Is module file attribute absolute or relative

Question

I m having trouble understanding   file    From what I understand    file   returns the absolute path from which the module was loaded    I m having problem producing this  I have a abc py with one statement print   file    running from  d projects  python abc py returns abc py  running from  d  returns projects abc py  Any reasons why

User · Answer

With the help of the of Guido mail provided by @kindall, we can understand the standard import process as trying to find the module in each member of sys.path, and file as the result of this lookup (more details in PyMOTW Modules and Imports.). So if the module is located in an absolute path in sys.path the result is absolute, but if it is located in a relative path in sys.path the result is relative.

Now the site.py startup file takes care of delivering only absolute path in sys.path, except the initial '', so if you don't change it by other means than setting the PYTHONPATH (whose path are also made absolute, before prefixing sys.path), you will get always an absolute path, but when the module is accessed through the current directory.

Now if you trick sys.path in a funny way you can get anything.

As example if you have a sample module foo.py in /tmp/ with the code:

import sys
print(sys.path)
print (__file__)

If you go in /tmp you get:

>>> import foo
['', '/tmp', '/usr/lib/python3.3', ...]
./foo.py

When in in /home/user, if you add /tmp your PYTHONPATH you get:

>>> import foo
['', '/tmp', '/usr/lib/python3.3', ...]
/tmp/foo.py

Even if you add ../../tmp, it will be normalized and the result is the same.

But if instead of using PYTHONPATH you use directly some funny path you get a result as funny as the cause.

>>> import sys
>>> sys.path.append('../../tmp')
>>> import foo
['', '/usr/lib/python3.3', .... , '../../tmp']
../../tmp/foo.py

Guido explains in the above cited thread, why python do not try to transform all entries in absolute paths:

we don't want to have to call getpwd() on every import .... getpwd() is relatively slow and can sometimes fail outright,

So your path is used as it is.

User · Answer

Late simple example   from os import path  getcwd  chdir  def print my path        print  cwd          format getcwd         print    file       format   file         print  abspath      format path abspath   file       print my path    chdir        print my path     Under Python-2    the second call incorrectly determines the path abspath   file    based on the current directory   cwd      C  codes py   file   cwd mayhem py abspath  C  codes py cwd mayhem py cwd      C  codes   file   cwd mayhem py abspath  C  codes cwd mayhem py   As noted by  techtonik  in Python 3 4   this will work fine since   file   returns an absolute path

User · Answer

From the documentation        file   is the pathname of the file from which the module was loaded  if it was loaded from a file  The   file   attribute is not present for C modules that are statically linked into the interpreter  for extension modules loaded dynamically from a shared library  it is the pathname of the shared library file    From the mailing list thread linked by  kindall in a comment to the question      I haven t tried to repro this particular example  but the reason is   that we don t want to have to call getpwd   on every import nor do we   want to have some kind of in-process variable to cache the current   directory   getpwd   is relatively slow and can sometimes fail   outright  and trying to cache it has a certain risk of being wrong        What we do instead  is code in site py that walks over the elements of   sys path and turns them into absolute paths  However this code runs   before    is inserted in the front of sys path  so that the initial   value of sys path is       For the rest of this  consider sys path not to include      So  if you are outside the part of sys path that contains the module  you ll get an absolute path  If you are inside the part of sys path that contains the module  you ll get a relative path   If you load a module in the current directory  and the current directory isn t in sys path  you ll get an absolute path   If you load a module in the current directory  and the current directory is in sys path  you ll get a relative path

User · Answer

file   is absolute since Python 3 4  except when executing a script directly using a relative path      Module   file   attributes  and related values  should now always contain absolute paths by default  with the sole exception of   main     file   when a script has been executed directly using a relative path   Contributed by Brett Cannon in bpo-18416     Not sure if it resolves symlinks though   Example of passing a relative path     python script py

[python] Is module file attribute absolute or relative?

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