It would be better to move all tests for internal functions in separate TestCase class. It should be clear that this is testing of implementation details and we are free to change tests when change the implementation.

There are some tests for find_function(). They should be moved too. Note that there is a bug in find_function() (it opens a file with default encoding).

Tests for internal functions don't free us from testing the same functionality as a part of high level functions.