Python code coverage and reverse engineering

I just found this Python coding style guidelines and I want to comment on the following paragraph regarding changing a function/method’s behaviour:

Even with unit testing, it’s really hard to track dependencies like this. We would need code coverage tools and exhaustive tests, neither of which we had.

I also find it hard to track dependencies in Python code and I’m aware of, at least, one code coverage tool for Python useful for improving test suites (but try to avoid some common pitfalls).