pythongh-91349: Adjust default compression level to 6 (down from 9) in gzip tarfile and

bzip.

It is the default level used by most compression tools and a
reasonable tradeoff between speed and performance. level 9 is very slow, in
the order of 1 MB/s, vary with the machine, and was likely a major
bottleneck for any software that forgot to specify the compression level.

Author: rmmancom

Doc/library/bz2.rst

@@ -29,7 +29,7 @@ The :mod:`bz2` module contains:
 (De)compression of files
 ------------------------
 
-.. function:: open(filename, mode='rb', compresslevel=9, encoding=None, errors=None, newline=None)
+.. function:: open(filename, mode='rb', compresslevel=6, encoding=None, errors=None, newline=None)
 
    Open a bzip2-compressed file in binary or text mode, returning a :term:`file
    object`.
@@ -63,7 +63,7 @@ The :mod:`bz2` module contains:
       Accepts a :term:`path-like object`.
 
 
-.. class:: BZ2File(filename, mode='r', *, compresslevel=9)
+.. class:: BZ2File(filename, mode='r', *, compresslevel=6)
 
    Open a bzip2-compressed file in binary mode.
 
@@ -198,7 +198,7 @@ The :mod:`bz2` module contains:
 Incremental (de)compression
 ---------------------------
 
-.. class:: BZ2Compressor(compresslevel=9)
+.. class:: BZ2Compressor(compresslevel=6)
 
    Create a new compressor object. This object may be used to compress data
    incrementally. For one-shot compression, use the :func:`compress` function
@@ -288,7 +288,7 @@ Incremental (de)compression
 One-shot (de)compression
 ------------------------
 
-.. function:: compress(data, compresslevel=9)
+.. function:: compress(data, compresslevel=6)
 
    Compress *data*, a :term:`bytes-like object <bytes-like object>`.
 

Doc/library/gzip.rst

@@ -26,7 +26,7 @@ Note that additional file formats which can be decompressed by the
 The module defines the following items:
 
 
-.. function:: open(filename, mode='rb', compresslevel=9, encoding=None, errors=None, newline=None)
+.. function:: open(filename, mode='rb', compresslevel=6, encoding=None, errors=None, newline=None)
 
    Open a gzip-compressed file in binary or text mode, returning a :term:`file
    object`.
@@ -67,7 +67,7 @@ The module defines the following items:
 
    .. versionadded:: 3.8
 
-.. class:: GzipFile(filename=None, mode=None, compresslevel=9, fileobj=None, mtime=None)
+.. class:: GzipFile(filename=None, mode=None, compresslevel=6, fileobj=None, mtime=None)
 
    Constructor for the :class:`GzipFile` class, which simulates most of the
    methods of a :term:`file object`, with the exception of the :meth:`~io.IOBase.truncate`
@@ -184,7 +184,7 @@ The module defines the following items:
       attribute instead.
 
 
-.. function:: compress(data, compresslevel=9, *, mtime=0)
+.. function:: compress(data, compresslevel=6, *, mtime=0)
 
    Compress the *data*, returning a :class:`bytes` object containing
    the compressed data.  *compresslevel* and *mtime* have the same meaning as in

Lib/bz2.py

@@ -34,7 +34,7 @@ class BZ2File(_compression.BaseStream):
     returned as bytes, and data to be written should be given as bytes.
     """
 
-    def __init__(self, filename, mode="r", *, compresslevel=9):
+    def __init__(self, filename, mode="r", *, compresslevel=6):
         """Open a bzip2-compressed file.
 
         If filename is a str, bytes, or PathLike object, it gives the
@@ -276,7 +276,7 @@ def tell(self):
         return self._pos
 
 
-def open(filename, mode="rb", compresslevel=9,
+def open(filename, mode="rb", compresslevel=6,
          encoding=None, errors=None, newline=None):
     """Open a bzip2-compressed file in binary or text mode.
 
@@ -318,7 +318,7 @@ def open(filename, mode="rb", compresslevel=9,
         return binary_file
 
 
-def compress(data, compresslevel=9):
+def compress(data, compresslevel=6):
     """Compress a block of data.
 
     compresslevel, if given, must be a number between 1 and 9.

Lib/tarfile.py

@@ -1911,7 +1911,7 @@ def taropen(cls, name, mode="r", fileobj=None, **kwargs):
         return cls(name, mode, fileobj, **kwargs)
 
     @classmethod
-    def gzopen(cls, name, mode="r", fileobj=None, compresslevel=9, **kwargs):
+    def gzopen(cls, name, mode="r", fileobj=None, compresslevel=6, **kwargs):
         """Open gzip compressed tar archive name for reading or writing.
            Appending is not allowed.
         """
@@ -1944,7 +1944,7 @@ def gzopen(cls, name, mode="r", fileobj=None, compresslevel=9, **kwargs):
         return t
 
     @classmethod
-    def bz2open(cls, name, mode="r", fileobj=None, compresslevel=9, **kwargs):
+    def bz2open(cls, name, mode="r", fileobj=None, compresslevel=6, **kwargs):
         """Open bzip2 compressed tar archive name for reading or writing.
            Appending is not allowed.
         """

Misc/NEWS.d/next/Library/2025-03-19-12-41-42.gh-issue-91349.8eTOCP.rst

@@ -0,0 +1,5 @@
+Adjust default compression level to 6 (down from 9) in gzip tarfile and
+bzip. It is the default level used by most compression tools and a
+reasonable tradeoff between speed and performance. level 9 is very slow, in
+the order of 1 MB/s, vary with the machine, and was likely a major
+bottleneck for any software that forgot to specify the compression level.