude: ^Tools/(build|clinic|i18n|peg_generator|wasm)/
       - id: ruff-check
         name: Run Ruff (lint) on Tools/build/
         args: [--exit-non-zero-on-fix, --config=Tools/build/.ruff.toml]

Doc/c-api/init_config.rst

@@ -623,6 +623,10 @@ Some options are read from the :mod:`sys` attributes. For example, the option
 
    .. versionadded:: 3.14
 
+   .. versionchanged:: next
+      The function now replaces :data:`sys.flags` (create a new object),
+      instead of modifying :data:`sys.flags` in-place.
+
 
 .. _pyconfig_api:
 

Include/internal/pycore_critical_section.h

@@ -196,10 +196,9 @@ _PyCriticalSection2_Begin(PyThreadState *tstate, PyCriticalSection2 *c, PyObject
 static inline void
 _PyCriticalSection2_End(PyThreadState *tstate, PyCriticalSection2 *c)
 {
-    // if mutex1 is NULL, we used the fast path in
-    // _PyCriticalSection_BeginSlow for mutexes that are already held,
-    // which should only happen when mutex1 and mutex2 were the same mutex,
-    // and mutex2 should also be NULL.
+    // if mutex1 is NULL, we used the fast path in either
+    // _PyCriticalSection_BeginSlow or _PyCriticalSection2_BeginSlow for mutexes
+    // that are already held, and mutex2 should also be NULL.
     if (c->_cs_base._cs_mutex == NULL) {
         assert(c->_cs_mutex2 == NULL);
         return;

Lib/.ruff.toml

@@ -0,0 +1,21 @@
+extend = "../.ruff.toml"  # Inherit the project-wide settings
+
+# Unlike Tools/, stdlib can use newer syntax than PYTHON_FOR_REGEN
+target-version = "py315"
+
+[lint]
+select = [
+    "F401",  # Unused import
+]
+
+[lint.per-file-ignores]
+"ctypes/__init__.py" = ["F401"]  # Re-exports from _ctypes
+"ensurepip/__init__.py" = ["F401"]  # `import zlib` availability check
+"idlelib/idle_test/htest.py" = ["F401"]  # Import for Windows DPI side effect
+"idlelib/idle_test/test_iomenu.py" = ["F401"]  # Imports checked for existence
+"importlib/_abc.py" = ["F401"]  # Bootstrap-sensitive _bootstrap import
+"importlib/machinery.py" = ["F401"]  # NamespacePath re-export
+"importlib/metadata/__init__.py" = ["F401"]  # Synced from importlib_metadata
+"profiling/sampling/sample.py" = ["F401"]  # Re-exports PROFILING_MODE_* constants
+"ssl.py" = ["F401"]  # Re-exports from _ssl
+"warnings.py" = ["F401"]  # Re-exports from _py_warnings

Lib/_pyrepl/completing_reader.py

@@ -32,7 +32,7 @@
 # types
 Command = commands.Command
 if TYPE_CHECKING:
-    from .types import CommandName, CompletionAction, Keymap, KeySpec
+    from .types import CompletionAction, Keymap
 
 
 def prefix(wordlist: list[str], j: int = 0) -> str:

Lib/_pyrepl/reader.py

@@ -38,7 +38,7 @@
 )
 from .layout import LayoutMap, LayoutResult, LayoutRow, WrappedRow, layout_content_lines
 from .render import RenderCell, RenderLine, RenderedScreen, ScreenOverlay
-from .utils import ANSI_ESCAPE_SEQUENCE, ColorSpan, THEME, StyleRef, wlen, gen_colors
+from .utils import ANSI_ESCAPE_SEQUENCE, ColorSpan, THEME, StyleRef, gen_colors
 from .trace import trace
 
 

Lib/_pyrepl/unix_console.py

@@ -34,7 +34,7 @@
 from collections.abc import Callable
 from dataclasses import dataclass
 from fcntl import ioctl
-from typing import TYPE_CHECKING, cast, overload
+from typing import TYPE_CHECKING, overload
 
 from . import terminfo
 from .console import Console, Event

Lib/email/contentmanager.py

@@ -174,7 +174,8 @@ def set_text_content(msg, string, subtype="plain", charset='utf-8', cte=None,
                      params=None, headers=None):
     _prepare_set(msg, 'text', subtype, headers)
 
-    charset = email.charset.Charset(charset).input_charset
+    cs = email.charset.Charset(charset)
+    charset = cs.output_charset
     cte, payload = _encode_text(string, charset, cte, msg.policy)
     msg.set_payload(payload)
     msg.set_param('charset', charset, replace=True)

Lib/idlelib/idle_test/template.py

@@ -1,6 +1,5 @@
 "Test , coverage %."
 
-from idlelib import zzdummy
 import unittest
 from test.support import requires
 from tkinter import Tk

Lib/idlelib/idle_test/test_help.py

@@ -4,7 +4,6 @@
 import unittest
 from test.support import requires
 requires('gui')
-from os.path import abspath, dirname, join
 from tkinter import Tk