gzip – Read and write GNU zip files¶
| Purpose: | Read and write gzip files. |
|---|---|
| Available In: | 1.5.2 and later |
The gzip module provides a file-like interface to GNU zip files, using zlib to compress and uncompress the data.
Writing Compressed Files¶
The module-level function open() creates an instance of the file-like class GzipFile. The usual methods for writing and reading data are provided. To write data into a compressed file, open the file with mode 'w'.
import gzip
import os
outfilename = 'example.txt.gz'
output = gzip.open(outfilename, 'wb')
try:
output.write('Contents of the example file go here.\n')
finally:
output.close()
print outfilename, 'contains', os.stat(outfilename).st_size, 'bytes of compressed data'
os.system('file -b --mime %s' % outfilename)
$ python gzip_write.py
application/x-gzip; charset=binary
example.txt.gz contains 68 bytes of compressed data
Different amounts of compression can be used by passing a compresslevel argument. Valid values range from 1 to 9, inclusive. Lower values are faster and result in less compression. Higher values are slower and compress more, up to a point.
import gzip
import os
import hashlib
def get_hash(data):
return hashlib.md5(data).hexdigest()
data = open('lorem.txt', 'r').read() * 1024
cksum = get_hash(data)
print 'Level Size Checksum'
print '----- ---------- ---------------------------------'
print 'data %10d %s' % (len(data), cksum)
for i in xrange(1, 10):
filename = 'compress-level-%s.gz' % i
output = gzip.open(filename, 'wb', compresslevel=i)
try:
output.write(data)
finally:
output.close()
size = os.stat(filename).st_size
cksum = get_hash(open(filename, 'rb').read())
print '%5d %10d %s' % (i, size, cksum)
The center column of numbers in the output of the script is the size in bytes of the files produced. As you see, for this input data, the higher compression values do not necessarily pay off in decreased storage space. Results will vary, depending on the input data.
$ python gzip_compresslevel.py
Level Size Checksum
----- ---------- ---------------------------------
data 754688 e4c0f9433723971563f08a458715119c
1 9839 1ff2fc4d9e466f01aa78856bf50fdad5
2 8260 342d621c603a409214172edd0e8e6838
3 8221 75a64632782808feb2b7519034d3e6c1
4 4160 c445f8379b44186bba1c2cf38ca4109a
5 4160 cdc096dc168637a7bae6174147294d6f
6 4160 841ef69720326a32fd8e28281cdd8f7e
7 4160 a05f486792e120aaf1621aef527fa8ad
8 4160 c03900a4c0d24198d1582e0239784873
9 4160 a6fb55fe8cbaaa10ee7427788839f89b
A GzipFile instance also includes a writelines() method that can be used to write a sequence of strings.
import gzip
import itertools
import os
output = gzip.open('example_lines.txt.gz', 'wb')
try:
output.writelines(itertools.repeat('The same line, over and over.\n', 10))
finally:
output.close()
os.system('gzcat example_lines.txt.gz')
$ python gzip_writelines.py
The same line, over and over.
The same line, over and over.
The same line, over and over.
The same line, over and over.
The same line, over and over.
The same line, over and over.
The same line, over and over.
The same line, over and over.
The same line, over and over.
The same line, over and over.
Reading Compressed Data¶
To read data back from previously compressed files, simply open the file with mode 'r'.
import gzip
input_file = gzip.open('example.txt.gz', 'rb')
try:
print input_file.read()
finally:
input_file.close()
This example reads the file written by gzip_write.py from the previous section.
$ python gzip_read.py
Contents of the example file go here.
While reading a file, it is also possible to seek and read only part of the data.
import gzip
input_file = gzip.open('example.txt.gz', 'rb')
try:
print 'Entire file:'
all_data = input_file.read()
print all_data
expected = all_data[5:15]
# rewind to beginning
input_file.seek(0)
# move ahead 5 bytes
input_file.seek(5)
print 'Starting at position 5 for 10 bytes:'
partial = input_file.read(10)
print partial
print
print expected == partial
finally:
input_file.close()
The seek() position is relative to the uncompressed data, so the caller does not even need to know that the data file is compressed.
$ python gzip_seek.py
Entire file:
Contents of the example file go here.
Starting at position 5 for 10 bytes:
nts of the
True
Working with Streams¶
When working with a data stream instead of a file, use the GzipFile class directly to compress or uncompress it. This is useful when the data is being transmitted over a socket or from read an existing (already open) file handle. A StringIO buffer can also be used.
import gzip
from cStringIO import StringIO
import binascii
uncompressed_data = 'The same line, over and over.\n' * 10
print 'UNCOMPRESSED:', len(uncompressed_data)
print uncompressed_data
buf = StringIO()
f = gzip.GzipFile(mode='wb', fileobj=buf)
try:
f.write(uncompressed_data)
finally:
f.close()
compressed_data = buf.getvalue()
print 'COMPRESSED:', len(compressed_data)
print binascii.hexlify(compressed_data)
inbuffer = StringIO(compressed_data)
f = gzip.GzipFile(mode='rb', fileobj=inbuffer)
try:
reread_data = f.read(len(uncompressed_data))
finally:
f.close()
print
print 'RE-READ:', len(reread_data)
print reread_data
Note
When re-reading the previously compressed data, I pass an explicit length to read(). Leaving the length off resulted in a CRC error, possibly because StringIO returned an empty string before reporting EOF. If you are working with streams of compressed data, you may want to prefix the data with an integer representing the actual amount of data to be read.
$ python gzip_StringIO.py
UNCOMPRESSED: 300
The same line, over and over.
The same line, over and over.
The same line, over and over.
The same line, over and over.
The same line, over and over.
The same line, over and over.
The same line, over and over.
The same line, over and over.
The same line, over and over.
The same line, over and over.
COMPRESSED: 51
1f8b08001d06215102ff0bc94855284ecc4d55c8c9cc4bd551c82f4b2d5248cc4b0133f4b8424665916401d3e717802c010000
RE-READ: 300
The same line, over and over.
The same line, over and over.
The same line, over and over.
The same line, over and over.
The same line, over and over.
The same line, over and over.
The same line, over and over.
The same line, over and over.
The same line, over and over.
The same line, over and over.
See also
- gzip
- The standard library documentation for this module.
- zlib
- The zlib module is a lower-level interface to gzip compression.
- zipfile
- The zipfile module gives access to ZIP archives.
- bz2
- The bz2 module uses the bzip2 compression format.
- tarfile
- The tarfile module includes built-in support for reading compressed tar archives.
