Files get closed automatically when they are no longer referenced, but it is not good practice to do so. In fact, in Python 3 you will now get warnings that the system had to close files for you if you didn't do it.

Moral: Clean up after yourself. :)

Some Pythons will close files automatically when they are no longer referenced, while others will not and it's up to the O/S to close files when the Python interpreter exits.

Even for the Pythons that will close files for you, the timing is not guaranteed: it could be immediately, or it could be seconds/minutes/hours/days later.

So, while you may not experience problems with the Python you are using, it is definitely not good practice to leave your files open. In fact, in cpython 3 you will now get warnings that the system had to close files for you if you didn't do it.

Moral: Clean up after yourself. :)