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 osprint(__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.

http://docs.python.org/reference/datamodel.html 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.pyprint 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.