Should I unit-test with data that should not be passed in a function (invalid input)?

Question

I am trying to use TDD for my coding practice. I would like to ask should I test with a data that should not happen in a function BUT this data may possibly break your progr

Answer 1

The programming by contract style of design and implementation draws attention to the fact that a single function (method) should be responsible for only some things, not for everything. The other functions that it calls (delegates to) and which call it also have responsibilities. This partition of responsibilities is at the heart of dividing the task of programming into smaller tasks that can be performed separately. The contract part of programming by contract is that the specification of a function says what a function must do if and only if the caller of the function fulfills the responsibilities placed on the caller by that specification. The requirement that the input integer is within the range [0,100] is that kind of requirement.

Now, unit tests should not test implementation details. They should test that the function conforms to its specification. This enables the implementation to change without the tests breaking. It makes refactoring possible.

Combining those two ideas, how can we write a test for a function that is given some particular invalid input? We should check that the function behaves _according to the specification*. But the specification does not say what the function must do in this case. So we can not write any checks of the program state after the invalid function call; the behaviour is undefined. So we can not write such a test at all.