The ConfigParser in Lib has a parameter called `interpolation`, that expects an instance of a subclass of Interpolation. However, when ConfigParser is given an argument of an uninstantiated subclass of Interpolation, the __init__ function of ConfigParser accepts it and continues on. This results in a later receiving an error message along the lines of `TypeError: before_set()
missing 1 required positional argument: 'value'` when functions are later called on the ConfigParser instance. This delay between the feedback and the original mistake has led to a few bugs open on the issue tracker (https://bugs.python.org/issue26831 and https://bugs.python.org/issue26469. Both of which were closed after a quick and simple explanation, which can be easily implemented in the library itself. 

I've created a PR for this work and will attach it shortly. Please let me know if there is a better name for the exception other than `InterpolationIsNotInstantiatedError`. It seems long, but also in line with the other Errors already in configparser.