[Python-Dev] bug in tarfile module?

[Petri Lehtinen]

Chris Withers wrote:
> Hi All,
> 
> This feels like a bug, but just wanted to check here before filing a
> report if I've missed something:
> 
> buzzkill$ python2.7
> Enthought Python Distribution -- www.enthought.com
> Version: 7.2-2 (32-bit)
> 
> Python 2.7.2 |EPD 7.2-2 (32-bit)| (default, Sep  7 2011, 09:16:50)
> [GCC 4.0.1 (Apple Inc. build 5493)] on darwin
> Type "packages", "demo" or "enthought" for more information.
> >>> import tarfile
> >>> source = open('/src/Python-2.6.7.tgz', 'rb')
> >>> tar = tarfile.open(fileobj=source, mode='r|*')
> >>> member = tar.extractfile('Python-2.6.7/Lib/genericpath.py')
> >>> data = member.read()
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
>   File "/Library/Frameworks/Python.framework/Versions/7.2/lib/python2.7/tarfile.py",
> line 815, in read
>     buf += self.fileobj.read()
>   File "/Library/Frameworks/Python.framework/Versions/7.2/lib/python2.7/tarfile.py",
> line 735, in read
>     return self.readnormal(size)
>   File "/Library/Frameworks/Python.framework/Versions/7.2/lib/python2.7/tarfile.py",
> line 742, in readnormal
>     self.fileobj.seek(self.offset + self.position)
>   File "/Library/Frameworks/Python.framework/Versions/7.2/lib/python2.7/tarfile.py",
> line 554, in seek
>     raise StreamError("seeking backwards is not allowed")
> tarfile.StreamError: seeking backwards is not allowed
> 
> The key is the "mode='r*|" which I understood to be specifically for
> reading blocks from a stream without seeking that would cause
> problems.

When discussing "filemode|[compression]" modes, the docs say:

    However, such a TarFile object is limited in that it does not
    allow to be accessed randomly

I'm not a tarfile expert, but extracting a single file sounds like
random access to me. If it was the first file in the archive (or there
was only one file) it probably wouldn't count as random access.

Petri