Sync pre-release CPython main branch from GitHub (2026-07-15)

Summary:
Imported python/cpython `3.16.0a0` from upstream rev [`87411d0`](https://www.github.com/python/cpython/commit/87411d0742f070b1424c08b41cf3af49d377b215) (committed 2026-07-15 03:06:24+00:00).

# Commit Info

- Base: (`3.16.0a0`) - [`2dc1a91`](https://www.github.com/python/cpython/commit/2dc1a91af801ea896e21f6c6e6cc41f57e8268d9) (commit date: 2026-07-13 22:38:43+00:00)
- Imported: (`3.16.0a0`) - [`87411d0`](https://www.github.com/python/cpython/commit/87411d0742f070b1424c08b41cf3af49d377b215) (commit date: 2026-07-15 03:06:24+00:00)

# Noteworthy file changes

- Low-signal files (7 added) (NEWS.d, docs, .github)

Complete list of added/removed files: https://www.internalfb.com/intern/everpaste/?color=0&handle=GNxi1SErxX7n1ZMFAOn3_Fu84codbr0LAAAz

Reviewed By: itamaro, alexmalyshev

Differential Revision: D112059877

fbshipit-source-id: dbd06cdecbf2bb48ec45e824b394daeca652c14a

Author: ambv

.github/workflows/reusable-emscripten.yml

@@ -77,3 +77,5 @@ jobs:
       run: python3 Platforms/emscripten pythoninfo-host
     - name: "Test"
       run: python3 Platforms/emscripten run --test
+    - name: "Test Repl"
+      run: Platforms/emscripten/browser_test/run_test.sh

Doc/library/asyncio-eventloop.rst

@@ -1263,7 +1263,9 @@ Working with pipes
    *protocol_factory* must be a callable returning an
    :ref:`asyncio protocol <asyncio-protocol>` implementation.
 
-   *pipe* is a :term:`file-like object <file object>`.
+   *pipe* is a :term:`file-like object <file object>`.  See
+   :ref:`Supported pipe objects <asyncio-pipe-objects>` for the objects
+   supported as *pipe*.
 
    Return pair ``(transport, protocol)``, where *transport* supports
    the :class:`ReadTransport` interface and *protocol* is an object
@@ -1280,7 +1282,9 @@ Working with pipes
    *protocol_factory* must be a callable returning an
    :ref:`asyncio protocol <asyncio-protocol>` implementation.
 
-   *pipe* is :term:`file-like object <file object>`.
+   *pipe* is a :term:`file-like object <file object>`.  See
+   :ref:`Supported pipe objects <asyncio-pipe-objects>` for the objects
+   supported as *pipe*.
 
    Return pair ``(transport, protocol)``, where *transport* supports
    :class:`WriteTransport` interface and *protocol* is an object
@@ -1289,6 +1293,33 @@ Working with pipes
    With :class:`SelectorEventLoop` event loop, the *pipe* is set to
    non-blocking mode.
 
+.. _asyncio-pipe-objects:
+
+.. rubric:: Supported pipe objects
+
+These methods only work with objects the operating system can poll for
+readiness or perform overlapped I/O on.  Regular files on disk are **not**
+supported on any platform.  There is no asynchronous file I/O in asyncio;
+use :meth:`loop.run_in_executor` to read and write regular files without
+blocking the event loop.
+
+On Unix, with :class:`SelectorEventLoop`, *pipe* must wrap one of the
+following:
+
+* a pipe, such as an end of an :func:`os.pipe` pair or a FIFO created with
+  :func:`os.mkfifo`;
+* a socket;
+* a character device, such as a terminal.
+
+On Windows, where only :class:`ProactorEventLoop` implements these methods,
+*pipe* must wrap a handle opened for overlapped I/O (that is, created with the
+``FILE_FLAG_OVERLAPPED`` flag), since the handle has to be associated with an
+I/O completion port.  Handles that were not opened for overlapped I/O are
+rejected.  In particular, the standard streams (:data:`sys.stdin`,
+:data:`sys.stdout` and :data:`sys.stderr`), console handles, and the pipes
+created by :func:`os.pipe` are **not** opened for overlapped I/O and therefore
+cannot be used with these methods.
+
 .. note::
 
    :class:`SelectorEventLoop` does not support the above methods on

Doc/library/asyncio-platforms.rst

@@ -19,6 +19,11 @@ All Platforms
 * :meth:`loop.add_reader` and :meth:`loop.add_writer`
   cannot be used to monitor file I/O.
 
+* :meth:`loop.connect_read_pipe` and :meth:`loop.connect_write_pipe`
+  cannot be used with regular files.  See :ref:`Supported pipe objects
+  <asyncio-pipe-objects>` for the objects that are accepted on each
+  platform.
+
 
 Windows
 =======
@@ -62,6 +67,10 @@ All event loops on Windows do not support the following methods:
 * The :meth:`loop.add_reader` and :meth:`loop.add_writer`
   methods are not supported.
 
+* :meth:`loop.connect_read_pipe` and :meth:`loop.connect_write_pipe` only
+  accept a handle opened for overlapped I/O.
+  See :ref:`Supported pipe objects <asyncio-pipe-objects>` for which objects are supported.
+
 The resolution of the monotonic clock on Windows is usually around 15.6
 milliseconds.  The best resolution is 0.5 milliseconds. The resolution depends on the
 hardware (availability of `HPET

Doc/library/struct.rst

@@ -68,6 +68,7 @@ The module defines the following exception and functions:
    Pack the values *v1*, *v2*, ... according to the format string *format* and
    write the packed bytes into the writable buffer *buffer* starting at
    position *offset*.  Note that *offset* is a required argument.
+   A negative *offset* counts from the end of *buffer*.
 
 
 .. function:: unpack(format, buffer)
@@ -84,6 +85,7 @@ The module defines the following exception and functions:
    string *format*.  The result is a tuple even if it contains exactly one
    item.  The buffer's size in bytes, starting at position *offset*, must be at
    least the size required by the format, as reflected by :func:`calcsize`.
+   A negative *offset* counts from the end of *buffer*.
 
 
 .. function:: iter_unpack(format, buffer)

Doc/library/weakref.rst

@@ -63,12 +63,15 @@ exposed by the :mod:`!weakref` module for the benefit of advanced uses.
 Not all objects can be weakly referenced. Objects which support weak references
 include class instances, functions written in Python (but not in C), instance methods,
 sets, frozensets, some :term:`file objects <file object>`, :term:`generators <generator>`,
-type objects, sockets, arrays, deques, regular expression pattern objects, and code
-objects.
+type objects, sockets, arrays, deques, regular expression pattern objects, code
+objects, and frame objects.
 
 .. versionchanged:: 3.2
    Added support for thread.lock, threading.Lock, and code objects.
 
+.. versionchanged:: 3.16
+   Added support for frame objects.
+
 Several built-in types such as :class:`list` and :class:`dict` do not directly
 support weak references but can add support through subclassing::
 

Doc/using/windows.rst

@@ -162,7 +162,9 @@ omitted in cases where the tag refers to an official release and starts with
    $> py -V:3-arm64 ...
 
 Runtimes from other distributors may require the *company* to be included as
-well. This should be separated from the tag by a slash, and may be a prefix.
+well.
+It should be separated from the tag by a slash (either ``/`` or ``\``),
+and may be shortened to any prefix of its full value.
 Specifying the company is optional when it is ``PythonCore``, and specifying the
 tag is optional (but not the slash) when you want the latest release from a
 specific company.

Doc/whatsnew/3.16.rst

@@ -75,6 +75,11 @@ New features
 Other language changes
 ======================
 
+* :ref:`Frame objects <frame-objects>` now support :mod:`weak references
+  <weakref>`.  This allows associating extra data with active frames,
+  for example in debuggers, without keeping the frames (and everything
+  they reference) alive indefinitely.
+  (Contributed by Łukasz Langa in :gh:`102960`.)
 
 
 New modules
@@ -263,8 +268,9 @@ imaplib
   (Contributed by Przemysław Buczkowski and Serhiy Storchaka in :gh:`89869`.)
 
 * Non-ASCII mailbox names are now automatically encoded as modified UTF-7
-  (:rfc:`3501`, section 5.1.3), so international mailbox names can be passed
-  as ordinary :class:`str`.
+  (:rfc:`3501`, section 5.1.3) in the default mode, so international mailbox
+  names can be passed as ordinary :class:`str` without enabling ``UTF8=ACCEPT``
+  (under which they are sent as UTF-8).
   (Contributed by Serhiy Storchaka in :gh:`49555`.)
 
 * The :meth:`~imaplib.IMAP4.copy`, :meth:`~imaplib.IMAP4.move`,

Include/internal/pycore_fileutils_windows.h

@@ -55,7 +55,11 @@ static inline BOOL _Py_GetFileInformationByName(
     static int GetFileInformationByName_init = -1;
 
     if (GetFileInformationByName_init < 0) {
+#ifdef MS_WINDOWS_DESKTOP
         HMODULE hMod = LoadLibraryW(L"api-ms-win-core-file-l2-1-4");
+#else
+        HMODULE hMod = LoadPackagedLibrary(L"api-ms-win-core-file-l2-1-4", 0);
+#endif
         GetFileInformationByName_init = 0;
         if (hMod) {
             GetFileInformationByName = (PGetFileInformationByName)GetProcAddress(

Include/internal/pycore_frame.h

@@ -34,6 +34,7 @@ struct _frame {
      * "support" for the borrowed references, ensuring that they remain valid.
      */
     PyObject *f_overwritten_fast_locals;
+    PyObject *f_weakreflist;    /* List of weak references */
     /* The frame data, if this frame object owns the frame */
     PyObject *_f_frame_data[1];
 };

Lib/asyncio/proactor_events.py

@@ -62,6 +62,7 @@ def __init__(self, loop, sock, protocol, waiter=None,
         self._closing = False  # Set when close() called.
         self._called_connection_lost = False
         self._eof_written = False
+        self._empty_waiter = None
         if self._server is not None:
             self._server._attach(self)
         self._loop.call_soon(self._protocol.connection_made, self)
@@ -331,10 +332,6 @@ class _ProactorBaseWritePipeTransport(_ProactorBasePipeTransport,
 
     _start_tls_compatible = True
 
-    def __init__(self, *args, **kw):
-        super().__init__(*args, **kw)
-        self._empty_waiter = None
-
     def write(self, data):
         if not isinstance(data, (bytes, bytearray, memoryview)):
             raise TypeError(
@@ -465,7 +462,6 @@ class _ProactorDatagramTransport(_ProactorBasePipeTransport,
     def __init__(self, loop, sock, protocol, address=None,
                  waiter=None, extra=None):
         self._address = address
-        self._empty_waiter = None
         self._buffer_size = 0
         # We don't need to call _protocol.connection_made() since our base
         # constructor does it for us.