Hi,

I consider this to be a bug in os.path.isabs():

PythonWin 2.5 (r25:51908, Sep 19 2006, 09:52:17) [MSC v.1310 32 bit (Intel)] on win32.
>>> import os.path
>>> s = '\\Python25'
>>> os.path.isabs(s)
True
>>> os.path.abspath(s)
'C:\\Python25'
>>> os.chdir('d:')
>>> os.path.abspath(s)
'D:\\Python25'
>>> 

If s is really an absolute path as isabs() claims, then why does abspath() return a different path (i.e. not s)?  And worse, note that a call to os.chdir() changes the meaning of s!  So s is clearly not an absolute path, and isabs() is wrong.

It turns out that on Windows there are really 4 different kinds of paths:

1) Completely relative, e.g. foo\bar
2) Completely absolute, e.g. c:\foo\bar or \\server\share
3) Halfbreeds with no drive, e.g. \foo\bar
4) Halfbreeds relative to the current working directory on a specific drive, e.g. c:foo\bar

Python 2.5's os.path.isabs() method considers both (2) and (3) to be absolute; I agree with the classification of (2) but strongly disagree about case (3).

Oh, and os.path.join is also broken, e.g. os.path.join('foo', 'a:bar') gives 'foo\\a:bar', which is an invalid path.

Another consequence of this is that os.path.isabs() is not enough to classify paths.  Sometimes you really need a relative path, so we really need (at least) a new os.path.isrelative(), which can return "not isabs(s)" on POSIX platforms, and do the right thing on Win32.

The attached patch:
- Changes the behaviour of os.path.isabs() and os.path.join() on Win32, to classify pathnames as described above (including adding UNC support)
- Adds os.path.isrelative() on all platforms
- Changes a couple of Win32 os.path tests where I have deliberately broken backward compatibility
- Adds lots of new tests for these 3 functions on Win32

This does change the behaviour of isabs(), and it is possible that existing applications might be depending on the current behaviour.  Silently changing the behaviour might break those applications.  I'm not sure what the best course of action is - applying this for 2.6, putting a new API in (os.path.isreallyabs()?), or waiting for Python 3000...?

Kind regards,

Jon Foster