Sync pre-release CPython main branch from GitHub (2026-08-05)

Summary:
Imported python/cpython `3.16.0a0` from upstream rev [`ca4518b`](https://www.github.com/python/cpython/commit/ca4518b7f73bcd79d49ea20390f45827ddd0cb67) (committed 2026-08-05 04:43:08+00:00).

# Commit Info

- Base: (`3.16.0a0`) - [`fd9feab`](https://www.github.com/python/cpython/commit/fd9feabfb0fc80ee714b97140006d3af431d591a) (commit date: 2026-08-03 19:50:04+00:00)
- Imported: (`3.16.0a0`) - [`ca4518b`](https://www.github.com/python/cpython/commit/ca4518b7f73bcd79d49ea20390f45827ddd0cb67) (commit date: 2026-08-05 04:43:08+00:00)

# Noteworthy file changes

- Low-signal files (5 added) (NEWS.d, docs, .github)

Complete list of added/removed files: https://www.internalfb.com/intern/everpaste/?color=0&handle=GODW6ChuKSTOtWoGABfjiJOI0rJQbr0LAAAz

Differential Revision: D114837018

fbshipit-source-id: 1d7d910d770b62f14edf25e8a0aa478c04b08c41

Author: generatedunixname1734921407115435

Doc/library/asyncio-task.rst

@@ -288,6 +288,17 @@ Creating tasks
               # completion:
               task.add_done_callback(background_tasks.discard)
 
+      Note that this approach never awaits the tasks, so if a task
+      fails, its exception is never retrieved and asyncio logs a
+      "Task exception was never retrieved" message when the task is
+      garbage collected.  To avoid this, use :class:`asyncio.TaskGroup`
+      which keeps a strong reference to each task, awaits them and
+      propagates their exceptions::
+
+          async with asyncio.TaskGroup() as tg:
+              for i in range(10):
+                  tg.create_task(some_coro(param=i))
+
    .. versionadded:: 3.7
 
    .. versionchanged:: 3.8

Lib/argparse.py

@@ -1964,6 +1964,9 @@ def _prog_name(prog=None):
     if modspec is None:
         # simple script
         return _os.path.basename(arg0)
+    if modspec.name != '__main__' and arg0 != modspec.origin:
+        # named module executed as main without altering sys.argv[0]
+        return _os.path.basename(arg0)
     py = _os.path.basename(_sys.executable)
     if modspec.name != '__main__':
         # imported module or package

Lib/asyncio/windows_events.py

@@ -760,6 +760,46 @@ def _get_accept_socket(self, family):
         s.settimeout(0)
         return s
 
+    def _process_completion_status(self, status):
+        """Process a single status from the completion port.
+
+        A caller that waits on the completion port itself can pass each
+        status it receives here.
+        """
+        err, transferred, key, address = status
+        try:
+            f, ov, obj, callback = self._cache.pop(address)
+        except KeyError:
+            if self._loop.get_debug():
+                self._loop.call_exception_handler({
+                    'message': ('GetQueuedCompletionStatus() returned an '
+                                'unexpected event'),
+                    'status': ('err=%s transferred=%s key=%#x address=%#x'
+                               % (err, transferred, key, address)),
+                })
+
+            # key is either zero, or it is used to return a pipe
+            # handle which should be closed to avoid a leak.
+            if key not in (0, _overlapped.INVALID_HANDLE_VALUE):
+                _winapi.CloseHandle(key)
+            return
+
+        if obj in self._stopped_serving:
+            f.cancel()
+        # Don't call the callback if _register() already read the result or
+        # if the overlapped has been cancelled
+        elif not f.done():
+            try:
+                value = callback(transferred, key, ov)
+            except OSError as e:
+                f.set_exception(e)
+                self._results.append(f)
+            else:
+                f.set_result(value)
+                self._results.append(f)
+            finally:
+                f = None
+
     def _poll(self, timeout=None):
         if timeout is None:
             ms = INFINITE
@@ -778,39 +818,8 @@ def _poll(self, timeout=None):
                 break
             ms = 0
 
-            err, transferred, key, address = status
-            try:
-                f, ov, obj, callback = self._cache.pop(address)
-            except KeyError:
-                if self._loop.get_debug():
-                    self._loop.call_exception_handler({
-                        'message': ('GetQueuedCompletionStatus() returned an '
-                                    'unexpected event'),
-                        'status': ('err=%s transferred=%s key=%#x address=%#x'
-                                   % (err, transferred, key, address)),
-                    })
-
-                # key is either zero, or it is used to return a pipe
-                # handle which should be closed to avoid a leak.
-                if key not in (0, _overlapped.INVALID_HANDLE_VALUE):
-                    _winapi.CloseHandle(key)
-                continue
-
-            if obj in self._stopped_serving:
-                f.cancel()
-            # Don't call the callback if _register() already read the result or
-            # if the overlapped has been cancelled
-            elif not f.done():
-                try:
-                    value = callback(transferred, key, ov)
-                except OSError as e:
-                    f.set_exception(e)
-                    self._results.append(f)
-                else:
-                    f.set_result(value)
-                    self._results.append(f)
-                finally:
-                    f = None
+            # gh-154971: split out so custom event loops can call it directly
+            self._process_completion_status(status)
 
         # Remove unregistered futures
         for ov in self._unregistered:

Lib/mimetypes.py

@@ -250,7 +250,7 @@ def read(self, filename, strict=True):
         list of standard types, else to the list of non-standard
         types.
         """
-        with open(filename, encoding='utf-8') as fp:
+        with open(filename, encoding='utf-8', errors='surrogateescape') as fp:
             self.readfp(fp, strict)
 
     def readfp(self, fp, strict=True):
@@ -444,7 +444,7 @@ def init(files=None):
 
 def read_mime_types(file):
     try:
-        f = open(file, encoding='utf-8')
+        f = open(file, encoding='utf-8', errors='surrogateescape')
     except OSError:
         return None
     with f:

Lib/test/_isolated_sample.py

@@ -5,6 +5,8 @@
 a subprocess.  Several of these tests fail, error or are skipped on purpose.
 """
 
+import atexit
+import os
 import time
 import unittest
 from test.support import isolation
@@ -109,3 +111,33 @@ class BrokenSubclassSample(SubclassingSample):
     @classmethod
     def setUpClass(cls):
         pass
+
+
+# The exit code the samples below die with, after their tests have run.
+EXIT_CODE = 3
+
+
+def _die_at_exit():
+    atexit.register(os._exit, EXIT_CODE)
+
+
+class MethodExitSample(unittest.TestCase):
+
+    @isolation.runInSubprocess()
+    def test_passes_then_dies(self):
+        _die_at_exit()
+
+    @isolation.runInSubprocess()
+    def test_fails_and_dies(self):
+        _die_at_exit()
+        self.fail('the test itself failed')
+
+
+@isolation.runInSubprocess()
+class ClassExitSample(unittest.TestCase):
+
+    def test_pass(self):
+        pass
+
+    def test_dies(self):
+        _die_at_exit()

Lib/test/list_tests.py

@@ -6,7 +6,7 @@
 from functools import cmp_to_key
 
 from test import seq_tests
-from test.support import ALWAYS_EQ, NEVER_EQ, skip_if_huge_c_stack
+from test.support import ALWAYS_EQ, NEVER_EQ, run_with_limited_c_stack
 from test.support import skip_emscripten_stack_overflow, skip_wasi_stack_overflow
 
 
@@ -60,7 +60,7 @@ def test_repr(self):
         self.assertEqual(str(a2), "[0, 1, 2, [...], 3]")
         self.assertEqual(repr(a2), "[0, 1, 2, [...], 3]")
 
-    @skip_if_huge_c_stack(200_000)
+    @run_with_limited_c_stack(200_000)
     @skip_wasi_stack_overflow()
     @skip_emscripten_stack_overflow()
     def test_repr_deep(self):

Lib/test/mapping_tests.py

@@ -629,7 +629,7 @@ def __repr__(self):
         d = self._full_mapping({1: BadRepr()})
         self.assertRaises(Exc, repr, d)
 
-    @support.skip_if_huge_c_stack()
+    @support.run_with_limited_c_stack()
     @support.skip_wasi_stack_overflow()
     @support.skip_emscripten_stack_overflow()
     @support.skip_if_sanitizer("requires deep stack", ub=True)

Lib/test/support/__init__.py

@@ -46,6 +46,7 @@
     "check_disallow_instantiation", "check_sanitizer", "skip_if_sanitizer",
     "requires_limited_api", "requires_specialization", "thread_unsafe",
     "skip_if_unlimited_stack_size", "skip_if_huge_c_stack",
+    "run_with_limited_c_stack",
     # sys
     "MS_WINDOWS", "is_jython", "is_android", "is_emscripten", "is_wasi",
     "is_apple_mobile", "check_impl_detail", "unix_shell", "setswitchinterval",
@@ -2839,30 +2840,94 @@ def exceeds_recursion_limit():
     return 150_000
 
 
+def _has_huge_c_stack(depth):
+    """Check that *depth* recursive calls cannot exhaust the C stack."""
+    try:
+        from _testinternalcapi import get_c_recursion_remaining
+    except ImportError:
+        # Fall back to checking for an unlimited stack size.
+        if is_emscripten or is_wasi or os.name == "nt":
+            return False
+        import resource
+        soft, hard = resource.getrlimit(resource.RLIMIT_STACK)
+        return soft == hard and soft in (-1, 0xFFFF_FFFF_FFFF_FFFF)
+    else:
+        remaining = get_c_recursion_remaining()
+        # A negative value means integer overflow in the estimate
+        # (e.g. with an unlimited RLIMIT_STACK).  The estimate is based on
+        # the size of the interpreter loop frame, so it is only a lower
+        # bound for recursion with smaller C frames.
+        return remaining >= depth or remaining < 0
+
+
 def skip_if_huge_c_stack(depth=150_000):
     """Skip decorator for tests which cannot overflow the C stack.
 
     Tests exhausting the C stack with *depth* recursive calls cannot
     trigger the recursion protection if the C stack is too large (e.g.
     with a large or unlimited RLIMIT_STACK), and either fail, or run
     for a very long time, or crash, or consume all memory.
+
+    Prefer run_with_limited_c_stack() for tests recursing to a fixed depth.
     """
-    try:
-        from _testinternalcapi import get_c_recursion_remaining
-    except ImportError:
-        # Fall back to checking for an unlimited stack size.
-        huge = False
-        if not (is_emscripten or is_wasi) and os.name != "nt":
-            import resource
-            soft, hard = resource.getrlimit(resource.RLIMIT_STACK)
-            huge = soft == hard and soft in (-1, 0xFFFF_FFFF_FFFF_FFFF)
-    else:
-        remaining = get_c_recursion_remaining()
-        # A negative value means integer overflow in the estimate
-        # (e.g. with an unlimited RLIMIT_STACK).
-        huge = remaining >= depth or remaining < 0
     return unittest.skipIf(
-        huge, f"the C stack is large enough for {depth} recursive calls")
+        _has_huge_c_stack(depth),
+        f"the C stack is large enough for {depth} recursive calls")
+
+
+# Small enough to be exhausted by tens of thousands of recursive calls,
+# but not smaller than Py_C_STACK_SIZE (4 MiB) which the interpreter
+# assumes if it cannot query the thread stack size.
+C_STACK_SIZE = 8 * 1024 * 1024
+
+
+def run_with_limited_c_stack(depth=150_000, size=C_STACK_SIZE):
+    """Decorator for tests exhausting the C stack with *depth* recursive calls.
+
+    Run the test in a separate thread with the C stack of *size* bytes, so
+    that the outcome does not depend on the C stack size of the main thread
+    (which can be large or unlimited, see RLIMIT_STACK).
+
+    If a thread with the limited C stack cannot be created, run the test in
+    the current thread, but skip it if the C stack is too large.
+    """
+    reason = f"the C stack is large enough for {depth} recursive calls"
+    def decorator(test):
+        @functools.wraps(test)
+        def wrapper(*args, **kwargs):
+            def run_test():
+                # The C stack can still be too large if limiting it failed.
+                if _has_huge_c_stack(depth):
+                    raise unittest.SkipTest(reason)
+                test(*args, **kwargs)
+
+            try:
+                import threading
+                old_size = threading.stack_size(size)
+            except (ImportError, ValueError, RuntimeError):
+                # Setting the thread stack size is not supported.
+                return run_test()
+
+            exceptions = []
+            def run():
+                try:
+                    run_test()
+                except BaseException as exc:
+                    exceptions.append(exc)
+
+            thread = threading.Thread(target=run)
+            try:
+                thread.start()
+            except RuntimeError:
+                # Threads are not supported.
+                return run_test()
+            finally:
+                threading.stack_size(old_size)
+            thread.join()
+            if exceptions:
+                raise exceptions[0]
+        return wrapper
+    return decorator
 
 
 # Windows doesn't have os.uname() but it doesn't support s390x.

Lib/test/support/isolation.py

@@ -163,6 +163,16 @@ def _raise_fixture_outcome(outcome):
     raise exc from _remote(outcome['detail'])
 
 
+def _check_returncode(returncode, output, what):
+    # The subprocess writes its result before exiting, so a non-zero exit code
+    # means it died afterwards, during finalization, unnoticed by the result.
+    if returncode:
+        exc = _SubprocessTestError(
+            f'the subprocess exited with code {returncode} '
+            f'after running the {what}')
+        raise exc from _remote(output)
+
+
 def _isolate_method(func):
     @functools.wraps(func)
     def wrapper(self, /, *args, **kwargs):
@@ -180,7 +190,9 @@ def wrapper(self, /, *args, **kwargs):
             raise exc from _remote(output)
         # The parent measures this method's own duration (the real cost of the
         # isolated run, subprocess startup included), so nothing to forward here.
+        # Replay the outcomes first: a failure of the test itself is more useful.
         _replay_outcomes(self, payload['outcomes'])
+        _check_returncode(returncode, output, 'test')
     return wrapper
 
 
@@ -219,13 +231,20 @@ def setUpClass(cls):
             by_id.setdefault(outcome['id'], []).append(outcome)
         cls._isolated_outcomes = by_id
         cls._isolated_durations = dict(payload.get('durations', ()))
+        # Report the crash from tearDownClass(), after replaying the outcomes.
+        cls._isolated_exit = (returncode, output)
 
     def tearDownClass(cls):
         if runningInSubprocess:
             orig_tearDownClass(cls)
-        else:
-            cls._isolated_outcomes = None
-            cls._isolated_durations = None
+            return
+        cls._isolated_outcomes = None
+        cls._isolated_durations = None
+        # Missing if an overriding setUpClass() bypassed the subprocess.
+        exited = getattr(cls, '_isolated_exit', None)
+        cls._isolated_exit = None
+        if exited is not None:
+            _check_returncode(*exited, 'class')
 
     def _callSetUp(self):
         # In the parent the real test does not run, so neither should setUp().

Lib/test/test_argparse.py

@@ -7346,6 +7346,19 @@ def test_module(self, compiled=False):
     def test_module_compiled(self):
         self.test_module(compiled=True)
 
+    def test_module_as_main_without_altering_argv(self):
+        basename = 'module' + os_helper.FS_NONASCII
+        modulename = f'{self.dirname}.{basename}'
+        self.make_script(self.dirname, basename)
+        runner_source = textwrap.dedent(f'''\
+            import runpy
+            runpy._run_module_as_main({modulename!r}, alter_argv=False)
+        ''')
+        runner = script_helper.make_script(
+            self.dirname, 'runner', runner_source)
+        self.check_usage(os.path.basename(runner), runner,
+                         PYTHONPATH=os.curdir)
+
     def test_package(self, compiled=False):
         basename = 'subpackage' + os_helper.FS_NONASCII
         packagename = f'{self.dirname}.{basename}'