Fix Python – Why use Abstract Base Classes in Python?

Because I am used to the old ways of duck typing in Python, I fail to understand the need for ABC (abstract base classes). The help is good on how to use them.
I tried to read the rationale in the PEP, but it went over my head. If I was looking for a mutable sequence container, I would check for __setitem__, or more likely try to use it (EAFP). I ….