Probably my example didn’t emphasize it, but ‘outer’ function could be nested into another function (thus no global state modification) and all I was caring about was a variable in the outside scope and how to modify it.

Maybe the following example explains my intentions better than the old one:

# main function
def outer():
    outer.error = False

    # inner function does all the job and modifies the flag
    def inner(index):
        if index == 5:
            outer.error = True

    for index in range(10):
        inner(index)

    return 'FAILURE' if outer.error else 'SUCCESS'

>>> outer()

Quite often I would use classes and avoid workarounds altogether by storing state in a class instance.

I often use a 1-element list as the state box for a mutating closure.

*sigh* I love closures.