I disagree. The intention in foo is not for x and y to be numpy arrays, it is to add 2 objects together. "If it walks like a duck and it quacks like a duck, th

I disagree. The intention in foo is not for x and y to be numpy arrays, it is to add 2 objects together. "If it walks like a duck and it quacks like a duck, then it must be a duck". That function can work on anything implementing +, this is the philosophy of Python. It enables the whole numpy, dask, TensorFlow ecosystem. If you intend to limit it to numpy arrays, you either use numpy 1.20's static types, or you add more checks at the beginning of the function, such as np.asarray(x).