I concur with Raymond.

NumPy is a third-party library, Python builtins can't depend on it. This could be solved by introducing a special protocol, but this isn't free. It adds a burden on writing and maintaining code and tests, and adds a runtime overhead of checking if the protocol is supported. This will slowdown sum() for every other collection. It can even slowdown it for short NumPy arrays.

And, as Raymond has noted, if introduce the __sum__ protocol, why not introduce protocols for min(), max(), sort(), all(), and for any other builtin? This has the same drawbacks as for sum(), but multiplied by many times.

If you use NumPy arrays just use NumPy array methods. Otherwise what is the reason of using NumPy?