While trying to get a PEP302 import hook to function properly, I found
that the default traceback picks up the wrong sourcecode for PEP302
loaded modules.

The testcase pep302_traceback.py tries to emulate the behavior of the
files in ordinary, which generates the following output when run:

  (cd ordinary ; python2.6.2 main.py )   

  A.__name__= a
  B.__name__ a.b
  Traceback (most recent call last):
    File "main.py", line 6, in <module>
      a.A()
  File "/tmp/ordinary/a/__init__.py", line 6, in __init__
    b.B()
  File "/tmp/ordinary/a/b/__init__.py", line 4, in __init__
    raise Exception()
  Exception

But when i run the testcase, I get the following:

  python2.6.2 pep302_traceback.py 
  ### Show possible bug in default linecache (works in 2.6.2)
  A.__name__= a
  B.__name__= a.b
  Traceback (most recent call last):
    File "pep302_traceback.py", line 82, in <module>
      i.load_module("__main__")
    File "pep302_traceback.py", line 58, in load_module
      exec(code, mod.__dict__)
    File "<__main__.Importer>/main.py", line 6, in <module>
      a.A()
    File "<__main__.Importer>/a/__init__.py", line 6, in __init__
      b.B()
    File "<__main__.Importer>/a/b/__init__.py", line 4, in __init__
      raise Exception()
  Exception
  ### Show possible bug in default traceback (does not work in 2.6.2)
  A.__name__= a
  B.__name__= a.b
  Traceback (most recent call last):
    File "pep302_traceback.py", line 88, in <module>
      i.load_module("__main__")
    File "pep302_traceback.py", line 58, in load_module
      exec(code, mod.__dict__)
    File "<__main__.Importer>/main.py", line 6, in <module>
      # main.py picked up from somewhere in sys.path
    File "<__main__.Importer>/a/__init__.py", line 6, in __init__
      # __init__.py picked up from somewhere in sys.path
    File "<__main__.Importer>/a/b/__init__.py", line 4, in __init__
      # __init__.py picked up from somewhere in sys.path
  Exception

I.e. in python 2.6 the linecache module correctly picks up PEP302 loaded
code, while the default traceback picks up any source file from sys.path
that happens to match the name of the file that generated the exception.
When run with python2.5.2, the linecache module also locates the wrong
file from sys.path.