What Does the __File__ Variable Mean/Do?

When a module is loaded from a file in Python, __file__ is set to its path. You can then use that with other functions to find the directory that the file is located in.

Taking your examples one at a time:

A = os.path.join(os.path.dirname(__file__), '..')
# A is the parent directory of the directory where program resides.

B = os.path.dirname(os.path.realpath(__file__))
# B is the canonicalised (?) directory where the program resides.

C = os.path.abspath(os.path.dirname(__file__))
# C is the absolute path of the directory where the program resides.

You can see the various values returned from these here:

import os
print(__file__)
print(os.path.join(os.path.dirname(__file__), '..'))
print(os.path.dirname(os.path.realpath(__file__)))
print(os.path.abspath(os.path.dirname(__file__)))

and make sure you run it from different locations (such as ./text.py, ~/python/text.py and so forth) to see what difference that makes.

I just want to address some confusion first. __file__ is not a wildcard it is an attribute. Double underscore attributes and methods are considered to be "special" by convention and serve a special purpose.

shows many of the special methods and attributes, if not all of them.

In this case __file__ is an attribute of a module (a module object). In Python a .py file is a module. So import amodule will have an attribute of __file__ which means different things under difference circumstances.

Taken from the docs:

__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.

In your case the module is accessing it's own __file__ attribute in the global namespace.

To see this in action try:

# file: test.py

print globals()
print __file__

And run:

python test.py

{'__builtins__': <module '__builtin__' (built-in)>, '__name__': '__main__', '__file__':
 'test_print__file__.py', '__doc__': None, '__package__': None}
test_print__file__.py

Per 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.

and also:

__file__ is to be the “path” to the file unless the module is built-in (and thus listed in sys.builtin_module_names) in which case the attribute is not set.

Just going to add a quick note here (mostly answering the question's title rather than its description) about a change which can confuse some people. As of Python 3.4 there has been a slight change in how the __file__ behaves:

• It's set to the relative path of the module in which it's used, if that module is executed directly.
• It's set to the absolute path of the file otherwise.

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 issue 18416.)

Example:

Calling module x directly and module y indirectly:

# x.py:
from pathlib import Path
import y
print(__file__)
print(Path(__file__))
print(Path(__file__).resolve())

# y.py:
from pathlib import Path
print(__file__)
print(Path(__file__))

Running python3 x.py will output:

/home/aderchox/mytest/y.py                                                                                                                       
/home/aderchox/mytest/y.py                                                                                                                       
x.py                                                                                                                                             
x.py                                                                                                                                             
/home/aderchox/mytest/x.py

Using __file__ combined with various os.path modules lets all paths be relative the current module's directory location. This allows your modules/projects to be portable to other machines.

In your project you do:

A = '/Users/myname/Projects/mydevproject/somefile.txt'

and then try to deploy it to your server with a deployments directory like /home/web/mydevproject/ then your code won't be able to find the paths correctly.