Opening zipfile of unsupported compression-type silently returns empty filestream, instead of throwing exception

Seem to be knocking my head off a newbie error and I am not a newbie. I have a 1.2G known-good zipfile ‘train.zip’ containing a 3.5G file ‘train.csv’. I open the zipfile and file itself without any exceptions (no LargeZipFile), but the resulting filestream appears to be empty. (UNIX ‘unzip -c …’ confirms it is good) The file objects returned by Python ZipFile.open() are not seek’able or tell’able, so I can’t check that.

Python distribution is 2.7.3 EPD-free 7.3-1 (32-bit) ; but should be ok for large zips. OS is MacOS 10.6.6

import csv
import zipfile as zf

zip_pathname = os.path.join('/my/data/path/.../', 'train.zip')
#with zf.ZipFile(zip_pathname).open('train.csv') as z:
z = zf.ZipFile(zip_pathname, 'r', zf.ZIP_DEFLATED, allowZip64=True) # I tried all permutations
z.debug = 1
z.testzip() # zipfile integrity is ok

z1 = z.open('train.csv', 'r') # our file keeps coming up empty?

# Check the info to confirm z1 is indeed a valid 3.5Gb file...
z1i = z.getinfo(file_name)
for att in ('filename', 'file_size', 'compress_size', 'compress_type', 'date_time',  'CRC', 'comment'):
    print '%s:t' % att, getattr(z1i,att)
# ... and it looks ok. compress_type = 9 ok?
#filename:  train.csv
#file_size: 3729150126
#compress_size: 1284613649
#compress_type: 9
#date_time: (2012, 8, 20, 15, 30, 4)
#CRC:   1679210291

# All attempts to read z1 come up empty?!
# z1.readline() gives ''
# z1.readlines() gives []
# z1.read() takes ~60sec but also returns '' ?

# code I would want to run is:
reader = csv.reader(z1)
header = reader.next()
return reader

JavaScript
​x
 
import csv
import zipfile as zf
​
zip_pathname = os.path.join('/my/data/path/.../', 'train.zip')
#with zf.ZipFile(zip_pathname).open('train.csv') as z:
z = zf.ZipFile(zip_pathname, 'r', zf.ZIP_DEFLATED, allowZip64=True) # I tried all permutations
z.debug = 1
z.testzip() # zipfile integrity is ok
​
z1 = z.open('train.csv', 'r') # our file keeps coming up empty?
​
# Check the info to confirm z1 is indeed a valid 3.5Gb file...
z1i = z.getinfo(file_name)
for att in ('filename', 'file_size', 'compress_size', 'compress_type', 'date_time',  'CRC', 'comment'):
    print '%s:t' % att, getattr(z1i,att)
# ... and it looks ok. compress_type = 9 ok?
#filename:  train.csv
#file_size: 3729150126
#compress_size: 1284613649
#compress_type: 9
#date_time: (2012, 8, 20, 15, 30, 4)
#CRC:   1679210291
​
# All attempts to read z1 come up empty?!
# z1.readline() gives ''
# z1.readlines() gives []
# z1.read() takes ~60sec but also returns '' ?
​
# code I would want to run is:
reader = csv.reader(z1)
header = reader.next()
return reader
​

Answer

The cause is the combination of:

this file’s compression type is type 9: Deflate64/Enhanced Deflate (PKWare’s proprietary format, as opposed to the more common type 8)
and a zipfile bug: it will not throw an exception for unsupported compression-types. It used to just silently return a bad file object [Section 4.4.5 compression method]. Aargh. How bogus. UPDATE: I filed bug 14313 and it was fixed back in 2012 so it now raises NotImplementedError when the compression type is unknown.

A command-line Workaround is to unzip, then rezip, to get a plain type 8: Deflated.

zipfile will throw an exception in 2.7 , 3.2+ I guess zipfile will never be able to actually handle type 9, for legal reasons. The Python doc makes no mention whatsoever that zipfile cannot handle other compression types :(

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Answer