[Python-Dev] __file__ is not always an absolute path

[Ezio Melotti]

In #7712 I was trying to change regrtest to always run the tests in a 
temporary CWD (e.g. /tmp/@test_1234_cwd/).
The patches attached to the issue add a context manager that changes the 
CWD, and it works fine when I run ./python -m test.regrtest from trunk/. 
However, when I try from trunk/Lib/ it fails with ImportErrors (note 
that the latest patch by Florent Xicluna already tries to workaround the 
problem). The traceback points to "the_package = __import__(abstest, 
globals(), locals(), [])" in runtest_inner (in regrtest.py), and a 
"print __import__('test').__file__" there returns 'test/__init__.pyc'.
This can be reproduced quite easily:

trunk$ ./python
Python 2.7a2+ (trunk:77941M, Feb  3 2010, 06:40:49)
[GCC 4.4.1] on linux2
Type "help", "copyright", "credits" or "license" for more information.
 >>> import os, sys
 >>> os.getcwd()
'/home/wolf/dev/trunk'
 >>> import test
 >>> test.__file__  # absolute
'/home/wolf/dev/trunk/Lib/test/__init__.pyc'
 >>> os.chdir('/tmp')
 >>> test.__file__
'/home/wolf/dev/trunk/Lib/test/__init__.pyc'
 >>> from test import test_unicode  # works
 >>> test_unicode.__file__
'/home/wolf/dev/trunk/Lib/test/test_unicode.pyc'
 >>>
[21]+  Stopped                 ./python

trunk$ cd Lib/
trunk/Lib$ ../python
Python 2.7a2+ (trunk:77941M, Feb  3 2010, 06:40:49)
[GCC 4.4.1] on linux2
Type "help", "copyright", "credits" or "license" for more information.
 >>> import os, sys
 >>> os.getcwd()
'/home/wolf/dev/trunk/Lib'
 >>> import test
 >>> test.__file__  # relative
'test/__init__.pyc'
 >>> os.chdir('/tmp')
 >>> from test import test_unicode  # fails
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
ImportError: cannot import name test_unicode

Is there a reason why in the second case test.__file__ is relative?