python zlib produces a -3 error when inflating data through a Decompress() object -

i have created decoder parse, decompress , extract single file zlib encoded file downloaded through urllib2 file-like object. idea utilize little memory , disk space possible, using reader / writer pattern "decoder" in middle uncompress data coming urllib2, feed cpio subprocess , write file data disk:

with closing(builder.open()) reader:     open(component, "w+b") writer:          decoder = decoder()           while true:              data = reader.read(10240)              if len(data) == 0:                  break               writer.write(decoder.decode(data))          final = decoder.flush()         if final not none:             writer.write(final)          writer.flush() 

the decoder pretty simple too:

class decoder(object):     def __init__(self):         self.__zcat = zlib.decompressobj()         # cpio initialisation      def decode(self, data_in):         return self.__consume(self.__zcat.decompress(data_in))      def __consume(self, zcat_data_in):         # cpio operations         return data_out      def flush(self):         return self.__consume(self.__zcat.flush()) 

i seeing error before passed cpio pipe, felt omitting here sensible clarity.

the interesting thing, verify data in fact uncompressed zlib, wrote raw data data_in being passed decode() stdout:

def decode(self, data_in):     sys.stdout.write(data_in)     return self.__consume(self.__zcat.decompress(data_in)) 

then ran:

$ bin/myprog.py 2>/dev/null | zcat - | file - /dev/stdin: cpio archive 

as can see, zcat quite happy data given on stdin , resultant file cpio archive. zlib decompress method reporting:

error: error -3 while decompressing: incorrect header check 

\x1f\x9d first 2 bytes of old unix compress format. zlib can't decompress it. gzip can decompress compatible old compress utility.

you can pull the code pigz decompressing format , use directly.