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The attached script works in 2.6(.4) but not in 2.7(.3); the script cannot locate its own source code if invoked by relative path and work directory has changed. If line 8 uncommented, everything is fine due to caching. 
I think this is related to issue #4050 changes in inspect.py. 
~ kgabor$ python ./instest.py
None ./instest.py
Traceback (most recent call last):
 File "./instest.py", line 10, in <module>
 print inspect.getsourcefile(A), inspect.getfile(A),inspect.findsource(A)
 File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/inspect.py", line 529, in findsource
 raise IOError('source code not available')
IOError: source code not available
~ kgabor$ pwd
/Users/kgabor
~ kgabor$ python /Users/kgabor/instest.py
/Users/kgabor/instest.py /Users/kgabor/instest.py (['import inspect\n', 'import os\n', '\n', 'class A:\n', ' def __init__(self):\n', ' self.a=1\n', '\n', '#print inspect.getsourcefile(A),inspect.getfile(A),inspect.findsource(A)\n', "os.chdir('/')\n", 'print inspect.getsourcefile(A), inspect.getfile(A),inspect.findsource(A)\n'], 3)

Sadly, there is nothing we can do about it, as we simply don't have absolute paths in __file__ attributes in your case.

Note that this is fixed in 3.4. That is, in 3.4 __file__ paths are made absolute when the import happens.

Hm, maybe just for the imports? This simple script, run it directly:
test.py:
 def foo(): pass
 print(foo.__code__.co_filename)
"$ python3.4 test.py" will still print relative path "test.py".
Hence, if you do 'os.chdir' in the above script, the introspection code will break. There are many similar issues on the tracker, btw.

__file__ is absolute for imports, yes. I didn't check co_filename, maybe that needs to be fixed too.

Yes. I'll create an issue for that.