The documentation for contextlib.ExitStack (https://docs.python.org/3.7/library/contextlib.html#replacing-any-use-of-try-finally-and-flag-variables) shows an helper class (Callback) to perform cleanup using a callback.
Problem is, Callback.cancel() calls ExitStack.pop_all(), which in turn calls type(self)(), this is not a valid constructor for Callback, since it always expects at least one element.

(I have marked only Python 3.4 and 3.6 since those are the versions where I verified this, but it likely applies to every version.)