New submission from Eduardo Robles Elvira:
I think I have found a small typo-bug in tarfile.py, that seems to
be present in cpython upstream, which makes tarfile compression slower.
The issue can be seen here, in line 415 [1] of tarfile.py:
self.cmp = self.zlib.compressobj(9, self.zlib.DEFLATED,
-self.zlib.MAX_WBITS,
self.zlib.DEF_MEM_LEVEL,
0)
The minus sign doesn't make sense to me, because zlib.MAX_WBITS is 15,
and according to the documentation [2] wbits is by default 15 and
"This should be an integer from 8 to 15. Higher values give better
compression, but use more memory". -15 is not even in range. So I guess the
expected value should be the zlib default, 15. Or maybe another value, but it
should at least be in range.
I marked it as "performance" type bug as this only really affects performance.
I might have gotten it all wrong and it's fine the way it is, but I thought
it'd be best to report it, as it looked fishy to me.
--
[1] http://hg.python.org/cpython/file/94d0e842b9ea/Lib/tarfile.py#l415
[2] https://docs.python.org/3.4/library/zlib.html#zlib.compressobj
----------
components: Library (Lib)
files: max_wbits.patch
keywords: patch
messages: 225006
nosy: edulix
priority: normal
severity: normal
status: open
title: max_wbits set incorrectly to -zlib.MAX_WBITS in tarfile, shouldn't be
negative
type: performance
versions: Python 2.7, Python 3.1, Python 3.2, Python 3.3, Python 3.4, Python 3.5
Added file: http://bugs.python.org/file36299/max_wbits.patch
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<http://bugs.python.org/issue22163>
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