gh-160860: Fix value of GzipFile.mtime of writable stream

Doc/library/gzip.rst

@@ -163,6 +163,11 @@ The module defines the following items:
       since the Unix epoch (00:00:00 UTC, January 1, 1970).
       The initial value before reading any headers is ``None``.
 
+      When compressing, this attribute is set to the value passed to the constructor.
+
+      .. versionchanged:: 3.16
+         In previous versions, an :exc:`AttributeError` occured for compressing streams.
+
    .. attribute:: name
 
       The path to the gzip file on disk, as a :class:`str` or :class:`bytes`.

Doc/whatsnew/3.16.rst

@@ -461,6 +461,8 @@ gzip
 * :func:`gzip.open` now accepts an optional argument ``mtime``
   which is passed on to the constructor of the :class:`~gzip.GzipFile` class.
   (Contributed by Marin Misur in :gh:`91372`.)
+* The write-only property :attr:`gzip.GzipFile.mtime` now returns the
+  constructor value for :class:`~gzip.GzipFile` files in write mode.
 
 
 imaplib

Lib/gzip.py

@@ -262,6 +262,8 @@ def __init__(self, filename=None, mode=None,
     @property
     def mtime(self):
         """Last modification time read from stream, or None"""
+        if self.mode == WRITE:
+            return self._write_mtime
         return self._buffer.raw._last_mtime
 
     def __repr__(self):

Lib/test/test_gzip.py

@@ -343,6 +343,7 @@ def test_paddedfile_getattr(self):
     def test_mtime(self):
         mtime = 123456789
         with gzip.GzipFile(self.filename, 'w', mtime = mtime) as fWrite:
+            self.assertEqual(fWrite.mtime, mtime)
             fWrite.write(data1)
         with gzip.GzipFile(self.filename) as fRead:
             self.assertHasAttr(fRead, 'mtime')

Misc/NEWS.d/next/Library/2026-10-28-20-23-15.gh-issue-160860.ozPoph.rst

@@ -0,0 +1,3 @@
+The write-only property :attr:`gzip.GzipFile.mtime` now returns the
+constructor value for :class:`~gzip.GzipFile` files in write mode. Formerly,
+this caused an :exc:`AttributeError`.