gh-71253: Run unclosed file test on all io implementations

Update `test_io` `_check_warn_on_dealloc` to use `self.` to dispatch to
different I/O implementations.

Update the `_pyio` implementation to match expected behavior, using the
same `_dealloc_warn` design as the C implementation uses to report the
topmost `__del__` object.

Author: cmaloney

Lib/_pyio.py

@@ -407,6 +407,9 @@ def __del__(self):
         if closed:
             return
 
+        if dealloc_warn := getattr(self, "_dealloc_warn", None):
+            dealloc_warn(self)
+
         # If close() fails, the caller logs the exception with
         # sys.unraisablehook. close() must be called at the end at __del__().
         self.close()
@@ -853,6 +856,10 @@ def __repr__(self):
         else:
             return "<{}.{} name={!r}>".format(modname, clsname, name)
 
+    def _dealloc_warn(self, source):
+        if dealloc_warn := getattr(self.raw, "_dealloc_warn", None):
+            dealloc_warn(source)
+
     ### Lower-level APIs ###
 
     def fileno(self):
@@ -1600,11 +1607,15 @@ def __init__(self, file, mode='r', closefd=True, opener=None):
             raise
         self._fd = fd
 
-    def __del__(self):
+    def _dealloc_warn(self, source):
         if self._fd >= 0 and self._closefd and not self.closed:
             import warnings
-            warnings.warn('unclosed file %r' % (self,), ResourceWarning,
+            warnings.warn(f'unclosed file {source!r}', ResourceWarning,
                           stacklevel=2, source=self)
+
+    def __del__(self):
+        if self._fd >= 0 and self._closefd and not self.closed:
+            self._dealloc_warn(self)
             self.close()
 
     def __getstate__(self):
@@ -2689,6 +2700,10 @@ def readline(self, size=None):
     def newlines(self):
         return self._decoder.newlines if self._decoder else None
 
+    def _dealloc_warn(self, source):
+        if dealloc_warn := getattr(self.buffer, "_dealloc_warn", None):
+            dealloc_warn(source)
+
 
 class StringIO(TextIOWrapper):
     """Text I/O implementation using an in-memory buffer.

Lib/test/test_io.py

@@ -4417,7 +4417,7 @@ def test_abc_inheritance_official(self):
         self._check_abc_inheritance(io)
 
     def _check_warn_on_dealloc(self, *args, **kwargs):
-        f = open(*args, **kwargs)
+        f = self.open(*args, **kwargs)
         r = repr(f)
         with self.assertWarns(ResourceWarning) as cm:
             f = None
@@ -4446,7 +4446,7 @@ def cleanup_fds():
         r, w = os.pipe()
         fds += r, w
         with warnings_helper.check_no_resource_warning(self):
-            open(r, *args, closefd=False, **kwargs)
+            self.open(r, *args, closefd=False, **kwargs)
 
     @unittest.skipUnless(hasattr(os, "pipe"), "requires os.pipe()")
     def test_warn_on_dealloc_fd(self):

Misc/NEWS.d/next/Library/2025-05-17-19-56-43.gh-issue-71253.BJzTU7.rst

@@ -0,0 +1,3 @@
+Emit :exc:`RuntimeWarning` in the Python implementation of :mod:`io` when
+the :term:`file object` is not closed explicitly in the presence of multiple
+I/O layers.