Unzipping part of a .gz file using python

Question

So here's the problem. I have sample.gz file which is roughly 60KB in size. I want to decompress the first 2000 bytes off this file. I am running into CRC check failed error cuz I guess the gzip CRC field appears at the end of file, and it requires the entire gzipped file to decompress. Is there a way to get around this? I dont care about the CRC check. Even if I fail to decompress cuz of bad CRC, that is OK. Is there a way to get around this and unzip partial .gz files?

The code I have so far is

import gzip
import time
import StringIO

file = open('sample.gz', 'rb')
mybuf = MyBuffer(file)
mybuf = StringIO.StringIO(file.read(2000))
f = gzip.GzipFile(fileobj=mybuf)
data = f.read()
print data

The error encountered is

File "gunzip.py", line 27, in ?
    data = f.read()
File "/usr/local/lib/python2.4/gzip.py", line 218, in read
  self._read(readsize)
File "/usr/local/lib/python2.4/gzip.py", line 273, in _read
  self._read_eof()
File "/usr/local/lib/python2.4/gzip.py", line 309, in _read_eof
  raise IOError, "CRC check failed"
IOError: CRC check failed

Also is there any way to use zlib module to do this and ignore the gzip headers?

Answer 1

I seems that you need to look into Python zlib library instead

The GZIP format relies on zlib, but introduces a a file-level compression concept along with CRC checking, and this appears to be what you do not want/need at the moment.

See for example these code snippets from Dough Hellman

Edit: the code on Doubh Hellman's site only show how to compress or decompress with zlib. As indicated above, GZIP is "zlib with an envelope", and you'll need to decode the envellope before getting to the zlib-compressed data per se. Here's more info to go about it, it's really not that complicated:

• see RFC 1952 for details about the GZIP format
• This format starts with a 10 bytes header, followed by optional, non compressed elements such as the file name or a comment, followed by the zlib-compressed data, itself followed by a CRC-32 (precisely an "Adler32" CRC).
• By using Python's struct module, parsing the header should be relatively simple
• The zlib sequence (or its first few thousand bytes, since that is what you want to do) can then be decompressed with python's zlib module, as shown in the examples above
• Possible problems to handle: if there are more than one file in the GZip archive, and if the second file starts within the block of a few thousand bytes we wish to decompress.

Sorry to provide neither an simple procedure nor a ready-to-go snippet, however decoding the file with the indication above should be relatively quick and simple.

Answer 2

I can't see any possible reason why you would want to decompress the first 2000 compressed bytes. Depending on the data, this may uncompress to any number of output bytes.

Surely you want to uncompress the file, and stop when you have uncompressed as much of the file as you need, something like:

f = gzip.GzipFile(fileobj=open('postcode-code.tar.gz', 'rb'))
data = f.read(4000)
print data

AFAIK, this won't cause the whole file to be read. It will only read as much as is necessary to get the first 4000 bytes.