python - Running unit tests on nested functions

Question

I come from the Java world, where you can hide variables and functions and then run unit tests against them using reflection. I have used nested functions to hide implementation details of my classes so that only the public API is visible. I am trying to write unit tests against these nested functions to make sure that I don't break them as I develop. I have tried calling one of the nested functions like:

def outer():
    def inner():
        pass

outer.inner()

which results in the error message:

AttributeError: 'function' object has no attribute 'inner'

Is there a way for me to write unit tests against these nested functions? If not, is there a way to trigger the name munging for function names like you can for class variables by prefixing them with __?

Answer 1

inner doesn't exist until outer makes it. You should either move inner up to a toplevel function for testability, or have the outer test test all the possible execution paths of itself and inner.

Do note that the inner function isn't a simple function, it's a closure. Consider this case:

def outer(a):
    b = compute_something_from(a)
    def inner():
        do_something_with(a, b)

That's the standard testability trade-off. If your cyclomatic complexity is too high, your tests will be too numerous.