Why does python/numpy's += mutate the original array?

Question

import numpy as np

W = np.array([0,1,2])
W1 = W
W1 += np.array([2,3,4])
print W

W = np.array([0,1,2])
W1 = W
W1 = W1 + np.array([2,3,4])
print W

Answer 1

Essentially, + and += are different methods that any class can implement. In numpy, += is implemented to do in memory changes, while + returns a new array.

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