gh-81954: Warn when ZipFile is closed with unwritten data

Bring `zipfile.ZipFile` into alignment with other buffered file writers
like `GzipFile` and `TextIOWrapper` by emitting a `ResourceWarning` if
there is unwritten data and it is closed implicitly.

ZipFile should be closed either by using it as a context manager or
explicitly calling `.close()`.

Author: cmaloney

Doc/library/zipfile.rst

@@ -285,6 +285,10 @@ ZipFile objects
       Added support for specifying member name encoding for reading
       metadata in the zipfile's directory and file headers.
 
+   .. versionchanged:: next
+      Deleting a :class:`zipfile.ZipFile` which contains unwritten data before
+      it is closed now emits a :exc:`ResourceWarning`. Use as a
+      :term:`context manager` or call :meth:`~zipfile.ZipFile.close` explicitly.
 
 .. method:: ZipFile.close()
 

Lib/test/test_zipfile/_path/test_path.py

@@ -6,6 +6,7 @@
 import stat
 import sys
 import unittest
+import warnings
 import zipfile
 import zipfile._path
 
@@ -86,6 +87,12 @@ class TestPath(unittest.TestCase):
     def setUp(self):
         self.fixtures = contextlib.ExitStack()
         self.addCleanup(self.fixtures.close)
+        # These tests use zipfiles as fixtures which do not get closed. Ignore
+        # the ResourceWarning.
+        self.fixtures.enter_context(warnings.catch_warnings())
+        warnings.filterwarnings(
+            'ignore', category=ResourceWarning, message='unclosed ZipFile'
+        )
 
     def zipfile_ondisk(self, alpharep):
         tmpdir = pathlib.Path(self.fixtures.enter_context(temp_dir()))

Lib/test/test_zipfile/test_core.py

@@ -25,12 +25,13 @@
 from test.support import (
     findfile, requires_zlib, requires_bz2, requires_lzma,
     requires_zstd, captured_stdout, captured_stderr, requires_subprocess,
-    cpython_only
+    cpython_only, gc_collect
 )
 from test.support.os_helper import (
     TESTFN, unlink, rmtree, temp_dir, temp_cwd, fd_count, FakePath
 )
 from test.support.import_helper import ensure_lazy_imports
+from test.support.warnings_helper import check_no_resource_warning
 
 
 TESTFN2 = TESTFN + "2"
@@ -4051,6 +4052,28 @@ def test_close_on_exception(self):
         except zipfile.BadZipFile:
             self.assertIsNone(zipfp2.fp, 'zipfp is not closed')
 
+    def test_garbage_collection(self):
+        # gh-81954: Warn if a writable zipfile is closed by GC.
+        with self.assertWarns(ResourceWarning):
+            zipfile.ZipFile(io.BytesIO(), "w")
+            gc_collect()
+
+        # Only warn if there is possible data loss.
+        # Properly closed via context manager.
+        buf = io.BytesIO()
+        with zipfile.ZipFile(buf, "w") as zf:
+            zf.writestr("f.txt", b"data")
+
+        with check_no_resource_warning(self):
+            # Read mode: No possible data loss.
+            zipfile.ZipFile(buf, "r")
+
+            # Write with manual explicit close: No pending data.
+            zf = zipfile.ZipFile(io.BytesIO(), "w")
+            zf.writestr("f.txt", b"data")
+            zf.close()
+            del zf
+
     def test_unsupported_version(self):
         # File has an extract_version of 120
         data = (b'PK\x03\x04x\x00\x00\x00\x00\x00!p\xa1@\x00\x00\x00\x00\x00\x00'
@@ -5503,10 +5526,10 @@ def test_root_folder_in_zipfile(self):
         the zip file, this is a strange behavior, but we should support it.
         """
         in_memory_file = io.BytesIO()
-        zf = zipfile.ZipFile(in_memory_file, "w")
-        zf.mkdir('/')
-        zf.writestr('./a.txt', 'aaa')
-        zf.extractall(TESTFN2)
+        with zipfile.ZipFile(in_memory_file, "w") as zf:
+            zf.mkdir('/')
+            zf.writestr('./a.txt', 'aaa')
+            zf.extractall(TESTFN2)
 
     def tearDown(self):
         rmtree(TESTFN2)

Lib/zipfile/__init__.py

@@ -12,6 +12,7 @@
 import sys
 import threading
 import time
+lazy import warnings
 
 try:
     import zlib # We may need its compression method
@@ -2616,6 +2617,13 @@ def mkdir(self, zinfo_or_directory_name, mode=511):
 
     def __del__(self):
         """Call the "close()" method in case the user forgot."""
+        # gh-81954: Warn if ZipFile is implicitly closed with unwritten end
+        # records. GC cleanup order is non-deterministic and can result in data
+        # loss.
+        if (self.fp is not None and self.mode in ('w', 'x', 'a')
+                and self._didModify):
+            warnings.warn(f"unclosed ZipFile {self!r}",
+                          ResourceWarning, source=self, stacklevel=2)
         self.close()
 
     def close(self):

Misc/NEWS.d/next/Library/2026-06-26-16-15-55.gh-issue-81954.MfUjgS.rst

@@ -0,0 +1,3 @@
+Deleting a :class:`zipfile.ZipFile` which contains unwritten data before it
+is closed now emits a :exc:`ResourceWarning`. Use as a :term:`context manager`
+or call :meth:`~zipfile.ZipFile.close` explicitly.