python subprocess with gzip

I am trying to stream data through a subprocess, gzip it and write to a file. The following works. I wonder if it is possible to use python’s native gzip library instead.

fid = gzip.open(self.ipFile, 'rb') # input data
oFid = open(filtSortFile, 'wb') # output file
sort = subprocess.Popen(args="sort | gzip -c ", shell=True, stdin=subprocess.PIPE, stdout=oFid) # set up the pipe
processlines(fid, sort.stdin, filtFid) # pump data into the pipe

THE QUESTION: How do I do this instead .. where the gzip package of python is used? I’m mostly curious to know why the following gives me a text files (instead of a compressed binary version) … very odd.

fid = gzip.open(self.ipFile, 'rb')
oFid = gzip.open(filtSortFile, 'wb')
sort = subprocess.Popen(args="sort ", shell=True, stdin=subprocess.PIPE, stdout=oFid)
processlines(fid, sort.stdin, filtFid)

Answer

subprocess writes to oFid.fileno() but gzip returns fd of underlying file object:

def fileno(self):
    """Invoke the underlying file object's fileno() method."""
    return self.fileobj.fileno()

To enable compression use gzip methods directly:

import gzip
from subprocess import Popen, PIPE
from threading import Thread

def f(input, output):
    for line in iter(input.readline, ''):
        output.write(line)

p = Popen(["sort"], bufsize=-1, stdin=PIPE, stdout=PIPE)
Thread(target=f, args=(p.stdout, gzip.open('out.gz', 'wb'))).start()

for s in "cafebabe":
    p.stdin.write(s+"n")
p.stdin.close()

Example

$ python gzip_subprocess.py  && od -c out.gz && zcat out.gz 
0000000 037 213  b  b 251   E   t   N 002 377   o   u   t     K 344
0000020   J 344   J 002 302   d 256   T       L 343 002     j 017   j
0000040   k 020      
0000045
a
a
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b
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e
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