Is Module _File_ Attribute Absolute or Relative

Is module __file__ attribute absolute or relative?

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.

How to retrieve a module's path?

import a_module
print(a_module.__file__)

Will actually give you the path to the .pyc file that was loaded, at least on Mac OS X. So I guess you can do:

import os
path = os.path.abspath(a_module.__file__)

You can also try:

path = os.path.dirname(a_module.__file__)

To get the module's directory.

Reading file using relative path in python project

Relative paths are relative to current working directory.
If you do not your want your path to be, it must be absolute.

But there is an often used trick to build an absolute path from current script: use its __file__ special attribute:

from pathlib import Path

path = Path(__file__).parent / "../data/test.csv"
with path.open() as f:
    test = list(csv.reader(f))

This requires python 3.4+ (for the pathlib module).

If you still need to support older versions, you can get the same result with:

import csv
import os.path

my_path = os.path.abspath(os.path.dirname(__file__))
path = os.path.join(my_path, "../data/test.csv")
with open(path) as f:
    test = list(csv.reader(f))

[2020 edit: python3.4+ should now be the norm, so I moved the pathlib version inspired by jpyams' comment first]

python unipath: path to current file directory (ancestor) outputs nothing

Use os.path.abspath(__file__) instead of __file__.

This is because __file__ contains a relative path in your case.

__file__ can contain a relative path or an absolute one in different situations:

So, if you aren't inside 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.

(quote from Python __file__ attribute absolute or relative?)

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