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

The mock discussion still misses the real solution, which is to refactor the code so that you have a function that simple reads the file and returns json that is essentially a wrapper around open and doesn't need to be tested.

Then have your main function take in that json as a parameter (or class wrapping that json).

Then your code becomes the ideal code. Stateless and with no interaction with the outside world. Then it's trivial to test just like and other function that is simple inputs translated outputs (ie pure).

Every time you see the need for a mock, you're first thought should be "how can I take the 90% or 95% of this function that is pure and pull it out, and separate the impure portion (side effects and/or stateful) that now has almost no logic or complexity left in it and push it to the boundary of my codebase?"

Then the complex pure part you test the heck out of, and the stateful/side effectful impure part becomes barely a wrapper over system APIs.

Funnily enough, I am preparing a simple presentation at work to speak about exactly that. The idea of separating "logic" from I/O and side effects is an old one and can be found in many architectures (like hexagonal architecture). There is plenty of benefit doing this, but testing is a big one.

It should be obvious, but this is not something that seem to be thought in school or in most workplaces, and when it is, it's often through the lens of functional programming, which most just treat as a curiosity and not a practical thing to use at work. So I started to teach this simple design principle to all my junior dev because this is something that is actually quite easy to implement, does not need a complete shift of architecture/big refactor when working on existing code, and is actually practical and useful.