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Make a file called "test.py"
----
import pack
print pack.func.func_code.co_filename
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Make a directory called "pack". Put a file in it 
called "__init__.py" with the contents:
def func(): pass
Now run test.py. It will compile a relative path into 
these modules. Now you can change to any directory on 
the system and run test.py and it will return the 
original relative path. The problem is that the 
relative path is compiled into the .pyc. It should be 
an absolute path.

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I'm not clear on what the rules for co_filename are, but it
looks like the absolute path is only used for package
imports. If you cd into the package directory, run python
-c "import __init__", and then later import the package the
path name is relative.
The compiler package isn't connected to the import
mechanism, so there's no way for it to know whether it is
compiling a module or a package. I don't think there's a
way to do anything better.

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Just to clarify. The compiler in question is the builtin
compiler. It generates absolute path names unless the .py
file is in the current working directory.

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Why on earth was this assigned to Paul? He's not going to
make progress. Assigning back to nobody.

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I just ran into this today when trying to find out why 
pydoc wasn't working for a module of mine: 
inspect.getmodule calls os.abspath with the filename found 
in the code object of global functions. If the cwd is 
different than when the code object was compiled, 
inspect.getmodule fails.
Anyway, it looks to me like most of these relative paths 
could be caught in the find_module function (of import.c) 
if, when given an empty string for a path (while iterating 
over sys.path), find_module called getcwd() and used that 
instead of the empty string. Of course, this assumes that 
(on all platforms) opening a bare filename means open the 
file with that name in the current directory (is that a 
valid assumption?). Also, it appears from posixmodule.c 
that getcwd may not always be available.
Does this sound like a reasonable idea?

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I tried to reproduce the problem with python 2.4 and I'm
getting the absolute path....
So, I guess, the bug can be closed

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Closing as fixed, on the basis of isandlers report, until someone 
verifies that it is a current problem.

It's fairly easy to check this is still a problem:
$ ./python
Python 3.3.0a0 (default:a06ef7ab7321, Sep 22 2011, 13:41:29) 
[GCC 4.6.0 20110603 (Red Hat 4.6.0-10)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import setup
>>> setup.__file__
'setup.py'
That's a relative path rather than an absolute one. If you edit sys.path to use '.' instead of '', it becomes clear that the import machinery is just doing a blind join of the sys.path entry with the relative location of the file:
>>> del sys.modules['setup']
>>> sys.path[0] = '.'
>>> import setup
>>> setup.__file__
'./setup.py'

The reason why this is a problem:
$ cat test.py
def foo():
 pass
>>> import test, os, inspect
>>> os.chdir('/')
>>> inspect.getsource(test)
'def foo():\n pass\n'
But
>>> import test, os, inspect
>>> os.chdir('/')
>>> inspect.getsource(test)
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
IOError: source code not available

What is the status of this bug?
This is the oldest open bug.

So co_filename is absolute in Python 3.3, but __file__ on modules is not if a relative path is used from sys.path (as Nick pointed out). Changing this would possibly break code as this has been baked into the import system for quite some time. I think Nick as suggested making all paths absolute for __file__, but I can't remember where that discussion was held and why we never changed the semantics.
Regardless, __file__ being absolute is a different issue than co_filename, so I'm closing this at out of date.