Calling a lambda with a numpy array

Question

While familiarizing myself with numpy, I noticed an interesting behaviour in numpy arrays:

import numpy as np

arr = np.array([1, 2, 3])
scale = lambda x: x * 3

scale(arr) # Gives array([3, 6, 9])

Contrast this with normal Python lists:

arr = [1, 2, 3]
scale = lambda x: x * 3

scale(arr) # Gives [1, 2, 3, 1, 2, 3, 1, 2, 3]

I'm curious as to how this is possible. Does a numpy array override the multiplication operator or something?

Answer 1

numpy.ndarray overloads the * operator by defining its own __mul__ method. Likewise for +, -, etc. This allows for vector arithmetic.