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.
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