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[3.7] bpo-39389: gzip: fix compression level metadata (GH-18077) by miss-islington · Pull Request #18101 · python/cpython · GitHub

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[3.7] bpo-39389: gzip: fix compression level metadata (GH-18077) #18101

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Merged
ned-deily merged 2 commits into from
Mar 4, 2020
Conversation 8 Commits 2 Checks 0 Files changed
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[3.7] bpo-39389: gzip: fix compression level metadata (GH-18077) #18101

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45b9857
bpo-39389: gzip: fix compression level metadata (GH-18077)
wchargin Jan 21, 2020
a85095f
bpo-39389: add constants for 3.7 backport
ned-deily Mar 4, 2020
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17 changes: 14 additions & 3 deletions Lib/gzip.py
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Expand Up @@ -17,6 +17,11 @@

READ, WRITE = 1, 2

_COMPRESS_LEVEL_FAST = 1
_COMPRESS_LEVEL_TRADEOFF = 6
_COMPRESS_LEVEL_BEST = 9


def open(filename, mode="rb", compresslevel=9,
encoding=None, errors=None, newline=None):
"""Open a gzip-compressed file in binary or text mode.
Expand Down Expand Up @@ -191,7 +196,7 @@ def __init__(self, filename=None, mode=None,
self.fileobj = fileobj

if self.mode == WRITE:
self._write_gzip_header()
self._write_gzip_header(compresslevel)

@property
def filename(self):
Expand All @@ -218,7 +223,7 @@ def _init_write(self, filename):
self.bufsize = 0
self.offset = 0 # Current file offset for seek(), tell(), etc

def _write_gzip_header(self):
def _write_gzip_header(self, compresslevel):
self.fileobj.write(b'\037\213') # magic header
self.fileobj.write(b'\010') # compression method
try:
Expand All @@ -239,7 +244,13 @@ def _write_gzip_header(self):
if mtime is None:
mtime = time.time()
write32u(self.fileobj, int(mtime))
self.fileobj.write(b'\002')
if compresslevel == _COMPRESS_LEVEL_BEST:
xfl = b'\002'
elif compresslevel == _COMPRESS_LEVEL_FAST:
xfl = b'\004'
else:
xfl = b'\000'
self.fileobj.write(xfl)
self.fileobj.write(b'\377')
if fname:
self.fileobj.write(fname + b'\000')
Expand Down
20 changes: 20 additions & 0 deletions Lib/test/test_gzip.py
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Expand Up @@ -358,6 +358,26 @@ def test_metadata(self):
isizeBytes = fRead.read(4)
self.assertEqual(isizeBytes, struct.pack('<i', len(data1)))

def test_compresslevel_metadata(self):
# see RFC 1952: http://www.faqs.org/rfcs/rfc1952.html
# specifically, discussion of XFL in section 2.3.1
cases = [
('fast', 1, b'\x04'),
('best', 9, b'\x02'),
('tradeoff', 6, b'\x00'),
]
xflOffset = 8

for (name, level, expectedXflByte) in cases:
with self.subTest(name):
fWrite = gzip.GzipFile(self.filename, 'w', compresslevel=level)
with fWrite:
fWrite.write(data1)
with open(self.filename, 'rb') as fRead:
fRead.seek(xflOffset)
xflByte = fRead.read(1)
self.assertEqual(xflByte, expectedXflByte)

def test_with_open(self):
# GzipFile supports the context management protocol
with gzip.GzipFile(self.filename, "wb") as f:
Expand Down
2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Library/2020-01-20-00-56-01.bpo-39389.fEirIS.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
Write accurate compression level metadata in :mod:`gzip` archives, rather
than always signaling maximum compression.
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