Only for functions that have a clear input and output. You can nicely think of all the edge cases, and then get going with the actual code. But most of the tim

Only for functions that have a clear input and output. You can nicely think of all the edge cases, and then get going with the actual code. But most of the time, fixing some bug or implementing a feature is more of experimenting and prototyping at first. Writing tests for every futile attempt would be a waste of time. At best we design some small architecture first with interfaces, and then create the tests off that.