For example, let's assume we have a class that is imported three times from other classes. This would lead to a reusability of three. However, as there might be functions that are called only once, it would make more sense to count the called functions.
One general suggestion from measuring code reusability is to answer the following question: "How many functions are called from more than one place?"
What would be the best way to realise the solution, e.g. counting the functions that are called in other parts of the code with Python?