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.

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)