The current documentation of the gzip module should have its section "12.2.1. Examples of usage" updated to reflect the changes made to the module in Python3.2 (https://docs.python.org/3.2/whatsnew/3.2.html#gzip-and-zipfile).

Currently, the recipe given for gz-compressing a file is:

import gzip
with open('/home/joe/file.txt', 'rb') as f_in:
    with gzip.open('/home/joe/file.txt.gz', 'wb') as f_out:
        f_out.writelines(f_in)

which is clearly sub-optimal because it is line-based.

An equally simple, but more efficient recipe would be:

chunk_size = 1024
with open('/home/joe/file.txt', 'rb') as f_in:
    with gzip.open('/home/joe/file.txt.gz', 'wb') as f_out:
        while True:
            c = f_in.read(chunk_size)
            if not c: break
            d = f_out.write(c)

Comparing the two examples I find a >= 2x performance gain (both in terms of CPU time and wall time).

In the inverse scenario of file *de*-compression (which is not part of the docs though), the performance increase of substituting:

with gzip.open('/home/joe/file.txt.gz', 'rb') as f_in:
    with open('/home/joe/file.txt', 'wb') as f_out:
        f_out.writelines(f_in)

with:

with gzip.open('/home/joe/file.txt.gz', 'rb') as f_in:
    with open('/home/joe/file.txt', 'wb') as f_out:
        while True:
            c = f_in.read(chunk_size)
            if not c: break
            d = f_out.write(c)

is even higher (4-5x speed-ups).

In the de-compression case, another >= 2x speed-up can be achieved by avoiding the gzip module completely and going through a zlib.decompressobj instead, but of course this is a bit more complicated and should be documented in the zlib docs rather than the gzip docs (if you're interested, I could provide my code for it though).
Using the zlib library compression/decompression speed gets comparable to linux gzip/gunzip.

Maybe, shutil.copyfileobj() is good.

import gzip
import shutil

with open(src, 'rb') as f_in:
    with gzip.open(dst, 'wb') as f_out:
        shutil.copyfileobj(f_in, f_out)

>> with open(src, 'rb') as f_in:
>>     with gzip.open(dst, 'wb') as f_out:
>>        shutil.copyfileobj(f_in, f_out)

+1 !!
exactly as fast as my suggestion (with compression and de-compression), but a lot clearer !
Hadn't thought of it.

same speed is not surprising though as shutil.copyfileobj is implemented like this:

def copyfileobj(fsrc, fdst, length=16*1024):
    """copy data from file-like object fsrc to file-like object fdst"""
    while 1:
        buf = fsrc.read(length)
        if not buf:
            break
        fdst.write(buf)

which is essentially what I was proposing :)

ok, I've prepared the patch using the elegant shutil solution.

The patch looks good to me.

Isn’t there a buffering argument in open that can be used to avoid line buffering?

well, buffering is not the issue here. It's that the file iterator used in the current example is line-based, so whatever the buffer size you're doing unnecessary inspection to find and split on line terminators.

New changeset ae1528beae67 by Andrew Kuchling in branch 'default':
#21146: give a more efficient recipe in gzip docs
https://hg.python.org/cpython/rev/ae1528beae67

Applied to trunk.  Wolfgang Maier: thanks for your patch!