Bob Ippolito (@etrepum) on Haskell, Python, Erlang, JavaScript, etc.

xattr - Darwin 8 Extended Filesystem Attributes from Python


Darwin 8.0, the underpinnings of Mac OS X 10.4 (Tiger) ships with a new API at the filesystem layer: Extended Attributes. I haven't really seen much documentation thrown about, but effectively they can be used to associate arbitrary metadata with files (note that this is entirely separate from Spotlight Metadata Attributes). Extended attributes are the new way that the resource fork and finder information is looked up and stored. They're also used by the obscure copyfile that lives beneath all of the changes to cp, mv, tar, etc.

I whipped up a quick wrapper for the APIs today, xattr. This extension module exposes an xattr object that wraps paths or file descriptors (either as an int, or as an object with a fileno()), and it acts like a dict. Usage looks like this:

>>> import xattr
>>> x = xattr.xattr('tiger_8a428_userdvd.dmg')
>>> x
<xattr file='tiger_8a428_userdvd.dmg'>
>>> x.keys()
>>> dict(x)
{u'': 'i:969254 ...more stuff'}
>>> x['']
'i:969254 ...more stuff'
>>> x[''] = 'select'
>>> dict(x)
{u'': 'select',
 u'': 'i:969254 ...more stuff'}
>>> del x['']
>>> dict(x)
{u'': 'i:969254 ...more stuff'}

Note that keys are limited to 128 bytes and values appear to be limited to 3800 bytes. I think the resource fork is special cased to be longer, though.

You can download xattr 0.1 from for Tiger's Python 2.3.5 or Python 2.4.1 (installed on Tiger).

svn trunk:
0.1 source:
It looks like there is some relevant information and an xattr command line tool in the arstechnica tiger review.