bpo-36268: Change default tar format to pax from GNU. (GH-12355)

Doc/library/tarfile.rst

@@ -229,7 +229,11 @@ details.
 
 .. data:: DEFAULT_FORMAT
 
-   The default format for creating archives. This is currently :const:`GNU_FORMAT`.
+   The default format for creating archives. This is currently :const:`PAX_FORMAT`.
+
+    .. versionchanged:: 3.8
+       The default format for new archives was changed to
+       :const:`PAX_FORMAT` from :const:`GNU_FORMAT`.
 
 
 .. seealso::
@@ -820,8 +824,10 @@ There are three tar formats that can be created with the :mod:`tarfile` module:
 
 * The POSIX.1-2001 pax format (:const:`PAX_FORMAT`). It is the most flexible
   format with virtually no limits. It supports long filenames and linknames, large
-  files and stores pathnames in a portable way. However, not all tar
-  implementations today are able to handle pax archives properly.
+  files and stores pathnames in a portable way. Modern tar implementations,
+  including GNU tar, bsdtar/libarchive and star, fully support extended *pax*
+  features; some older or unmaintained libraries may not, but should treat
+  *pax* archives as if they were in the universally-supported *ustar* format.
 
   The *pax* format is an extension to the existing *ustar* format. It uses extra
   headers for information that cannot be stored otherwise. There are two flavours
@@ -871,7 +877,7 @@ converted. Possible values are listed in section :ref:`error-handlers`.
 The default scheme is ``'surrogateescape'`` which Python also uses for its
 file system calls, see :ref:`os-filenames`.
 
-In case of :const:`PAX_FORMAT` archives, *encoding* is generally not needed
+For :const:`PAX_FORMAT` archives (the default), *encoding* is generally not needed
 because all the metadata is stored using *UTF-8*. *encoding* is only used in
 the rare cases when binary pax headers are decoded or when strings with
 surrogate characters are stored.

Doc/whatsnew/3.8.rst

@@ -316,6 +316,16 @@ and manipulating normal distributions of a random variable.
     [7.672102882379219, 12.000027119750287, 4.647488369766392]
 
 
+tarfile
+-------
+
+The :mod:`tarfile` module now defaults to the modern pax (POSIX.1-2001)
+format for new archives, instead of the previous GNU-specific one.
+This improves cross-platform portability with a consistent encoding (UTF-8)
+in a standardized and extensible format, and offers several other benefits.
+(Contributed by C.A.M. Gerlach in :issue:`36268`.)
+
+
 tokenize
 --------
 

Lib/tarfile.py

@@ -105,7 +105,7 @@ SOLARIS_XHDTYPE = b"X"          # Solaris extended header
 USTAR_FORMAT = 0                # POSIX.1-1988 (ustar) format
 GNU_FORMAT = 1                  # GNU tar format
 PAX_FORMAT = 2                  # POSIX.1-2001 (pax) format
-DEFAULT_FORMAT = GNU_FORMAT
+DEFAULT_FORMAT = PAX_FORMAT
 
 #---------------------------------------------------------
 # tarfile constants

Lib/test/test_tarfile.py

@@ -2136,15 +2136,16 @@ class MiscTest(unittest.TestCase):
     def test_write_number_fields(self):
         self.assertEqual(tarfile.itn(1), b"0000001\x00")
         self.assertEqual(tarfile.itn(0o7777777), b"7777777\x00")
-        self.assertEqual(tarfile.itn(0o10000000),
+        self.assertEqual(tarfile.itn(0o10000000, format=tarfile.GNU_FORMAT),
                          b"\x80\x00\x00\x00\x00\x20\x00\x00")
-        self.assertEqual(tarfile.itn(0xffffffff),
+        self.assertEqual(tarfile.itn(0xffffffff, format=tarfile.GNU_FORMAT),
                          b"\x80\x00\x00\x00\xff\xff\xff\xff")
-        self.assertEqual(tarfile.itn(-1),
+        self.assertEqual(tarfile.itn(-1, format=tarfile.GNU_FORMAT),
                          b"\xff\xff\xff\xff\xff\xff\xff\xff")
-        self.assertEqual(tarfile.itn(-100),
+        self.assertEqual(tarfile.itn(-100, format=tarfile.GNU_FORMAT),
                          b"\xff\xff\xff\xff\xff\xff\xff\x9c")
-        self.assertEqual(tarfile.itn(-0x100000000000000),
+        self.assertEqual(tarfile.itn(-0x100000000000000,
+                                     format=tarfile.GNU_FORMAT),
                          b"\xff\x00\x00\x00\x00\x00\x00\x00")
 
         # Issue 32713: Test if itn() supports float values outside the

Misc/NEWS.d/next/Library/2019-03-14-16-25-17.bpo-36268.MDXLw6.rst

@@ -0,0 +1,3 @@
+Switch the default format used for writing tars with mod:`tarfile` to
+the modern POSIX.1-2001 pax standard, from the vendor-specific GNU.
+Contributed by C.A.M. Gerlach.