gh-156078: Prevent ZipFile.mkdir() during an active write

Lib/test/test_zipfile/test_core.py

@@ -4598,6 +4598,8 @@ def test_open_conflicting_handles(self):
                     zipf.writestr('str', 'abcde')
                 with self.assertRaises(ValueError):
                     zipf.write(__file__, 'file')
+                with self.assertRaises(ValueError):
+                    zipf.mkdir('directory')
                 with self.assertRaises(ValueError):
                     zipf.close()
                 w1.write(msg2)
@@ -5518,6 +5520,9 @@ def test_mkdir(self):
             zf.extractall(target)
             self.assertEqual(set(os.listdir(target)), {"directory", "directory2", "directory3", "directory4"})
 
+        with self.assertRaises(ValueError):
+            zf.mkdir("closed")
+
     def test_create_directory_with_write(self):
         with zipfile.ZipFile(TESTFN, "w") as zf:
             zf.writestr(zipfile.ZipInfo('directory/'), '')

Lib/zipfile/__init__.py

@@ -2596,6 +2596,14 @@ def mkdir(self, zinfo_or_directory_name, mode=511):
         else:
             raise TypeError("Expected type str or ZipInfo")
 
+        if not self.fp:
+            raise ValueError(
+                "Attempt to write to ZIP archive that was already closed")
+        if self._writing:
+            raise ValueError(
+                "Can't write to ZIP archive while an open writing handle exists."
+            )
+
         with self._lock:
             if self._seekable:
                 self.fp.seek(self.start_dir)

Misc/NEWS.d/next/Library/2026-08-20-09-56-45.gh-issue-156078.-E7gZc.rst

@@ -0,0 +1,3 @@
+Fix :meth:`zipfile.ZipFile.mkdir` to raise :exc:`ValueError` when called
+while a writable member handle is open, preventing corruption of the ZIP
+archive.