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.

Answer 2

The Python convention is to name "private" functions and methods with a leading underscore. When you see a leading underscore, you know not to try and use it.

Remember, Python is not Java.

Answer 3

I don't think that there is any chance to access inner() from the extern namespace.

However, in my opinion the fact that you keep inner() nested implies that the only "contract" that really matters is outer()'s one. inner() is part of the implementation, and you shouldn't want to test the implementation. If you really want to test inner(), do extensive tests on outer() with data that will involve all the functionalities of inner().