Why built-in functions like abs works on numpy array?

Question

I feel surprised that abs works on numpy array but not on lists. Why is that?

import numpy as np

abs(np.array((1,-2)))
array([1, 2])

abs([1,-1])
TypeError: bad operand type for abs(): 'list'

Also, built in functions like sum also works on numpy array. I guess it is because numpy array supports __getitem__? But in case of abs, if it depends on __getitem__ it should work for list as well, but it didn't.

Answer 1

That's because numpy.ndarray implements the __abs__(self) method. Just provide it for your own class, and abs() will magically work. For non-builtin types you can also provide this facility after-the-fact. E.g.

class A:
    "A class without __abs__ defined"
    def __init__(self, v):
        self.v = v

def A_abs(a):
    "An 'extension' method that will be added to `A`"
    return abs(a.v)

# Make abs() work with an instance of A
A.__abs__ = A_abs

However, this will not work for built-in types, such as list or dict.