What is the difference between np.sum and np.add.reduce?

Question

What is the difference between np.sum and np.add.reduce?
While the docs are quite explicit:

For example, add.reduce()

Answer 1

There is actually one difference that might bite you if you were to blindly refactor from one to the other:

>>> import numpy as np
>>> a = np.arange(4).reshape(2, 2)
>>> 
>>> np.sum(a)
6
>>> np.add.reduce(a)
array([2, 4])
>>> 

The axis default values are different!