gh-125260: gzip: let compress mtime default to 0

this follows GNU gzip, which defaults to
store 0 as mtime for compressing stdin, where no file mtime is involved

This makes gzip.compress(str) output deterministic by default
and greatly helps reproducible builds.

Author: bmwiedemann

Doc/library/gzip.rst

@@ -184,7 +184,7 @@ The module defines the following items:
       attribute instead.
 
 
-.. function:: compress(data, compresslevel=9, *, mtime=None)
+.. function:: compress(data, compresslevel=9, *, mtime=0)
 
    Compress the *data*, returning a :class:`bytes` object containing
    the compressed data.  *compresslevel* and *mtime* have the same meaning as in
@@ -203,6 +203,8 @@ The module defines the following items:
    .. versionchanged:: 3.13
       The gzip header OS byte is guaranteed to be set to 255 when this function
       is used as was the case in 3.10 and earlier.
+   .. versionchanged:: 3.14
+      The *mtime* parameter now defaults to 0 for reproducible output.
 
 .. function:: decompress(data)
 

Lib/gzip.py

@@ -579,8 +579,7 @@ def _rewind(self):
         super()._rewind()
         self._new_member = True
 
-
-def compress(data, compresslevel=_COMPRESS_LEVEL_BEST, *, mtime=None):
+def compress(data, compresslevel=_COMPRESS_LEVEL_BEST, *, mtime=0):
     """Compress data in one shot and return the compressed string.
 
     compresslevel sets the compression level in range of 0-9.

Lib/test/test_gzip.py

@@ -717,6 +717,10 @@ def test_compress_correct_level(self):
         for mtime in (0, 42):
             with self.subTest(mtime=mtime):
                 nocompress = gzip.compress(data1, compresslevel=0, mtime=mtime)
+                if mtime == 0:
+                    # test for gh-125260
+                    nocompressnomtime = gzip.compress(data1, compresslevel=0)
+                    self.assertEqual(nocompress, nocompressnomtime)
                 yescompress = gzip.compress(data1, compresslevel=1, mtime=mtime)
                 self.assertIn(data1, nocompress)
                 self.assertNotIn(data1, yescompress)