Why does np.array[None] update all values in that numpy arrray?

Question

I found a bug in my code and while fixing that bug, and I noticed a peculiar behavior in numpy.

a = np.arange(10) # ints 0 -> 9
a[None] = 20

print(a) # output --> [20, 20, 20, 20, 20, 20, ... 20]

Why do the values in every position get replaced with 20?

Quang Hoang · Accepted Answer · 2021-03-08 16:16:43Z

2

This is the expected behavior, not a bug. a[None] is not slicing/indexing but broadcasting. What it does does is to provide a different view of the underlying data of a.

And since you are modifying this view with a[None]=20, you also modify a's data, and therefore a itself.

answered Mar 8, 2021 at 16:16

Quang Hoang

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1 Comment

hpaulj Over a year ago

a[None] is the same as a[None,:] and produces a (1,10) shaped view. As you say it's that view that's being changed.

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