I think the default behavior of os.path.join() when None is passed as the first argument should be to translate it to '' by default.

>>> import os
>>> os.path.join(None, 'somewhere')
'somewhere'

vs the current

>>> import os
>>> os.path.join(None, 'somewhere')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.7/posixpath.py", line 68, in join
    elif path == '' or path.endswith('/'):
AttributeError: 'NoneType' object has no attribute 'endswith'

I believe this can be easily archived by adding the following changes:

64c64
<     path = a or ''
---
>

I think it is probably better to keep the error, myself.  It seems to me that a None creeping in is more likely to be an error in the program.   But I could be convinced otherwise :)

What is your use case?

I agree with David. This is a programming error, and should result in an exception.

Since most other os.path functions also choke (intentionally) on None values, I agree and would point out that if you want this behavior, it's easy to do the "or ''" dance on the caller side.