A common task in programs I've been working on lately is modifying a text file in some way. (Hey, I'm on Linux. Everything's a file. And I do large-scale system admin.)
But the file the code modifies may not exist on my desktop box. And I probably don't want to modify it if it IS on my desktop.
I've read about unit testing in Dive Into Python, and it's pretty clear what I want to do when testing an app that converts decimal to Roman Numerals (the example in DintoP). The testing is nicely self-contained. You don't need to verify that the program PRINTS the right thing, you just need to verify that the functions are returning the right output to a given input.
In my case, however, we need to test that the program is modifying its environment correctly. Here's what I've come up with:
1) Create the "original" file in a standard location, perhaps /tmp.
2) Run the function that modifies the file, passing it the path to the file in /tmp.
3) Verify that the file in /tmp was changed correctly; pass/fail unit test accordingly.
This seems kludgy to me. (Gets even kludgier if you want to verify that backup copies of the file are created properly, etc.)  Has anyone come up with a better way?
You might want to setup the test so that it runs inside a chroot jail, so you have all the environment the test needs, even if paths and file locations are hardcoded in the code [not really a good practice, but sometimes one gets the file locations from other places...] and then check the results via the exit code.
Unit Testing File Modifications