bpo-32695: Docs and tests for compresslevel and preset kwargs in tarfile (GH-21470)

Co-Authored-By: Bo Bayles <bbayles@gmail.com>
(cherry picked from commit eb2d4a66ff)

Co-authored-by: Zackery Spytz <zspytz@gmail.com>

Doc/library/tarfile.rst

@@ -102,6 +102,9 @@ Some facts and figures:
    ``'x:bz2'``, :func:`tarfile.open` accepts the keyword argument
    *compresslevel* (default ``9``) to specify the compression level of the file.
 
+   For modes ``'w:xz'`` and ``'x:xz'``, :func:`tarfile.open` accepts the
+   keyword argument *preset* to specify the compression level of the file.
+
    For special purposes, there is a second format for *mode*:
    ``'filemode|[compression]'``.  :func:`tarfile.open` will return a :class:`TarFile`
    object that processes its data as a stream of blocks.  No random seeking will

Lib/test/test_tarfile.py

@@ -1706,15 +1706,30 @@ class CreateTest(WriteTestBase, unittest.TestCase):
 
 
 class GzipCreateTest(GzipTest, CreateTest):
-    pass
+
+    def test_create_with_compresslevel(self):
+        with tarfile.open(tmpname, self.mode, compresslevel=1) as tobj:
+            tobj.add(self.file_path)
+        with tarfile.open(tmpname, 'r:gz', compresslevel=1) as tobj:
+            pass
 
 
 class Bz2CreateTest(Bz2Test, CreateTest):
-    pass
+
+    def test_create_with_compresslevel(self):
+        with tarfile.open(tmpname, self.mode, compresslevel=1) as tobj:
+            tobj.add(self.file_path)
+        with tarfile.open(tmpname, 'r:bz2', compresslevel=1) as tobj:
+            pass
 
 
 class LzmaCreateTest(LzmaTest, CreateTest):
-    pass
+
+    # Unlike gz and bz2, xz uses the preset keyword instead of compresslevel.
+    # It does not allow for preset to be specified when reading.
+    def test_create_with_preset(self):
+        with tarfile.open(tmpname, self.mode, preset=1) as tobj:
+            tobj.add(self.file_path)
 
 
 class CreateWithXModeTest(CreateTest):

Misc/NEWS.d/next/Library/2020-07-13-23-46-59.bpo-32695.tTqqXe.rst

@@ -0,0 +1,2 @@
+The *compresslevel* and *preset* keyword arguments of :func:`tarfile.open`
+are now both documented and tested.