为什么 python/numpy += 与 arrays 的行为类似？

Question

import numpy as np
x = np.arange(20)
x = np.reshape(x, (4,5))
dW = np.zeros(x.shape)
dWhy = dW
dW += np.sum(x)
dWhy += x

Why do the two methods results in the same result (dW = dWhy), when

dW = dW + np.sum(x)
dWhy = dWhy + x

does not?

Answer 1

Because dWhy = dW makes them the same array. += is not a redefinition (reassignment), it changes the values inplace.

dW += np.sum(x) # affects both since they are the same object
dWhy += x  # affects both since they are the same object

Unless you redefine (reassign) one of them later.

dW = dW + np.sum(x) # redefines dW
dWhy = dWhy + x # redefines dWhy

Another way to make dW and dWhy 2 different arrays that can then be changed inplace independently with += would be to define dWhy = dW.copy() at the start.