An overly aggressive mock can work fine, but break much later

This blog post talks as if mocking the `open` function is a good thing that people should be told how to do. If you are mocking anything in the standard library your code is probably structured poorly.

In the example the author walks through, a cleaner way would be to have the second function take the Options as a parameter and decouple those two functions. You can then test both in isolation.

Details matters, but good test doubles here are important. You want to capture all calls to IO and do something different. You don't want tests to break because someone has a different filesystem, didn't set their home directory as you want it setup, or worse is trying to run two different tests at the same time and the other test is changing files the other wants.

Note that I said test doubles. Mocks are a bit over specific - they are about verifying functions are called at the right time with the right arguments, but the easy ability to set return values makes it easy to abuse them for other things (this abuse is good, but it is still abuse of the intent).

In this case you want a fake: a smart service that when you are in a test setups a temporary directory tree that contains all the files you need in the state that particular test needs, and destroys that when the test is done (with an optional mode to keep it - useful if a test fails to see debug). Depending on your situation you may need something for network services, time, or other such things. Note that in most cases a filesystem itself is more than fast enough to use in tests, but you need isolation from other tests. There are a number of ways to create this fake, it could override open, or it could just be a GetMyProgramDir function that you override are two that I can think of.