[Python-Dev] Question on bz2 codec. Is this a bug?

M.-A. Lemburg mal at egenix.com
Wed Sep 29 23:05:38 CEST 2010

Chris Bergstresser wrote:
> Hi all --
>    I looked through the bug tracker, but I didn't see this listed.  I
> was trying to use the bz2 codec, but it seems like it's not very
> useful in the current form (and I'm not sure if it's getting added
> back to py3k, so maybe this is a moot point).  It looks like the codec
> writes every piece of data fed to it as a separate compressed block.
> This results in compressed files which are significantly larger than
> the uncompressed files, if you're writing a lot of small bursts of
> data.  It also leads to interesing oddities like this:
>     import codecs
>     with codecs.open('text.bz2', 'w', 'bz2') as f:
>         for x in xrange(20):
>             f.write('This is data %i\n' % x)
>     with codecs.open('text.bz2', 'r', 'bz2') as f:
>         print f.read()
> This prints "This is data 0" and exits, because the codec won't read
> beyond the first compressed block.
> My question is, is this known, intended behavior?  Should I open a bug
> report?  Is it going away in py3k, so there's no real point in fixing
> it?

The codec is scheduled to be added back to Python3.

However, it's main use is in working on whole chunks of
data rather than the line-by-line approach you're after.
This is provided by the codec's incremental encoder/decoders,
but these are currently not used by codecs.open() and
I'm not sure whether the io lib uses them, which could
be used via the regular open().

Marc-Andre Lemburg

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